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Speech perception differences in children with dyslexia and persistent speech delay
Accepted Manuscript
Speech perception differences in children with dyslexia and persistent speech delay Kathryn L. Cabbage , Tiffany P. Hogan , Thomas D. Carrell PII: DOI: Reference:
S0167-6393(16)30137-6 10.1016/j.specom.2016.05.002 SPECOM 2378
To appear in:
Speech Communication
Received date: Revised date: Accepted date:
10 July 2015 23 May 2016 25 May 2016
Please cite this article as: Kathryn L. Cabbage , Tiffany P. Hogan , Thomas D. Carrell , Speech perception differences in children with dyslexia and persistent speech delay, Speech Communication (2016), doi: 10.1016/j.specom.2016.05.002
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ACCEPTED MANUSCRIPT Running Head: SPEECH PERCEPTION DIFFERENCES
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Highlights
The degree to which speech perception deficits overlap is unknown in children with dyslexia and children with persistent speech delay. This study used sine-wave speech and amplitude-comodulated sine-wave speech to
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determine whether these children respond similarly to stimuli preserving spectral structure.
Amplitude comodulation improves speech perception in children with phonological
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impairments, particularly for children with dyslexia.
Children with persistent speech delay have difficulty perceiving words containing their
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erred sound.
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Speech perception differences in children with dyslexia and persistent speech delay Kathryn L. Cabbage1 Tiffany P. Hogan1
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Thomas D. Carrell2
MGH Institute of Health Professions, 36 1st Avenue, Boston, MA 02129; Cabbage email: [email protected]
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Hogan email: [email protected]
University of Nebraska – Lincoln, 357 Barkley Memorial Center, Lincoln, NE 68583;
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Carrell email: [email protected]
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Corresponding author: Kathryn Cabbage MGH Institute of Health Professions 36 1st Avenue Boston, MA 02129 phone: 402-309-3111 email: [email protected]
Abstract Purpose: Deficits in phonology, which are related to the organization and retrieval of speech sounds in the mental lexicon, are associated with two distinct clinical disorders, dyslexia and
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persistent speech delay. Research has shown associated speech perception deficits in individuals with these disorders, but the degree to which the underlying perceptual deficits overlap is unclear. In this study, we investigated how children with dyslexia, children with persistent speech delay and their typically-developing peers differed in speech perception when listening to
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speech stimuli that varied in non-linguistic temporal fine structure acoustic characteristics.
Method: Thirty-six children were classified into three groups (dyslexia, speech sound disorder, typically-developing; n=12 in each group) based on their reading and speech articulation skills. Novel single-word sine-wave speech stimuli was aurally presented to the children.
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Results: There were no group differences between children with dyslexia and their typicallydeveloping peers, but the children with persistent speech delay had more difficulty than the other two groups. In particular, the children with persistent speech delay had difficulty recognizing
misarticulated in their own speech.
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words with limited acoustic structure when the stimuli involved a phoneme that they
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Conclusions: These results suggest a specific difficulty in the perception of temporal fine
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structure in children with persistent speech delay.
Keywords: speech perception, persistent speech delay, speech sound disorder, dyslexia,
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phonological processing
I.
Introduction The development of spoken and written language is reliant on an intact phonological
system. In children, phonological deficits may manifest as difficulty mapping phonemes onto
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letters during the reading process, as is the case with dyslexia (Goswami, 2000). Alternatively, children may manifest phonological deficits in the form of a difficulty producing speech sounds accurately as seen in children with speech delay (Anthony et al., 2011; Rvachew, 2007). Poorly specified phonological representations have been implicated for both children with dyslexia
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(Boada & Pennington, 2006; Elbro et al., 1998; Goswami, 2000; Swan & Goswami, 1997) and children with speech delay (Anthony et al., 2011; Edwards, Fourakis, Beckman, & Fox, 1999; Rvachew et al., 2003; Sutherland & Gillon, 2005; Sutherland & Gillon, 2007). Phonological representations refer to the storage of the phonological characteristics of words in long-term
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memory, and it is plausible that deficits in a listener‟s speech perception may impact the quality of phonological representations necessary for speaking and reading processes. For example, if children have deficits in encoding and processing the acoustic-phonetic features of speech
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stimuli, this may interfere with the establishment of well-specified phonological representations in the mental lexicon (Fowler, 1991). In independent lines of research, speech perception skills
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of children with dyslexia and children with speech delay have been studied to better understand the nature of phonological deficits in these children. Although both populations have shown
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speech perception deficits, the degree to which the underlying perceptual deficit overlaps is
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unclear. The present study investigates the speech perception skills of both children with dyslexia and children with speech delay to determine whether these children differ in their
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speech perception for particular acoustic manipulations. In its natural form, running speech comprises multiple redundant cues that signal the
identity of phonemes and, thus, in typical listening situations, it is unlikely that a single cue is responsible for the accurate perception of any particular phoneme. This acoustic redundancy plays an important role in successful speech perception in everyday environments that are rarely
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free of noise, reverberation, or distortion (Ziegler et al., 2009). Individuals with dyslexia have difficulty perceiving speech when presented with a compromised speech signal such as speechin-noise, sine-wave speech, band-limited speech, or the multiple simultaneous distortions found in classrooms (Gruber, 2003; Johnson et al., 2011; Rosner et al., 2003; Ziegler et al., 2009).
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Signals that mask or omit the acoustic redundancy present in natural speech may interfere with a listener‟s ability to integrate relevant acoustic properties with their own internal representations. For listeners with poorly specified phonological representations, it may be especially difficult to accurately perceive a signal with limited acoustic detail.
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Compared to a naturally-spoken utterance, sine-wave speech is an acoustically-sparse, speech-based signal generated by creating sinusoidal tones that follow the center frequencies of the first four formants of a naturally-spoken utterance. This manipulation results in a signal that
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does not contain the fundamental frequencies, harmonic structure, or short-term spectral cues from the original spoken sentences. Thus, the resulting signal comprises several time-varying
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inharmonic frequency components that faithfully follow the original formant center frequencies yet the amplitude information of the original signal is so poorly specified that it is nearly binary.
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The listener‟s impression of this material is influenced by the instructions in the experiment
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(Remez et al., 1980). It ranges from not even recognizing the stimuli as speech-based, to hearing many of the words correctly. Overall, however, listener performance is low regardless of
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experimenter instructions. A simple acoustical manipulation, amplitude comodulation of the sine-wave sentences,
greatly increases their intelligibility. Amplitude comodulation refers to a digital manipulation of the signal which simultaneously modulates the sine-wave tones at the same frequency and phase. In other words, it is analogous to rapidly turning on and off each of the four component tones
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simultaneously, that is, to comodulate the four tones. The intelligibility of sine-wave speech increases significantly when the signal is amplitude comodulated for both adults (Carrell & Opie, 1992) and typically-developing children (Cabbage, Carrell & Hogan, 2011; Lewis & Carrell, 2007). Importantly, intelligibility increases despite the fact that no phonetic, syntactic, or
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semantic information has been added. Likewise, additional amplitude structure is unrelated to the amplitude envelope of the original utterance and only includes an arbitrary modulation of 80 Hz added to the signal. Amplitude comodulation does not refer to traditionally-defined
suprasegmental information such as intonation, tempo, and other forms of traditional
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suprasegmental information. Amplitude comodulation has been suggested to acoustically group components of sound together based on the phenomenon of comodulation masking release (Carrell & Opie, 1992). The grouping allows the simultaneous tones to be processed as one
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“object” rather than several for later, perhaps more linguistic, processes. Previous research has shown that typically-developing children have better perception for
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amplitude-modulated sine-wave speech signals than sine-wave speech (Lewis & Carrell, 2007). It is plausible that children with phonological impairments will not show these same gains
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because of an overall weak sensitivity to acoustic structure as compared to typical children.
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Children with unimpaired phonological representations, whether listening to either sine-wave speech or amplitude comodulated sine-wave speech, may use their own internal representations
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to “fill in” the gaps and recover missing acoustic structure to make sense of a signal. Children with phonological deficits, and thus likely underspecified phonological representations, may lack the necessary acoustic detail in their own representations to “fill in” the gaps for successful speech perception. Indeed, previous research has demonstrated the importance of long-term phonological representations for success in phonological serial recall tasks that are subject to
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short-term memory decay (Hulme et al., 1997). Thomson, Richardson, and Goswami (2005), investigating children with dyslexia, attributed the erred intrusion of real words during nonword recall as evidence these children have less well-specified phonological representations to draw on to reconstruct nonwords. It is possible that children with phonological deficits (i.e., dyslexia,
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speech sound disorder) may evidence similar less robust representations when presented with novel speech stimuli characterized by minimal acoustic structure. Separate lines of research suggest children with dyslexia and children with speech delay may respond differently to
particular acoustic cues during speech perception. Children with dyslexia appear to have more
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difficulty when speech perception is reliant on acoustic features that cue amplitude structure during speech perception (Goswami, 2011; Goswami et al., 2002; Goswami et al., 2011; Muneaux et al., 2004), whereas children with speech delay appear to have more difficulty when
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speech perception is reliant on acoustic features related to spectral cues (Johnson et al., 2011). Evidence suggests individuals with dyslexia show particular impairments for perception
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tasks relying on cues associated with amplitude, or global, structure such as amplitude envelope detection (Goswami, 2011), rise time discrimination (Goswami et al., 2002; Goswami et al.,
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2011; Muneaux et al., 2004), and amplitude modulation detection (Lorenzi et al., 2000;
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Rocheron et al., 2002). By design, sine-wave speech is devoid of most amplitude structure and comprises primarily spectral information. Thus, because spectral structure is comparatively
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preserved in sine-wave speech signals, it is possible that children with dyslexia do not show deficits perceiving sine-wave speech and amplitude-modulated sine-wave speech stimuli. By contrast, children with speech delay may exhibit difficulty with sine-wave speech and amplitudecomodulated sine-wave speech precisely because these signals are limited to primarily spectral information and lack additional acoustic redundancy, such as amplitude structure. Although there
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is considerably less research investigating speech perception skills in children with speech delay, Johnson and colleagues (2011) provided compelling evidence demonstrating that speech perception tasks for speech stimuli containing primarily global amplitude structure, i.e., vocoded speech, did not pose particular difficulty for children with a history of speech delay as compared
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to their peers with dyslexia. Children in their study, however, were given another task, syllable discrimination, that required the perception of spectral structure, i.e., formant transitions from fricatives /s, ʃ/ into a following vowel. Notably, the children with a history of speech delay, in a post-hoc comparison, were the only group shown to respond to this spectral cue more poorly
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than typical children. Neither children with dyslexia nor children with dyslexia and speech delay performed differently from typical children. Is it possible that children with speech sound disorder primarily attend to different acoustic cues both in perception and production, than peers
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with a different type of phonological impairment, such as dyslexia? If this is the case, we would predict that when these children are completing speech perception tasks involving sine-wave
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speech, which is characterized by the preservation of spectral structure of speech and very
speech delay.
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rudimentary amplitude information, children with dyslexia would outperform children with
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On the other hand, children with phonological deficits may simply have difficulty perceiving speech that is impoverished in some way, regardless of the type of preserved structure
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in the speech signal. Several studies have demonstrated that children with dyslexia show particular weakness when given poor or noisy speech signals, (Gruber, 2003; Johnson et al., 2011; Rosner et al., 2003; Ziegler et al., 2009), thus it is possible children with dyslexia simply exhibit deficits for perception of impoverished speech. Children with dyslexia performed poorer than their typically-developing peers on perception of speech-in-noise and band-limited speech
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(Ziegler et al., 2009). These deficits were found in comparison to both chronological agematched peers and reading-level matched peers, suggesting the perceptual problem was specifically associated with a disordered, dyslexic profile, instead of delayed reading skills. Rosner and colleagues (2003) presented adults with dyslexia with short sine-wave speech
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sentences and found that some, but not all, adults with dyslexia exhibited poor perception as compared to controls.
Although little research has investigated speech perception skills in children with speech sound disorder for speech stimuli that are impoverished in some way, evidence from a variety of
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speech perception tasks for syllables and words has shown these children exhibit particular
difficulty perceiving speech sounds they produce incorrectly (Broen et al., 1983; Hoffman et al., 1985; Locke, 1980; Monnin & Huntington, 1974; Rvachew & Jamieson, 1989; Rvachew et al.,
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2004). This may indicate a deficiency in the acoustic specificity for these erred phonemes in a child‟s own internal representations. Children who do not produce a particular phoneme
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correctly may exhibit particular difficulty perceiving that same phoneme in sine-wave speech or amplitude-comodulated sine-wave speech. As a result of this observation, in the present study we
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elected to include children with speech delay characterized by an erred production of /r/ to test
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whether these children had difficulty with a speech perception task specifically involving /r/. The /r/ phoneme is a late-developing speech sound in speech production and is often associated with
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a subset of children with speech delay who are classified as exhibiting persistent speech errors, or persistent speech delay (Shriberg et al., 2010). In the present study, we designed stimuli that removed the majority of acoustic features
normally present in natural speech and presented it to children with dyslexia, persistent speech delay, and their typically-developing peers. Our first research question asked whether children
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with dyslexia and children with persistent speech delay differed in their word recognition for sine-wave speech and amplitude-comodulated sine-wave speech. Because children with dyslexia have demonstrated difficulty with speech perception tasks involving global amplitude structure and children with speech delay have demonstrated particular difficulty with the perception of
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temporal fine structure (Johnson et al., 2011), we hypothesized that children with dyslexia would exhibit superior word recognition for sine-wave speech and amplitude-comodulated sine-wave speech as compared to their peers with persistent speech delay.
Our second research question asked whether children with persistent speech delay have
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more difficulty perceiving a phoneme erred in their own production than a phoneme that is
produced correctly in their own speech (Locke, 1980; Shuster, 1998). We hypothesized that children with persistent speech delay would have more difficulty perceiving a phoneme they did
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not produce correctly (in this study the erred /r/ phoneme) in both sine-wave speech and amplitude-comodulated sine-wave speech, as compared to their peers without speech delay.
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Because children with speech delay in the current study were selected to participate if they exhibited persistent speech errors in production of the /r/ phoneme, we predicted these children
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would demonstrate a particular difficulty perceiving sine-wave speech and amplitude-
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comodulated sine-wave speech tokens of the /r/ phoneme in words. 2. Method
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2.1 Participants
Thirty-six children ranging in age from 7;6 – 9;6 (years; months) participated in this
study. This age range was selected to include children who were old enough to have received reading instruction and exhibit word reading deficits, while also including children who were young enough to exhibit speech articulation deficits. Children were grouped into three groups
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(n=12 in each group): dyslexia, persistent speech delay and age-matched typically-developing peers. Children with dyslexia exhibited word reading abilities below the 30th percentile as compared to their peers and exhibited no speech articulation deficits, nor had a history of such deficits per written parent report. Children with persistent speech delay exhibited articulation
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deficits in speech production primarily characterized as difficulty producing the /r/ phoneme, described in detail below. Children with persistent speech delay exhibited word reading abilities in the average to above-average range. All children exhibited normal receptive and expressive language skills, were monolingual speakers of Standard American English and passed a pure
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tone hearing screening with thresholds at 20 dB or lower at the octave frequencies between 500 – 8000 Hz. 2.2 Assessment
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Articulation. Articulation skills were measured using the sounds-in-words subtest of the Goldman-Fristoe Test of Articulation-2 (GFTA-2, Goldman & Fristoe, 2000) and a probe
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designed to assess each child‟s production of the /r/ phoneme, which was adapted from a probe developed previously (Preston & Edwards, 2007). A speech-language pathologist transcribed
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participant speech production for each word using broad transcription with the International
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Phonetic Alphabet. A second transcriptionist independently transcribed a random 20% of samples. Inter-rater reliability was 98.4%. Children with persistent speech delay scored below
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the 16th percentile on the GFTA-2 and had difficulty producing /r/ in any or all positions of words. Of the 12 children with persistent speech delay, four children only exhibited difficulty with /r/, and eight children exhibited additional articulation errors including /s, z, sh, th/. All of these phonemes are also considered late-developing (Smit et al., 1990), and are common errors for those with a persistent speech delay. To further quantify each child‟s error pattern for
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production of /r/, a 46-word /r/ probe with words containing /r/ in both prevocalic (i.e. rat, rake) and post-vocalic (i.e. bird, bear) positions of words was administered to all children with persistent speech delay. The nature of /r/ distortions for children with persistent speech delay in this study was primarily characterized as gliding of /r/, /w/ for /r/ substitution, or vowelization of
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/r/. All but one child had difficulty producing /r/ in all positions of words. One child produced pre-vocalic /r/ accurately but did not produce /r/ in any post-vocalic contexts.
Reading. Word reading skills were assessed via the Basic Skills subtests of the
Woodcock Reading Mastery Test – Revised/Normative Update (WRMT-R/NU, Woodcock, 1998).
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All participants were administered the Word Identification and Word Attack subtests. The Word Identification subtest measures participants‟ reading abilities for real words that increase in difficulty from high to low frequency words. The Word Attack subtest measured each child‟s
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ability to apply phonics skills to nonwords and low-frequency words during reading. A composite Word Reading score was computed based on the combination of results from each
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subtest. Participants who received a standard score of 92 or below (< 30th percentile) (Manis et al., 1996) were considered potential candidates for the dyslexia group. Cut-off scores for the
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classification of dyslexia varies widely across studies ranging from the 7th percentile (Badian et
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al., 1990) to 30th percentile (Manis et al., 1996). Additionally, children in the persistent speech delay and typically-developing groups were required to receive a standard score of at least 100 to
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prevent the inclusion of any children in these groups with subclinical reading deficits. Non-verbal intelligence. Children were administered subtests of the Reynolds
Intellectual Assessment Scales (RIAS; Reynolds & Kamphaus, 2003) to verify normal non-verbal cognitive ability (standard score > 80).
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Receptive/Expressive vocabulary. Because of the positive relationship between vocabulary knowledge and the specificity of phonological representations (Metsala & Walley, 1998), all children were administered the Receptive One-Word Picture Vocabulary Test (ROWPVT; Brownell, 2000b) to assess receptive vocabulary skills and the Expressive One-Word
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Picture Vocabulary Test (EOWPVT; Brownell, 2000A) to assess expressive vocabulary skills. Phonological Awareness. Phonological awareness skills, which have been shown to be positively correlated with reading ability (Larivee & Catts, 1999), were assessed using selected subtests (i.e. Elision, Blending Words) of the Comprehensive Test of Phonological Processing
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(CTOPP; Wagner et al., 1999). The Elision subtest was a phoneme deletion task wherein
children were auditorily given a word and asked to delete a specific phoneme and say the resulting target word (ex. Say “meet” without /t/. Answer: “me”.). The Blending Words subtest
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involved auditorily presenting children with a series of phonemes and asking them to blend the
Answer: “mad”.). 2.3 Stimuli
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phonemes into an appropriate word (ex. “What word do these sounds make? /m/ /æ/ /d/.
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In this study, we used novel speech stimuli comprising rudimentary frequency and
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amplitude information (i.e. sine-wave speech, amplitude-comodulated sine-wave speech). The experimental word stimuli consisted of 12 pairs of consonant-vowel-consonant (CVC) or
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consonant-vowel (CV) rhyming words that began with the initial phonemes /r/ or /m/, to address hypothesized deficits for children with persistent speech delay (/r/) in contrast to a control phoneme (/m/) which was produced correctly by all children. An additional 12 words, all beginning with the phoneme /w/, were matched to and rhymed with the experimental stimuli to serve as foils. To determine eligible words for inclusion in this study, we generated a list of
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common CVC and CV real words with /r/ in the initial positions of words (ex. rake, row, run). We substituted /m/ and /w/ for /r/ in these words and rejected any rhymes that did not yield real words (ex. mun, wope). Of the resulting eligible triads, 12 were selected for inclusion in this study (see Table 1). We calculated word frequency values for each item using an online
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calculator based on a child corpus of Spoken American English (Storkel & Hoover, 2010).
Analysis of variance revealed that the words were similar across various word characteristics including word frequency (F(2,35) = 0.665, p = 0.521), part of speech (F(2,35) = 0.295, p =
(F(2,35) = 2.320, p = 0.114).
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0.746), duration (in milliseconds) (F(2,35) = 0.013, p = 0.987), and neighborhood density
Table 1. Stimulus words used for the word recognition task. Word Duration, in Number of Frequencya msb neighborsc
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raid rail rake rare rate ray raise real right rock rose row
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/m/ words 461 443 386 435 404 432 567 401 415 404 601 398 M=449.38 SD= (70.53)
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4.06 2.92 3.90 2.20 2.32 4.15 1.78 2.48 3.83 1.90 1.00 1.60 M=2.75 SD=(1.09)
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made mail make mare mate may maze meal might mock mows mow
/r/ words 2.00 507 2.20 436 2.04 440 2.61 428 3.32 428 2.28 594 2.72 424 1.30 415 3.86 361 2.88 397 2.93 574 2.56 395 M=2.508 SD= M=445.38
16 15 20 18 21 21 8 14 21 14 9 21 M=16.72 SD= (4.81) 22 16 22 16 25 26 8 14 19 16 15 28 M=18.54
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wade whale wake wear wait weigh ways wheel white walk woes whoa
SD= (70.04)
1.30 1.00 2.36 2.59 3.27 3.96 1.00 2.75 3.56 3.00 1.00 1.70 M=2.29 SD =1.07
SD= (5.73)
/w/ words 483 616 385 429 433 424 424 413 339 392 643 411 M=450.50 SD =90.57
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17 2 20 20 18 23 9 6 6 12 22 9 M=23 SD =7.18
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(0.67)
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Word frequency obtained from an online child corpus calculator (Storkel & Hoover, 2010). Word duration, in milliseconds, was calculated from the onset of phonation to offset of the final phoneme in each word. c Number of neighbors refers to the number of real words that can be obtained by a single phoneme omission, substitution calculated by an online child corpus calculator (Storkel & Hoover, 2010).
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The /r/ was selected as a target phoneme to address our research hypotheses regarding the
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perceptual deficit we expected for school-age children with persistent speech delay. The /m/ was
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selected as a control phoneme because it is an early developing sound in speech production and rare for school-age children (Smit et al., 1990) with speech delay to exhibit difficulty producing
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the /m/ at any point in development. Both phonemes were well-suited for representation in sinewave speech and amplitude-comodulated speech because of their respective acoustic structure,
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which depend on frequency relevant features such as formant transitions. Such acoustic structure is readily preserved in both sine-wave speech and amplitude-comodulated sine-wave speech.
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Moreover, rudimentary adjustments to amplitude levels enabled the simulation of naturally subtle amplitude differences (i.e. anti-resonance for nasality in /m/). For example, a key feature of the /r/, the height of the third formant frequency, is fully represented in both sine-wave speech and amplitude-comodulated sine-wave speech. The /m/ relies on the blending of the formant transition into the following vowel with minor adjustments to amplitude levels described below
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to mimic features of anti-resonance present in the production of nasal phonemes during natural speech production. Naturally-produced tokens of each word were recorded by an adult female speaker with a standard dialect of Midwest American English while seated in a single-walled isolated acoustic
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chamber. The words were randomized into three separate lists for recitation to eliminate order effects of reading during recording. All tokens were recorded at a sampling rate of 44.1 kHz and an amplitude resolution of 16 bits, using a desktop microphone (AKG C414B) and a Zoom H4N digital recorder. After recording was complete, audio files were digitally transferred to a personal
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computer and segmented into individual words using CoolEdit 2K. All words were screened for mispronunciations, peak clipping, and background noise and normalized at -.5 dB (re: 16 bits = 96 dB peak). Following this process, one of the remaining tokens of each word was randomly
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selected for inclusion in this study.
To create sine-wave speech from these tokens, a wide-band spectrogram of each
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recording was produced using standard audio software (WinSnoori 1.34). Algorithms within WinSnoori estimated and traced the center frequencies of each of the first four formants. Visual
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inspection, by the primary investigator, was used to examine the algorithm-produced tracings to
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confirm the estimations and to adjust parameters to better approximate the formant frequencies visible on the original spectrogram. Typically, only minor adjustments were required. Using the
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Klatt interface in the program, numerical values for each formant frequency were tabulated at 4 ms intervals. All values were entered into a custom-designed computer program that constructed waveforms made up of independent sinusoidal waves (TONE16, Tice & Carrell, 2011). Peak amplitudes for the tones representing the four formants were set to 90-, 84-, 76-, and 70-dB RMS (re: 16 bits = 96-dB peak) to approximate the spectral slope of speech. These amplitudes were
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lowered by 3-dB, 5-dB, 10-dB, or set to 0 to approximate the original formant amplitudes for specific phonemes. For example, for nasal phonemes, amplitudes of the first and second formants were lowered by 3-dB to simulate decreased amplitude as a result of anti-resonance in natural speech. All SWS tokens were generated with a sampling rate of 10 kHz. Amplitude-
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modulated sine-wave speech (AMSWS) words originated from the SWS tokens described above. Each sine-wave speech token was amplitude-comodulated at 80 Hz using a custom-designed modulating software, AMOD32 (Tice & Carrell, 2011). A modulation cycle of 80 Hz was
chosen as previous research has shown optimal intelligibility for amplitude co-modulation of
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SWS signals at rates between 50-100 Hz (Lewis & Carrell, 2007; Carrell & Opie, 1992). The modulating signal was a triangular wave with a duty cycle of 83%. On each 12.5-msec modulating cycle, the amplitude rose to 100% in 8 milliseconds (ms), dropped to 0% in 2 ms,
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and remained at 0% for the final 2.5 ms. All stimuli in the present experiment were equilibrated to 72-dB (re: 96 dB maximum level with 16-bits of amplitude resolution). See Figure 1 for
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sine-wave speech.
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spectrograms representative of natural speech, sine-wave speech, and amplitude-comodulated
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Figure 1. Spectrograms representative of natural speech, sine-wave speech, and amplitudecomodulated sine-wave speech. Panel A is a spectrogram of the naturally-spoken word “rare” showing tracings of the center frequencies of the formant frequencies using audio software Wavesurfer1.8.8. Panel B shows the spectrogram for the unmodulated sine-wave version of the same word based on the first four formants. Panel C shows the spectrogram of the amplitudecomodulated (80-Hz) sine-wave version of the same word.
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The sine-wave speech and amplitude-comodulated sine-wave speech stimuli were piloted on both typically-developing adult and child listeners because none of the words had been used in previous research. Consistent with previous findings, amplitude-comodulated sine-wave speech tokens were reliably more intelligible than the sine-wave speech tokens from which they
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were created (Carrell & Opie, 1992; Lewis & Carrell, 2007; Shapley, 2003). Listeners
consistently identified more phonemes in amplitude-comodulated sine-wave speech as compared to sine-wave speech tokens. As a perceptual reference for the acoustic quality of each of these types of novel stimuli, pilot children described sine-wave speech as sounding like “squeals”,
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“beeps”, or “R2D2 from Star Wars”, whereas amplitude-comodulated sine-wave speech was usually described as sounding “like a robot”. 2.4 Procedure
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Children were tested in two research sessions, the first of which involved the administration of the standardized assessment measures and the second of which involved completing the word
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recognition task. The session took place in a child-friendly room that was acoustically treated with absorbent foam designed to significantly reduce the ambient noise level of the testing
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environment. All participants were tested individually while seated at a computer running E-
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prime 2.0 experimental software. Children were fitted with insert earphones (Etymotic ER-1A) set to a comfortable listening level. Four children did not tolerate placement of the earphones in
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the ear canal and were thus fitted with noise-cancelling headphones (Sennheiser HD280Pro) set to a comfortable listening level. Throughout the experiment, a research assistant sat beside the child to monitor the child‟s attention to the task but was unable to hear the auditory stimuli due to presentation via headphones for the child.
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The word recognition task involved children listening to both types of novel word stimuli described above. Stimuli were presented in the context of novel talkers: an alien (sine-wave speech) or robot (amplitude-comodulated sine-wave speech). Pre-recorded instructions were provided via computer. Children were instructed that the alien and robot talkers “sounded funny”
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and the child‟s help was enlisted to determine what they were saying by “interpreting” what was said. Children responded verbally and their response was audio recorded using a desktop
microphone (AKG C414B) connected to a Marantz (PMD670) digital recorder. All responses were transcribed in real time. A second transcriber transcribed a random 20% of samples offline.
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Inter-rater reliability was 94.3%. Because some children, by definition, had difficulty producing speech sounds, children also pointed to the first letter of their chosen word on a provided letter strip secured to the table to eliminate ambiguity about the child‟s intended response. The letter
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strip contained 12 letters including the target letters, i.e. r/m, and 9 additional letters: b, w, s, n, p, g, d, l, f, c, and a “not here” option for any alternate letters the child might have wanted to select.
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Because of the simplicity of the words used, spelling deficiency was not expected to interfere with a child correctly identifying the first letter of their response. In all cases, if there was a
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mismatch between the initial phoneme of the child‟s initial response and the child‟s first-letter
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selection, the item was scored according to the letter the child pointed to on the letter strip. The task totaled 48 trials. Of children in the typically-developing group, one child had one mismatch
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between the spoken word and the selected first letter. All other typically-developing children exhibited 100% consistency between their transcribed verbal response and the selection of the first letter for all 48 trials. Of children in the dyslexia group, one child had two mismatches between the verbal response and the selected first letter. All other children in the dyslexia group exhibited 100% consistency between verbal responses and letter strip selections. Of children in
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the residual speech delay group, any verbal response potentially containing an /r/ was transcribed as the child produced it. As a result, for these children it is difficult to calculate the true mismatch rate between what the child said and what the child intended, hence this particular procedure. For verbal responses that did not contain a distorted phoneme, no child with a
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residual speech delay had any mismatch between their verbal response and the letter strip
response. Thus, it is assumed for distorted articulation in their verbal responses, these children confidently pointed to their intended first letter response.
The word recognition task began with a familiarization phase during which the child was
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introduced to each novel speaker, i.e. the alien or the robot, which individually appeared as a still image on the computer screen. Beginning with the alien, which spoke in sine-wave speech, the computer presented a sample item three times. The child was encouraged to guess the word that
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they heard. The child was told the word by the research assistant and then listened to the item a fourth time. This procedure was repeated using a different sample item for the robot who spoke
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with the amplitude-comodulated sine-wave speech. All children successfully completed the familiarization tasks. The experimental phase comprised four blocks of 12 words, for a total of
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48 words. Two blocks contained sine-wave speech tokens and two blocks contained amplitude-
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comodulated sine-wave speech tokens. All words were presented in both sine-wave speech and amplitude-comodulated sine-wave speech conditions. To control for order effects of word
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exposure, presentation order was counterbalanced across participants and groups. Within each block, all stimuli were randomized. Each stimulus was heard three times in succession, during or after which children provided their verbal and pointing responses. After the child‟s response, the research assistant advanced the program to the next trial. Children were not provided feedback regarding the accuracy of their responses. During the task, the alien or robot (depending on the
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block) advanced a step along a pathway towards a finish line, providing children a visual cue for their progress in the task. The dependent measure for these tasks was the number of initial phonemes (i.e. /r/, /m/) accurately recognized for each acoustic condition. In cases where the transcription of a child‟s
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response was not consistent with the letter to which they pointed indicating the first letter of the response (ex. The child said “wake” for “rake”, but pointed to the letter “r” as the first letter), the item was scored based on the child‟s pointing response.To ensure any obtained group differences were not attributable to knowledge of specific vocabulary used in this study, after the completion
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of the word recognition task, all participants were administered a vocabulary probe comprising words used in the tasks. Children were presented with picture plates with four pictures, one of which represented the target word. The other three pictures represented phonological, semantic,
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and unrelated foils. A one-way ANOVA revealed no difference between groups in performance on this probe, F(2,35) = 1.872 (p=.170). Correlation analysis further verified there were no
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statistically significant relationships between vocabulary probe accuracy and any of the word recognition dependent variables for any of the groups (all ps > 0.10). The data were analyzed
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using a series of repeated-measures ANOVAs to address our research questions. Throughout the
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analyses, post-hoc comparisons were used to decompose interactions. In all cases, an alpha level of .05 was used to determine statistical significance.
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3. Results
The purpose of this study was to determine how children with different phonological
deficits (i.e., dyslexia or persistent speech delay) perceive amplitude comodulation structure of words compared to novel sine-wave words during a word recognition task. Our first research question asked whether children with dyslexia or persistent speech delay exhibited more
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difficulty perceiving sine-wave speech and amplitude-comodulated sine-wave speech as compared to their typically-developing peers. Because individuals with persistent speech delay have demonstrated difficulty perceiving fine temporal structure in speech signals as compared to their peers with dyslexia, we predicted that children with dyslexia would have less difficulty
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perceiving sine-wave speech and amplitude-comodulated sine-wave speech. Our second
research question asked whether children with persistent speech delay have more difficulty
perceiving a phoneme erred in their own production (Locke, 1980; Shuster, 1998). We predicted that children with persistent speech delay would have more difficulty perceiving the /r/ phoneme
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in both sine-wave and amplitude-comodulated words, as compared to their peers with dyslexia and their typically-developing peers. 3.1 Descriptive Measures
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All children were administered a battery of articulation, language and reading assessments for assignment of participants to the appropriate group. Descriptive statistics
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regarding word reading, articulation, nonverbal intelligence, vocabulary and phonological awareness skills are displayed in Table 2. As selected, the groups differed in word reading such
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that children with dyslexia exhibited significantly lower word reading scores than those in the
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persistent speech delay and typically-developing groups, F (2, 35)=29.026, p<.001. Likewise, as selected the groups differed in speech articulation such that children in the persistent speech
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delay group exhibited significantly lower articulation skills than those in the dyslexia and typically-developing groups F(2,35)=51.839, p<.001. Furthermore, children in the persisent speech delay group performed significantly worse than children in the typically-developing and dyslexia groups on the R-probe, F(2,35)=990.398, p<.001. Of the 12 children with persistent speech delay, 11 failed to produce /r/ in any position of words in isolation or connected speech.
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One child with speech sound disorder exhibited a correct /r/ production in pre-vocalic positions of words (ex. rat, green), but failed to produce /r/ post-vocalically (ex. deer, fire). There were no group differences in receptive vocabulary, F(2,35) = .650, p=.528, expressive vocabulary, F(2,35) =2.805, p=.075, or nonverbal intelligence, F(2,35) = 1.927, p=.162. Although not
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statistically different, there were marginal differences in phonological awareness (F(2,35) = 3.003, p=.063) with the children with dyslexia performing numerically poorer than the other children.
SSD (n=12) Mean (SD) 100.25 (5.79)a 112.58 (8.69)a 77.83 (10.88)b 112.08 (9.61)a 105.42 (12.03)a
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DYS (n=12) Mean (SD) 106.26 (9.29)a 89.41 (3.96)b 102.42 (5.93)a 107.58 (12.23)a 96.25 (12.70)a
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Table 2. Descriptive Statistics. TD (n=12) Mean (SD) Age (in months) 103.3 (8.53)a WRMT-R/NU 111.75 (11.08)a 105.17 (1.95)a GFTA-2 ROWPVT 112.58 (13.33)a EOWPVT 107.92 (13.34)a
106.00 (14.41)a 94.1667 (7.22)a
101.5 (12.95)a
RIAS R-Probe (out of 46)
116.5 (13.37)a 46.00 (0.0)a
113.25 (16.45)a 1.42 (4.91)b
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CTOPP
106.00 (9.52)a 46.00 (0.0)a
Note. Means in the same row that do not share subscripts differ at p<.05.
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(TD = Typically-developing; DYS = Dyslexia; SSD = Speech sound disorder; WRMT-R/NU=Woodcock Reading Mastery Test-Revised/Normative Update, Basic Skills cluster; GFTA-2=Goldman-Fristoe Test of Articulation-2nd Ed., standard score (SS) reported; ROWPVT=Receptive One-Word Picture Vocabulary Test, SS reported; EOWPVT=Expressive One-Word Picture Vocabulary Test, SS reported; CTOPP=Comprehensive Test of Phonological Processing, SS reported; RIAS=Reynolds Intellectual Assessment Scales, SS reported; R-probe, raw score reported)
3.2 Experimental Measures The purpose of the word recognition task was to determine whether children differed in their recognition of impoverished speech signals in two acoustic conditions. One condition, sine-wave speech, presented words as four independent time-varying sinusoids designed to
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mimic center frequency values of the first four formants of a naturally-spoken word. The other condition, amplitude-comodulated sine-wave speech, applied a modulating waveform of 80 Hz to the sine-wave speech words. Previous research has demonstrated that both adults and children typically recognize more amplitude-comodulated sine-wave speech words than sine-wave speech
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words, when all other acoustic and linguistic structure is held constant (Carrell & Opie, 1992; Lewis & Carrell, 2007).
To analyze these data, we used a 2 (modulation: sine-wave speech, amplitude-
comodulated sine-wave speech) x 2 (phoneme: /r/, /m) x 3 (group: dyslexia, speech sound
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disorder, typically-developing) mixed factorial design repeated-measures ANOVA. Results revealed a main effect of modulation, F(1,33) = 27.920, p < .001, ηp2 = .458 such that on average, as expected, children recognized initial phonemes in the sine-wave speech condition
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with less accuracy than words in the amplitude-comodulated sine-wave speech condition. Figure 2 shows average performance on /m/ and /r/ words across groups.
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Figure 2. Initial phoneme accuracy for /m/ and /r/ collapsed across all participants. The plain white bar represents sine-wave speech (SWS) and the striped bar represents amplitudecomodulated sine-wave speech (AMSWS). Error bars reflect standard error of the mean.
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The main effect of group was not significant, F(2,33) = 2.268, p=0.119, ηp2 = .121, but the modulation by group interaction was significant, F(1,33) = 3.581, p=.039, ηp2 = .178, suggesting that the groups differed in their recognition accuracy across the two modulation conditions. Planned comparisons across the different phoneme conditions for each group
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revealed that the children with dyslexia recognized more amplitude-comodulated sine-wave speech words than sine-wave speech words for the /m/ phoneme, t(11) = -5.014, p<.001, and the /r/ phoneme, t(11) = -3.544, p=.005. By contrast, children with persistent speech delay
recognized more amplitude-comodulated sine-wave speech words than sine-wave speech words
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for the /m/ phoneme, t(11) = -2.785, p=.018, and there was no difference in accuracy between amplitude-comodulated sine-wave speech and sine-wave speech for the /r/ phoneme, t(11) = .951, p = .362. Children in the typically-developing group did not differ in accuracy between
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amplitude-comodulated sine-wave speech and sine-wave speech for the /m/ phoneme, t(11) 2.057, p = .064 nor the /r/ phoneme, t(11) -1.358, p = .202. Nonsignificant differences for the
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typically-developing group may be attributable to ceiling effects, as the typically-developing children recognized a high percentage of phonemes in the sine-wave speech condition and thus
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did not have the opportunity for substantial gains in phoneme recognition in the amplitude-
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comodulated sine-wave speech condition. Additionally, using word stimuli comprising simple CVC structures may have contributed to the high rates of accuracy in these children.
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There were no group differences in accuracy for sine-wave speech without comodulation for both the /m/ and /r/ phonemes (all ps > .05). All groups achieved similar levels of accuracy in the amplitude-comodulated sine-wave speech condition for the /m/ phoneme (all ps > .05), but children with persistent speech delay recognized significantly fewer /r/ phonemes in the amplitude-comodulated sine-wave speech condition, as compared to their peers with dyslexia
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(t(11) = 2.098, p = .048) and those who were typically-developing (t(-2.721, p=.012). The threeway interaction between modulation, phoneme and group was not significant, F(1,33) = .254, p=.777, ηp2 = .015, nor was the two-way interaction of modulation and phoneme significant, F(1,33) = 2.857, p=.100, ηp2 = .080. Figures 3 and 4 show performance level for each group by
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each phoneme and condition, and include individual data points for all participants (N=36) for the reader to more fully conceptualize performance across each group.
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Figure 3. Initial phoneme accuracy for /m/ for each participant and participant group. Each dot represents a child and connecting lines delineate performance in sine-wave speech (SWS) versus amplitude-comodulated sine-wave speech (AMSWS). Bolded lines indicate more than one child achieved this pattern of scores. Please see Appendix A for individual performance scores for each child in each group. The first panel shows children in the dyslexia (DYS) group with SWS performance on the left and AMSWS performance on the right. The second panel shows word recognition performance for children with persistent speech delay (PSD) and the third panel shows performance for typically-developing (TD) children. Hashed lines represent the group mean for each group for each condition. Asterisks indicate a statistically significant difference (p < .05) between SWS and AMSWS.
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Figure 4. Initial phoneme accuracy for /r/ for each participant and participant group. Each dot represents a child and connecting lines delineate performance in sine-wave speech (SWS) versus amplitude-comodulated sine-wave speech (AMSWS). Bolded lines indicate more than one child achieved this pattern of scores (e.g. in the TD group, 6 children achieved 100% accuracy for both SWS and AMSWS conditions). Please see Appendix A for individual performance scores for each child in each group. The first panel shows children in the dyslexia group with SWS performance on the left and AMSWS performance on the right. The second panel shows word recognition performance for children with persistent speech delay (PSD) and the third panel shows performance for typically-developing (TD) children. Hashed lines represent the group mean for each group for each condition. Asterisks indicate a statistically significant difference (p < .05) between SWS and AMSWS.
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As these figures show, for the /m/ phoneme, the majority of children across all three groups showed either improved word recognition from sine-wave speech to amplitude-comodulated sine-wave speech or exhibited a very similar performance between the two conditions. Visual inspection of individual performance for the /r/ phoneme, however, across the three groups of children shows a greater variability in performance for children with persistent speech delay
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versus children in the dyslexia and typically-developing group. Notably, several children in this group performed poorer in the amplitude-comodulated sine-wave speech condition. 4. Discussion The purpose of the present study was to determine the degree to which children with
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differing phonological deficit profiles, namely children with dyslexia and children with persistent speech delay, perceived impoverished speech signals containing varying amounts of acoustic information. Thirty-six children aged 7;6 – 9;6, based on word reading and speech articulation skills, were classified into three groups: 1) dyslexia, 2) persistent speech delay, and 3) typically-
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developing. All children listened to two types of impoverished speech: sine-wave speech and amplitude-comodulated sine-wave speech. We had two main hypotheses: 1) Children with dyslexia would exhibit superior word recognition for sine-wave speech and amplitude-
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comodulated sine-wave speech, as compared to their peers with persistent speech delay; and 2) children with persistent speech delay would exhibit particularly poor word recognition for
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stimuli containing a phoneme erred in their own production. We discuss two key findings here. 4.1 Amplitude comodulation improved speech perception in children with phonological
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impairments, particularly for children with dyslexia
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Regardless of the presence or absence of a phonological impairment, all children in this study, on average, performed better for the amplitude-comodulated stimuli as compared to the
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traditional sine-wave stimuli. While this response pattern has been reported for typical adults and children (Cabbage, Carrell, & Hogan, 2011; Carrell & Opie, 1992; Lewis & Carrell, 2007), it was unknown whether children with deficient phonological representations would perform similarly. As compared to typically-developing children, children with dyslexia exhibited substantial difficulty perceiving unmodulated sine-wave speech. When the stimuli were
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amplitude -comodulated, however, children with dyslexia achieved a performance at the same level as their typically-developing peers. That is, amplitude comodulation yielded substantial gains in perception for children with dyslexia. It is unsurprising that children with dyslexia showed poor perception of phonemes in the unmodulated sine-wave speech as this is consistent
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with previous work showing that adults with dyslexia struggle to perceive sine-wave speech as compared to typical adult listeners (Rosner, et al., 2003). Modifying the stimuli via amplitude comodulation, however, yielded substantial gains in perception for the children with dyslexia suggesting amplitude comodulation provided a particularly necessary cue for children with
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dyslexia.
Given the benefits in perception for sine-wave speech versus amplitude-comodulated sine-wave speech across all groups in this study, we hypothesize these stimuli are processed at an
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acoustic, or sensory, level as opposed to a more linguistic, or phonological, level. The two types of stimuli presented here contained identical phonetic parameters including formant frequency
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values, transitions, and duration. The only difference between the stimuli was the acoustic manipulation of amplitude comodulation suggesting that it is the only factor that is contributing
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to the improved intelligibility for these children. Some have argued the perceptual deficit in
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children with dyslexia and persistent speech delay is attributable to a deficit at a more phonetic, or linguistic, speech level of processing (Manis et al., 1997; Studdert-Kennedy & Mody, 1995),
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rather than a non-speech sensory auditory level of processing (Tallal, 1980; Vandermosten et al., 2011). While there has been much debate in the literature on this issue (see Rosen, 2003, for a review), it seems that while a subgroup of children with dyslexia may exhibit some low-level sensory processing deficiencies, it is not likely the basis of the phonological deficit seen in most
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children with dyslexia. From these results, it appears that children with dyslexia are indeed sensitive to acoustic manipulations that do not depend on linguistic, or phonetic, processing. The mechanism by which amplitude-comodulated sine-wave speech is usually more intelligible than sine-wave speech, despite the absence of any additional phonetic or linguistic
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structure is not well understood. One possibility for the better perception for amplitude-
comodulated sine-wave speech over unmodulated sine-wave speech is that in typical speech perception, neural fibers of the auditory nerve respond, in synchrony, to the harmonic elements of a spoken utterance. According to Koch et al. (1999), it is precisely the timing pattern of the
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neural impulses and the synchronization of neural firing that are critical for speech perception in compromised situations, such as a noisy background. Such timing and synchronization may also be important when listening to impoverished speech signals. Because sine-wave speech
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comprises independent sinusoids, it lacks any harmonic structure and thus, synchronous neural firing in the brain during speech perception is unlikely. It is possible that amplitude
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comodulation, which applies a simultaneous modulating signal to the independent sinusoids comprising the sine-wave speech signal, mimics the harmonic structure found in natural spoken
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utterances. The brain may process the sinusoids together for synchronous neural firing during
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processing, which results in more “speech-like” perception of amplitude-comodulated sine-wave speech as compared to sine-wave speech. While this was not specifically tested in this study, it
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would be an important future direction to address whether amplitude-comodulated sine-wave speech induces a level of neural synchrony that boosts intelligibility as compared to sine-wave speech.
Studies investigating amplitude modulation detection at very slow rates (i.e., 4 Hz, 10 Hz) have demonstrated that individuals with dyslexia show different patterns of response, both
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behaviorally (Goswami et al., 2002) and at the level of neuronal processing (Goswami, 2011; Hamalainen et al., 2012). These slow rates of amplitude modulation have been thought to be relevant to the extraction of syllable-level and prosodic elements, whereas faster rates of modulation are related more to the extraction of acoustic-phonetic information (Hamalainen et
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al., 2012). If neural synchrony plays a significant role in the improved intelligibility of
amplitude-comodulated sine-wave speech, which provides a faster level of amplitude modulation (i.e., 80 Hz, in the present study), the present findings would lend support to the notion that
dyslexic individuals have specific deficits in realms of slower, as opposed to rapid, temporal
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processing (Goswami, 2002; Goswami, 2011; Hamalainen et al., 2012). This follows from tasks that emphasize the salience of amplitude structure, such as amplitude envelope detection and band-limited speech tasks, for which dyslexic listeners show difficulty (Goswami, 2002; Lorenzi
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et al., 2000; Pasquini, Corriveau, & Goswami, 2007; Rocheron et al., 2002). The temporal sampling framework for developmental dyslexia posits that children with
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dyslexia exhibit specific difficulty for the perception of amplitude, or „global‟, structure in complex signals such as speech (Goswami, 2011). Although the current study sought to
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determine the degree to which these children responded to stimuli constrained to include
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primarily spectral detail, it would be important to understand how these same children respond to stimuli constrained by the removal of most spectral detail, such as vocoded speech. Vocoded
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speech is composed of an utterance that has been band-pass filtered into a selected number of channels, which are then processed with a designated type of noise (e.g. speech-shaped noise, white noise, etc), in order to remove temporal fine structure detail. In a study using vocoded speech sentences with 10- to 11-year-old children, Johnson et al. (2011) showed marginal effects of a perceptual deficit for children with dyslexia-only and statistically significant deficits for
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children with a history of both speech sound disorder and dyslexia. Children with a history of speech sound disorder-only (with no dyslexia) performed more similarly to their typical peers when listening to band-limited speech (Johnson et al., 2011). Because listeners with dyslexia have reportedly exhibited particular difficulty with band-
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limited speech (Ziegler et al., 2009; Johnson et al., 2011), it suggests they may more heavily rely on spectral structure for speech perception accuracy. The results of the current study suggest this indeed may be the case. In contrast to band-limited speech, sine-wave speech comprises
reasonably preserved spectral structure and so it is plausible that children with dyslexia may
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show a particular aptitude for sine-wave speech and amplitude-comodulated sine-wave speech as compared to their peers with persistent speech delay, because of the preserved temporal fine structure present in the signal. Further evidence suggests individuals with dyslexia show
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particular impairments for perception tasks relying on cues associated with amplitude structure such as amplitude envelope detection (Goswami, 2011), rise time discrimination (Goswami et
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al., 2002; Goswami et al., 2011; Muneaux et al., 2004), and amplitude modulation detection (Lorenzi et al., 2000; Rocheron et al., 2002). All of these tasks preserve amplitude structure,
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which primarily transmits information regarding energy variation over time cueing syllable and
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word-level features, as opposed to fine spectral detail that cues specific phonetic information, such as formant transitions.
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4.2 Children with persistent speech delay have difficulty perceiving words containing their erred sound
Previous evidence has suggested children with speech sound disorder show particular
perceptual vulnerability for phonemes which are erred in their own speech production (Hoffman et al., 1985; Rvachew & Jamieson, 1989). Children were presented with words beginning with
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the /m/ phoneme, a phoneme not produced in error by children with persistent speech delay, and were also presented with words beginning with the /r/ phoneme, a phoneme which was produced incorrectly by all children with speech sound disorder in our study. Children in the speech sound disorder group exhibited comparable recognition of /m/ words for sine-wave speech and
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amplitude-comodulated sine-wave speech, as compared to their peers. For words beginning with the /r/ phoneme, however, children with persistent speech delay did not show a gain in accuracy between sine-wave speech and amplitude-comodulated sine-wave speech, while their peers did, thus supporting our second hypothesis. While children with persistent speech delay did not show
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an overall difficulty perceiving phonemes presented in the context of impoverished speech, they showed specific difficulty perceiving a phoneme that was produced in error. This is consistent with previous research that has demonstrated some children with speech sound disorder have
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difficulty perceiving words containing phonemes that are distorted in their own speech production (Hoffman et al., 1985; Rvachew & Jamieson, 1989). Some have argued that
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imprecise articulation skills may be attributable to a weakness in the development of robust phonological representations (Carroll, Snowling, Hulme, & Stevenson, 2003; Raitano et al.,
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2004) for adequate speech production. Deficits in speech perception suggest that children with
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speech sound disorder may exhibit weak phonological representations, particularly for erred sounds in their own speech.
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By definition, sine-wave speech and amplitude-comodulated sine-wave speech comprise primarily spectral structure. Notably, the children in the persistent speech delay group only exhibited difficulty perceiving the /r/ phoneme, but did not show the same difficulty for the correctly produced /m/ phoneme. While this may lead to the conclusion that these children do not have difficulty perceiving fine temporal structure, it is also possible that these children
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capitalized on the fact that rudimentary amplitude structure was mimicked and provided in the creation of the /m/ stimuli in this study. The /m/ stimuli were created by reducing the overall intensity of the independent sinusoids by 5 dB to mimic reduced amplitude caused by antiresonance in the natural production of nasal phonemes. To further test this hypothesis,
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children with persistent speech delay should be given sine-wave speech and amplitude-
comodulated sine-wave speech containing phonemes that are not produced in error, but contain acoustic cues similarly constrained by fine temporal structure as the /r/ phoneme such as /l/. This study, like all studies, is not without limitations that should fuel future work. The
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present study investigated speech perception skills in a small subset of children with dyslexia or persistent speech delay. Importantly, the children included here met stringent criteria in both word reading and articulation skills to isolate phonological deficits specific to dyslexia or
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persistent speech delay. To date, only one known study of speech perception in children has accounted for both reading and speech production skill (Johnson et al., 2011) and the results of
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the present study likewise noted that children with dyslexia and children with speech sound disorder exhibit distinct profiles of speech perception skill. In our study, children provided
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verbal responses as well as pointing responses in an attempt to circumvent ambiguous speech
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production second to speech articulation errors in the group of children with persistent speech delay. Arguably, having children provide a verbal response during a perceptual task may have
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inadvertently interfered with the speech perception process. Future research should seek to mitigate this interference by designing tasks that require no verbal response via spelling responses or alternate paradigms that do not require generative responses. Additionally, in this study we investigated speech perception performance on a limited set of phonemes (i.e., /m/, /r/) in sine-wave speech and amplitude-comodulated sine-wave speech. Future work should include
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additional phonemes as well as additional acoustic manipulations to further describe the nature of the perceptual deficits in these children. 5. Conclusion The nature of the underlying perceptual deficits in children with phonological
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impairment, whether dyslexia or speech sound disorder, is not well understood. The present study sought to disentangle the commonalities and differences in these children by specifically addressing whether children with dyslexia and children with speech sound disorder responded to spectral structure in speech signals with the same degree of sensitivity. Although all children
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benefitted from added structure (i.e., sine-wave speech vs. amplitude-comodulated sine-wave speech) to combine acoustic cues into meaning, the present study lends support to the notion that children with dyslexia benefit particularly from this additional acoustic structure prior to the
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engagement of higher-level linguistic processes, as this gain was seen without the addition of phonetic, syntactic, or semantic context. Importantly, when subjected to a perceptual task that
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involved an erred phoneme in their own speech production, children with speech sound disorder failed to show significant improvements in perception from sine-wave speech to amplitude-
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comodulated sine-wave speech. The present results suggest that children with dyslexia and
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speech sound disorder exhibit distinct perceptual strengths and weaknesses when subjected to
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impoverished speech stimuli that preserves spectral structure.
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Acknowledgements This research was supported by funds from the Barkley Memorial Trust at the University of Nebraska-Lincoln (Cabbage), the American Speech-Language-Hearing Foundation (Cabbage),
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and the National Institutes of Health, R03 DC9667 (Hogan).
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46 Appendix A
Table 1. Word recognition accuracy for each child in each group (dyslexia, persistent speech delay, typically-developing) for sine-wave speech (SWS) and amplitude co-modulated sine-wave speech (AMSWS) for /m/ and /r/ phonemes. Scores are reported as percent correct (out of 12 words in each condition). /m/
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/r/ AMSWS 100.00% 83.33% 91.67% 100.00% 100.00% 100.00% 83.33% 66.67% 100.00% 75.00% 91.67% 100.00%
SWS 100.00% 83.33% 58.33% 75.00% 16.67% 41.67% 66.67% 66.67% 91.67% 16.67% 41.67% 83.33%
AMSWS 100.00% 100.00% 100.00% 100.00% 100.00% 100.00% 50.00% 100.00% 100.00% 75.00% 100.00% 100.00%
100.00% 66.67% 25.00% 91.67% 100.00% 41.67% 91.67% 0.00% 100.00% 16.67% 41.67% 91.67%
100.00% 91.67% 66.67% 91.67% 100.00% 75.00% 100.00% 100.00% 100.00% 25.00% 83.00% 91.67%
46.17% 33.33% 25.00% 66.67% 66.67% 91.67% 83.33% 0.00% 100.00% 91.67% 100.00% 100.00%
33.33% 75.00% 58.33% 100.00% 75.00% 58.33% 100.00% 58.33% 83.33% 100.00% 83.33% 58.33%
100.00% 83.33% 100.00% 91.67% 91.67% 100.00% 83.33% 91.67% 100.00% 66.67% 83.33% 50.00%
83.33% 100.00% 50.00% 100.00% 91.67% 91.67% 100.00% 100.00% 100.00% 100.00% 50.00% 83.33%
83.33% 100.00% 91.67% 100.00% 100.00% 75.00% 100.00% 100.00% 100.00% 100.00% 83.33% 91.67%
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ED 66.67% 100.00% 58.33% 100.00% 75.00% 100.00% 75.00% 75.00% 100.00% 83.33% 41.67% 0.00%
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SWS 100.00% 41.67% 66.67% 66.67% 83.33% 41.67% 0.00% 41.67% 33.33% 50.00% 16.67% 75.00%
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Child ID Dyslexia D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 Persistent Speech Delay SD1 SD2 SD3 SD4 SD5 SD6 SD7 SD8 SD9 SD10 SD11 SD12 Typically-Developing TD1 TD2 TD3 TD4 TD5 TD6 TD7 TD8 TD9 TD10 TD11 TD12
Speech perception differences in children with dyslexia and persistent speech delay Kathryn L. Cabbage , Tiffany P. Hogan , Thomas D. Carrell PII: DOI: Reference:
S0167-6393(16)30137-6 10.1016/j.specom.2016.05.002 SPECOM 2378
To appear in:
Speech Communication
Received date: Revised date: Accepted date:
10 July 2015 23 May 2016 25 May 2016
Please cite this article as: Kathryn L. Cabbage , Tiffany P. Hogan , Thomas D. Carrell , Speech perception differences in children with dyslexia and persistent speech delay, Speech Communication (2016), doi: 10.1016/j.specom.2016.05.002
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ACCEPTED MANUSCRIPT Running Head: SPEECH PERCEPTION DIFFERENCES
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Highlights
The degree to which speech perception deficits overlap is unknown in children with dyslexia and children with persistent speech delay. This study used sine-wave speech and amplitude-comodulated sine-wave speech to
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determine whether these children respond similarly to stimuli preserving spectral structure.
Amplitude comodulation improves speech perception in children with phonological
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impairments, particularly for children with dyslexia.
Children with persistent speech delay have difficulty perceiving words containing their
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erred sound.
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Speech perception differences in children with dyslexia and persistent speech delay Kathryn L. Cabbage1 Tiffany P. Hogan1
1
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Thomas D. Carrell2
MGH Institute of Health Professions, 36 1st Avenue, Boston, MA 02129; Cabbage email: [email protected]
2
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Hogan email: [email protected]
University of Nebraska – Lincoln, 357 Barkley Memorial Center, Lincoln, NE 68583;
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Carrell email: [email protected]
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Corresponding author: Kathryn Cabbage MGH Institute of Health Professions 36 1st Avenue Boston, MA 02129 phone: 402-309-3111 email: [email protected]
Abstract Purpose: Deficits in phonology, which are related to the organization and retrieval of speech sounds in the mental lexicon, are associated with two distinct clinical disorders, dyslexia and
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persistent speech delay. Research has shown associated speech perception deficits in individuals with these disorders, but the degree to which the underlying perceptual deficits overlap is unclear. In this study, we investigated how children with dyslexia, children with persistent speech delay and their typically-developing peers differed in speech perception when listening to
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speech stimuli that varied in non-linguistic temporal fine structure acoustic characteristics.
Method: Thirty-six children were classified into three groups (dyslexia, speech sound disorder, typically-developing; n=12 in each group) based on their reading and speech articulation skills. Novel single-word sine-wave speech stimuli was aurally presented to the children.
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Results: There were no group differences between children with dyslexia and their typicallydeveloping peers, but the children with persistent speech delay had more difficulty than the other two groups. In particular, the children with persistent speech delay had difficulty recognizing
misarticulated in their own speech.
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words with limited acoustic structure when the stimuli involved a phoneme that they
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Conclusions: These results suggest a specific difficulty in the perception of temporal fine
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structure in children with persistent speech delay.
Keywords: speech perception, persistent speech delay, speech sound disorder, dyslexia,
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phonological processing
I.
Introduction The development of spoken and written language is reliant on an intact phonological
system. In children, phonological deficits may manifest as difficulty mapping phonemes onto
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letters during the reading process, as is the case with dyslexia (Goswami, 2000). Alternatively, children may manifest phonological deficits in the form of a difficulty producing speech sounds accurately as seen in children with speech delay (Anthony et al., 2011; Rvachew, 2007). Poorly specified phonological representations have been implicated for both children with dyslexia
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(Boada & Pennington, 2006; Elbro et al., 1998; Goswami, 2000; Swan & Goswami, 1997) and children with speech delay (Anthony et al., 2011; Edwards, Fourakis, Beckman, & Fox, 1999; Rvachew et al., 2003; Sutherland & Gillon, 2005; Sutherland & Gillon, 2007). Phonological representations refer to the storage of the phonological characteristics of words in long-term
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memory, and it is plausible that deficits in a listener‟s speech perception may impact the quality of phonological representations necessary for speaking and reading processes. For example, if children have deficits in encoding and processing the acoustic-phonetic features of speech
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stimuli, this may interfere with the establishment of well-specified phonological representations in the mental lexicon (Fowler, 1991). In independent lines of research, speech perception skills
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of children with dyslexia and children with speech delay have been studied to better understand the nature of phonological deficits in these children. Although both populations have shown
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speech perception deficits, the degree to which the underlying perceptual deficit overlaps is
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unclear. The present study investigates the speech perception skills of both children with dyslexia and children with speech delay to determine whether these children differ in their
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speech perception for particular acoustic manipulations. In its natural form, running speech comprises multiple redundant cues that signal the
identity of phonemes and, thus, in typical listening situations, it is unlikely that a single cue is responsible for the accurate perception of any particular phoneme. This acoustic redundancy plays an important role in successful speech perception in everyday environments that are rarely
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free of noise, reverberation, or distortion (Ziegler et al., 2009). Individuals with dyslexia have difficulty perceiving speech when presented with a compromised speech signal such as speechin-noise, sine-wave speech, band-limited speech, or the multiple simultaneous distortions found in classrooms (Gruber, 2003; Johnson et al., 2011; Rosner et al., 2003; Ziegler et al., 2009).
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Signals that mask or omit the acoustic redundancy present in natural speech may interfere with a listener‟s ability to integrate relevant acoustic properties with their own internal representations. For listeners with poorly specified phonological representations, it may be especially difficult to accurately perceive a signal with limited acoustic detail.
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Compared to a naturally-spoken utterance, sine-wave speech is an acoustically-sparse, speech-based signal generated by creating sinusoidal tones that follow the center frequencies of the first four formants of a naturally-spoken utterance. This manipulation results in a signal that
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does not contain the fundamental frequencies, harmonic structure, or short-term spectral cues from the original spoken sentences. Thus, the resulting signal comprises several time-varying
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inharmonic frequency components that faithfully follow the original formant center frequencies yet the amplitude information of the original signal is so poorly specified that it is nearly binary.
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The listener‟s impression of this material is influenced by the instructions in the experiment
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(Remez et al., 1980). It ranges from not even recognizing the stimuli as speech-based, to hearing many of the words correctly. Overall, however, listener performance is low regardless of
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experimenter instructions. A simple acoustical manipulation, amplitude comodulation of the sine-wave sentences,
greatly increases their intelligibility. Amplitude comodulation refers to a digital manipulation of the signal which simultaneously modulates the sine-wave tones at the same frequency and phase. In other words, it is analogous to rapidly turning on and off each of the four component tones
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simultaneously, that is, to comodulate the four tones. The intelligibility of sine-wave speech increases significantly when the signal is amplitude comodulated for both adults (Carrell & Opie, 1992) and typically-developing children (Cabbage, Carrell & Hogan, 2011; Lewis & Carrell, 2007). Importantly, intelligibility increases despite the fact that no phonetic, syntactic, or
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semantic information has been added. Likewise, additional amplitude structure is unrelated to the amplitude envelope of the original utterance and only includes an arbitrary modulation of 80 Hz added to the signal. Amplitude comodulation does not refer to traditionally-defined
suprasegmental information such as intonation, tempo, and other forms of traditional
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suprasegmental information. Amplitude comodulation has been suggested to acoustically group components of sound together based on the phenomenon of comodulation masking release (Carrell & Opie, 1992). The grouping allows the simultaneous tones to be processed as one
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“object” rather than several for later, perhaps more linguistic, processes. Previous research has shown that typically-developing children have better perception for
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amplitude-modulated sine-wave speech signals than sine-wave speech (Lewis & Carrell, 2007). It is plausible that children with phonological impairments will not show these same gains
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because of an overall weak sensitivity to acoustic structure as compared to typical children.
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Children with unimpaired phonological representations, whether listening to either sine-wave speech or amplitude comodulated sine-wave speech, may use their own internal representations
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to “fill in” the gaps and recover missing acoustic structure to make sense of a signal. Children with phonological deficits, and thus likely underspecified phonological representations, may lack the necessary acoustic detail in their own representations to “fill in” the gaps for successful speech perception. Indeed, previous research has demonstrated the importance of long-term phonological representations for success in phonological serial recall tasks that are subject to
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short-term memory decay (Hulme et al., 1997). Thomson, Richardson, and Goswami (2005), investigating children with dyslexia, attributed the erred intrusion of real words during nonword recall as evidence these children have less well-specified phonological representations to draw on to reconstruct nonwords. It is possible that children with phonological deficits (i.e., dyslexia,
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speech sound disorder) may evidence similar less robust representations when presented with novel speech stimuli characterized by minimal acoustic structure. Separate lines of research suggest children with dyslexia and children with speech delay may respond differently to
particular acoustic cues during speech perception. Children with dyslexia appear to have more
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difficulty when speech perception is reliant on acoustic features that cue amplitude structure during speech perception (Goswami, 2011; Goswami et al., 2002; Goswami et al., 2011; Muneaux et al., 2004), whereas children with speech delay appear to have more difficulty when
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speech perception is reliant on acoustic features related to spectral cues (Johnson et al., 2011). Evidence suggests individuals with dyslexia show particular impairments for perception
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tasks relying on cues associated with amplitude, or global, structure such as amplitude envelope detection (Goswami, 2011), rise time discrimination (Goswami et al., 2002; Goswami et al.,
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2011; Muneaux et al., 2004), and amplitude modulation detection (Lorenzi et al., 2000;
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Rocheron et al., 2002). By design, sine-wave speech is devoid of most amplitude structure and comprises primarily spectral information. Thus, because spectral structure is comparatively
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preserved in sine-wave speech signals, it is possible that children with dyslexia do not show deficits perceiving sine-wave speech and amplitude-modulated sine-wave speech stimuli. By contrast, children with speech delay may exhibit difficulty with sine-wave speech and amplitudecomodulated sine-wave speech precisely because these signals are limited to primarily spectral information and lack additional acoustic redundancy, such as amplitude structure. Although there
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is considerably less research investigating speech perception skills in children with speech delay, Johnson and colleagues (2011) provided compelling evidence demonstrating that speech perception tasks for speech stimuli containing primarily global amplitude structure, i.e., vocoded speech, did not pose particular difficulty for children with a history of speech delay as compared
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to their peers with dyslexia. Children in their study, however, were given another task, syllable discrimination, that required the perception of spectral structure, i.e., formant transitions from fricatives /s, ʃ/ into a following vowel. Notably, the children with a history of speech delay, in a post-hoc comparison, were the only group shown to respond to this spectral cue more poorly
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than typical children. Neither children with dyslexia nor children with dyslexia and speech delay performed differently from typical children. Is it possible that children with speech sound disorder primarily attend to different acoustic cues both in perception and production, than peers
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with a different type of phonological impairment, such as dyslexia? If this is the case, we would predict that when these children are completing speech perception tasks involving sine-wave
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speech, which is characterized by the preservation of spectral structure of speech and very
speech delay.
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rudimentary amplitude information, children with dyslexia would outperform children with
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On the other hand, children with phonological deficits may simply have difficulty perceiving speech that is impoverished in some way, regardless of the type of preserved structure
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in the speech signal. Several studies have demonstrated that children with dyslexia show particular weakness when given poor or noisy speech signals, (Gruber, 2003; Johnson et al., 2011; Rosner et al., 2003; Ziegler et al., 2009), thus it is possible children with dyslexia simply exhibit deficits for perception of impoverished speech. Children with dyslexia performed poorer than their typically-developing peers on perception of speech-in-noise and band-limited speech
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(Ziegler et al., 2009). These deficits were found in comparison to both chronological agematched peers and reading-level matched peers, suggesting the perceptual problem was specifically associated with a disordered, dyslexic profile, instead of delayed reading skills. Rosner and colleagues (2003) presented adults with dyslexia with short sine-wave speech
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sentences and found that some, but not all, adults with dyslexia exhibited poor perception as compared to controls.
Although little research has investigated speech perception skills in children with speech sound disorder for speech stimuli that are impoverished in some way, evidence from a variety of
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speech perception tasks for syllables and words has shown these children exhibit particular
difficulty perceiving speech sounds they produce incorrectly (Broen et al., 1983; Hoffman et al., 1985; Locke, 1980; Monnin & Huntington, 1974; Rvachew & Jamieson, 1989; Rvachew et al.,
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2004). This may indicate a deficiency in the acoustic specificity for these erred phonemes in a child‟s own internal representations. Children who do not produce a particular phoneme
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correctly may exhibit particular difficulty perceiving that same phoneme in sine-wave speech or amplitude-comodulated sine-wave speech. As a result of this observation, in the present study we
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elected to include children with speech delay characterized by an erred production of /r/ to test
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whether these children had difficulty with a speech perception task specifically involving /r/. The /r/ phoneme is a late-developing speech sound in speech production and is often associated with
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a subset of children with speech delay who are classified as exhibiting persistent speech errors, or persistent speech delay (Shriberg et al., 2010). In the present study, we designed stimuli that removed the majority of acoustic features
normally present in natural speech and presented it to children with dyslexia, persistent speech delay, and their typically-developing peers. Our first research question asked whether children
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with dyslexia and children with persistent speech delay differed in their word recognition for sine-wave speech and amplitude-comodulated sine-wave speech. Because children with dyslexia have demonstrated difficulty with speech perception tasks involving global amplitude structure and children with speech delay have demonstrated particular difficulty with the perception of
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temporal fine structure (Johnson et al., 2011), we hypothesized that children with dyslexia would exhibit superior word recognition for sine-wave speech and amplitude-comodulated sine-wave speech as compared to their peers with persistent speech delay.
Our second research question asked whether children with persistent speech delay have
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more difficulty perceiving a phoneme erred in their own production than a phoneme that is
produced correctly in their own speech (Locke, 1980; Shuster, 1998). We hypothesized that children with persistent speech delay would have more difficulty perceiving a phoneme they did
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not produce correctly (in this study the erred /r/ phoneme) in both sine-wave speech and amplitude-comodulated sine-wave speech, as compared to their peers without speech delay.
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Because children with speech delay in the current study were selected to participate if they exhibited persistent speech errors in production of the /r/ phoneme, we predicted these children
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would demonstrate a particular difficulty perceiving sine-wave speech and amplitude-
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comodulated sine-wave speech tokens of the /r/ phoneme in words. 2. Method
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2.1 Participants
Thirty-six children ranging in age from 7;6 – 9;6 (years; months) participated in this
study. This age range was selected to include children who were old enough to have received reading instruction and exhibit word reading deficits, while also including children who were young enough to exhibit speech articulation deficits. Children were grouped into three groups
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(n=12 in each group): dyslexia, persistent speech delay and age-matched typically-developing peers. Children with dyslexia exhibited word reading abilities below the 30th percentile as compared to their peers and exhibited no speech articulation deficits, nor had a history of such deficits per written parent report. Children with persistent speech delay exhibited articulation
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deficits in speech production primarily characterized as difficulty producing the /r/ phoneme, described in detail below. Children with persistent speech delay exhibited word reading abilities in the average to above-average range. All children exhibited normal receptive and expressive language skills, were monolingual speakers of Standard American English and passed a pure
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tone hearing screening with thresholds at 20 dB or lower at the octave frequencies between 500 – 8000 Hz. 2.2 Assessment
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Articulation. Articulation skills were measured using the sounds-in-words subtest of the Goldman-Fristoe Test of Articulation-2 (GFTA-2, Goldman & Fristoe, 2000) and a probe
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designed to assess each child‟s production of the /r/ phoneme, which was adapted from a probe developed previously (Preston & Edwards, 2007). A speech-language pathologist transcribed
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participant speech production for each word using broad transcription with the International
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Phonetic Alphabet. A second transcriptionist independently transcribed a random 20% of samples. Inter-rater reliability was 98.4%. Children with persistent speech delay scored below
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the 16th percentile on the GFTA-2 and had difficulty producing /r/ in any or all positions of words. Of the 12 children with persistent speech delay, four children only exhibited difficulty with /r/, and eight children exhibited additional articulation errors including /s, z, sh, th/. All of these phonemes are also considered late-developing (Smit et al., 1990), and are common errors for those with a persistent speech delay. To further quantify each child‟s error pattern for
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production of /r/, a 46-word /r/ probe with words containing /r/ in both prevocalic (i.e. rat, rake) and post-vocalic (i.e. bird, bear) positions of words was administered to all children with persistent speech delay. The nature of /r/ distortions for children with persistent speech delay in this study was primarily characterized as gliding of /r/, /w/ for /r/ substitution, or vowelization of
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/r/. All but one child had difficulty producing /r/ in all positions of words. One child produced pre-vocalic /r/ accurately but did not produce /r/ in any post-vocalic contexts.
Reading. Word reading skills were assessed via the Basic Skills subtests of the
Woodcock Reading Mastery Test – Revised/Normative Update (WRMT-R/NU, Woodcock, 1998).
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All participants were administered the Word Identification and Word Attack subtests. The Word Identification subtest measures participants‟ reading abilities for real words that increase in difficulty from high to low frequency words. The Word Attack subtest measured each child‟s
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ability to apply phonics skills to nonwords and low-frequency words during reading. A composite Word Reading score was computed based on the combination of results from each
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subtest. Participants who received a standard score of 92 or below (< 30th percentile) (Manis et al., 1996) were considered potential candidates for the dyslexia group. Cut-off scores for the
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classification of dyslexia varies widely across studies ranging from the 7th percentile (Badian et
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al., 1990) to 30th percentile (Manis et al., 1996). Additionally, children in the persistent speech delay and typically-developing groups were required to receive a standard score of at least 100 to
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prevent the inclusion of any children in these groups with subclinical reading deficits. Non-verbal intelligence. Children were administered subtests of the Reynolds
Intellectual Assessment Scales (RIAS; Reynolds & Kamphaus, 2003) to verify normal non-verbal cognitive ability (standard score > 80).
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Receptive/Expressive vocabulary. Because of the positive relationship between vocabulary knowledge and the specificity of phonological representations (Metsala & Walley, 1998), all children were administered the Receptive One-Word Picture Vocabulary Test (ROWPVT; Brownell, 2000b) to assess receptive vocabulary skills and the Expressive One-Word
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Picture Vocabulary Test (EOWPVT; Brownell, 2000A) to assess expressive vocabulary skills. Phonological Awareness. Phonological awareness skills, which have been shown to be positively correlated with reading ability (Larivee & Catts, 1999), were assessed using selected subtests (i.e. Elision, Blending Words) of the Comprehensive Test of Phonological Processing
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(CTOPP; Wagner et al., 1999). The Elision subtest was a phoneme deletion task wherein
children were auditorily given a word and asked to delete a specific phoneme and say the resulting target word (ex. Say “meet” without /t/. Answer: “me”.). The Blending Words subtest
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involved auditorily presenting children with a series of phonemes and asking them to blend the
Answer: “mad”.). 2.3 Stimuli
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phonemes into an appropriate word (ex. “What word do these sounds make? /m/ /æ/ /d/.
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In this study, we used novel speech stimuli comprising rudimentary frequency and
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amplitude information (i.e. sine-wave speech, amplitude-comodulated sine-wave speech). The experimental word stimuli consisted of 12 pairs of consonant-vowel-consonant (CVC) or
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consonant-vowel (CV) rhyming words that began with the initial phonemes /r/ or /m/, to address hypothesized deficits for children with persistent speech delay (/r/) in contrast to a control phoneme (/m/) which was produced correctly by all children. An additional 12 words, all beginning with the phoneme /w/, were matched to and rhymed with the experimental stimuli to serve as foils. To determine eligible words for inclusion in this study, we generated a list of
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common CVC and CV real words with /r/ in the initial positions of words (ex. rake, row, run). We substituted /m/ and /w/ for /r/ in these words and rejected any rhymes that did not yield real words (ex. mun, wope). Of the resulting eligible triads, 12 were selected for inclusion in this study (see Table 1). We calculated word frequency values for each item using an online
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calculator based on a child corpus of Spoken American English (Storkel & Hoover, 2010).
Analysis of variance revealed that the words were similar across various word characteristics including word frequency (F(2,35) = 0.665, p = 0.521), part of speech (F(2,35) = 0.295, p =
(F(2,35) = 2.320, p = 0.114).
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0.746), duration (in milliseconds) (F(2,35) = 0.013, p = 0.987), and neighborhood density
Table 1. Stimulus words used for the word recognition task. Word Duration, in Number of Frequencya msb neighborsc
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raid rail rake rare rate ray raise real right rock rose row
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/m/ words 461 443 386 435 404 432 567 401 415 404 601 398 M=449.38 SD= (70.53)
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4.06 2.92 3.90 2.20 2.32 4.15 1.78 2.48 3.83 1.90 1.00 1.60 M=2.75 SD=(1.09)
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made mail make mare mate may maze meal might mock mows mow
/r/ words 2.00 507 2.20 436 2.04 440 2.61 428 3.32 428 2.28 594 2.72 424 1.30 415 3.86 361 2.88 397 2.93 574 2.56 395 M=2.508 SD= M=445.38
16 15 20 18 21 21 8 14 21 14 9 21 M=16.72 SD= (4.81) 22 16 22 16 25 26 8 14 19 16 15 28 M=18.54
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wade whale wake wear wait weigh ways wheel white walk woes whoa
SD= (70.04)
1.30 1.00 2.36 2.59 3.27 3.96 1.00 2.75 3.56 3.00 1.00 1.70 M=2.29 SD =1.07
SD= (5.73)
/w/ words 483 616 385 429 433 424 424 413 339 392 643 411 M=450.50 SD =90.57
a
17 2 20 20 18 23 9 6 6 12 22 9 M=23 SD =7.18
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(0.67)
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Word frequency obtained from an online child corpus calculator (Storkel & Hoover, 2010). Word duration, in milliseconds, was calculated from the onset of phonation to offset of the final phoneme in each word. c Number of neighbors refers to the number of real words that can be obtained by a single phoneme omission, substitution calculated by an online child corpus calculator (Storkel & Hoover, 2010).
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b
The /r/ was selected as a target phoneme to address our research hypotheses regarding the
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perceptual deficit we expected for school-age children with persistent speech delay. The /m/ was
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selected as a control phoneme because it is an early developing sound in speech production and rare for school-age children (Smit et al., 1990) with speech delay to exhibit difficulty producing
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the /m/ at any point in development. Both phonemes were well-suited for representation in sinewave speech and amplitude-comodulated speech because of their respective acoustic structure,
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which depend on frequency relevant features such as formant transitions. Such acoustic structure is readily preserved in both sine-wave speech and amplitude-comodulated sine-wave speech.
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Moreover, rudimentary adjustments to amplitude levels enabled the simulation of naturally subtle amplitude differences (i.e. anti-resonance for nasality in /m/). For example, a key feature of the /r/, the height of the third formant frequency, is fully represented in both sine-wave speech and amplitude-comodulated sine-wave speech. The /m/ relies on the blending of the formant transition into the following vowel with minor adjustments to amplitude levels described below
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to mimic features of anti-resonance present in the production of nasal phonemes during natural speech production. Naturally-produced tokens of each word were recorded by an adult female speaker with a standard dialect of Midwest American English while seated in a single-walled isolated acoustic
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chamber. The words were randomized into three separate lists for recitation to eliminate order effects of reading during recording. All tokens were recorded at a sampling rate of 44.1 kHz and an amplitude resolution of 16 bits, using a desktop microphone (AKG C414B) and a Zoom H4N digital recorder. After recording was complete, audio files were digitally transferred to a personal
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computer and segmented into individual words using CoolEdit 2K. All words were screened for mispronunciations, peak clipping, and background noise and normalized at -.5 dB (re: 16 bits = 96 dB peak). Following this process, one of the remaining tokens of each word was randomly
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selected for inclusion in this study.
To create sine-wave speech from these tokens, a wide-band spectrogram of each
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recording was produced using standard audio software (WinSnoori 1.34). Algorithms within WinSnoori estimated and traced the center frequencies of each of the first four formants. Visual
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inspection, by the primary investigator, was used to examine the algorithm-produced tracings to
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confirm the estimations and to adjust parameters to better approximate the formant frequencies visible on the original spectrogram. Typically, only minor adjustments were required. Using the
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Klatt interface in the program, numerical values for each formant frequency were tabulated at 4 ms intervals. All values were entered into a custom-designed computer program that constructed waveforms made up of independent sinusoidal waves (TONE16, Tice & Carrell, 2011). Peak amplitudes for the tones representing the four formants were set to 90-, 84-, 76-, and 70-dB RMS (re: 16 bits = 96-dB peak) to approximate the spectral slope of speech. These amplitudes were
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lowered by 3-dB, 5-dB, 10-dB, or set to 0 to approximate the original formant amplitudes for specific phonemes. For example, for nasal phonemes, amplitudes of the first and second formants were lowered by 3-dB to simulate decreased amplitude as a result of anti-resonance in natural speech. All SWS tokens were generated with a sampling rate of 10 kHz. Amplitude-
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modulated sine-wave speech (AMSWS) words originated from the SWS tokens described above. Each sine-wave speech token was amplitude-comodulated at 80 Hz using a custom-designed modulating software, AMOD32 (Tice & Carrell, 2011). A modulation cycle of 80 Hz was
chosen as previous research has shown optimal intelligibility for amplitude co-modulation of
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SWS signals at rates between 50-100 Hz (Lewis & Carrell, 2007; Carrell & Opie, 1992). The modulating signal was a triangular wave with a duty cycle of 83%. On each 12.5-msec modulating cycle, the amplitude rose to 100% in 8 milliseconds (ms), dropped to 0% in 2 ms,
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and remained at 0% for the final 2.5 ms. All stimuli in the present experiment were equilibrated to 72-dB (re: 96 dB maximum level with 16-bits of amplitude resolution). See Figure 1 for
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sine-wave speech.
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spectrograms representative of natural speech, sine-wave speech, and amplitude-comodulated
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Figure 1. Spectrograms representative of natural speech, sine-wave speech, and amplitudecomodulated sine-wave speech. Panel A is a spectrogram of the naturally-spoken word “rare” showing tracings of the center frequencies of the formant frequencies using audio software Wavesurfer1.8.8. Panel B shows the spectrogram for the unmodulated sine-wave version of the same word based on the first four formants. Panel C shows the spectrogram of the amplitudecomodulated (80-Hz) sine-wave version of the same word.
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The sine-wave speech and amplitude-comodulated sine-wave speech stimuli were piloted on both typically-developing adult and child listeners because none of the words had been used in previous research. Consistent with previous findings, amplitude-comodulated sine-wave speech tokens were reliably more intelligible than the sine-wave speech tokens from which they
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were created (Carrell & Opie, 1992; Lewis & Carrell, 2007; Shapley, 2003). Listeners
consistently identified more phonemes in amplitude-comodulated sine-wave speech as compared to sine-wave speech tokens. As a perceptual reference for the acoustic quality of each of these types of novel stimuli, pilot children described sine-wave speech as sounding like “squeals”,
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“beeps”, or “R2D2 from Star Wars”, whereas amplitude-comodulated sine-wave speech was usually described as sounding “like a robot”. 2.4 Procedure
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Children were tested in two research sessions, the first of which involved the administration of the standardized assessment measures and the second of which involved completing the word
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recognition task. The session took place in a child-friendly room that was acoustically treated with absorbent foam designed to significantly reduce the ambient noise level of the testing
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environment. All participants were tested individually while seated at a computer running E-
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prime 2.0 experimental software. Children were fitted with insert earphones (Etymotic ER-1A) set to a comfortable listening level. Four children did not tolerate placement of the earphones in
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the ear canal and were thus fitted with noise-cancelling headphones (Sennheiser HD280Pro) set to a comfortable listening level. Throughout the experiment, a research assistant sat beside the child to monitor the child‟s attention to the task but was unable to hear the auditory stimuli due to presentation via headphones for the child.
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The word recognition task involved children listening to both types of novel word stimuli described above. Stimuli were presented in the context of novel talkers: an alien (sine-wave speech) or robot (amplitude-comodulated sine-wave speech). Pre-recorded instructions were provided via computer. Children were instructed that the alien and robot talkers “sounded funny”
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and the child‟s help was enlisted to determine what they were saying by “interpreting” what was said. Children responded verbally and their response was audio recorded using a desktop
microphone (AKG C414B) connected to a Marantz (PMD670) digital recorder. All responses were transcribed in real time. A second transcriber transcribed a random 20% of samples offline.
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Inter-rater reliability was 94.3%. Because some children, by definition, had difficulty producing speech sounds, children also pointed to the first letter of their chosen word on a provided letter strip secured to the table to eliminate ambiguity about the child‟s intended response. The letter
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strip contained 12 letters including the target letters, i.e. r/m, and 9 additional letters: b, w, s, n, p, g, d, l, f, c, and a “not here” option for any alternate letters the child might have wanted to select.
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Because of the simplicity of the words used, spelling deficiency was not expected to interfere with a child correctly identifying the first letter of their response. In all cases, if there was a
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mismatch between the initial phoneme of the child‟s initial response and the child‟s first-letter
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selection, the item was scored according to the letter the child pointed to on the letter strip. The task totaled 48 trials. Of children in the typically-developing group, one child had one mismatch
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between the spoken word and the selected first letter. All other typically-developing children exhibited 100% consistency between their transcribed verbal response and the selection of the first letter for all 48 trials. Of children in the dyslexia group, one child had two mismatches between the verbal response and the selected first letter. All other children in the dyslexia group exhibited 100% consistency between verbal responses and letter strip selections. Of children in
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the residual speech delay group, any verbal response potentially containing an /r/ was transcribed as the child produced it. As a result, for these children it is difficult to calculate the true mismatch rate between what the child said and what the child intended, hence this particular procedure. For verbal responses that did not contain a distorted phoneme, no child with a
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residual speech delay had any mismatch between their verbal response and the letter strip
response. Thus, it is assumed for distorted articulation in their verbal responses, these children confidently pointed to their intended first letter response.
The word recognition task began with a familiarization phase during which the child was
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introduced to each novel speaker, i.e. the alien or the robot, which individually appeared as a still image on the computer screen. Beginning with the alien, which spoke in sine-wave speech, the computer presented a sample item three times. The child was encouraged to guess the word that
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they heard. The child was told the word by the research assistant and then listened to the item a fourth time. This procedure was repeated using a different sample item for the robot who spoke
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with the amplitude-comodulated sine-wave speech. All children successfully completed the familiarization tasks. The experimental phase comprised four blocks of 12 words, for a total of
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48 words. Two blocks contained sine-wave speech tokens and two blocks contained amplitude-
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comodulated sine-wave speech tokens. All words were presented in both sine-wave speech and amplitude-comodulated sine-wave speech conditions. To control for order effects of word
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exposure, presentation order was counterbalanced across participants and groups. Within each block, all stimuli were randomized. Each stimulus was heard three times in succession, during or after which children provided their verbal and pointing responses. After the child‟s response, the research assistant advanced the program to the next trial. Children were not provided feedback regarding the accuracy of their responses. During the task, the alien or robot (depending on the
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block) advanced a step along a pathway towards a finish line, providing children a visual cue for their progress in the task. The dependent measure for these tasks was the number of initial phonemes (i.e. /r/, /m/) accurately recognized for each acoustic condition. In cases where the transcription of a child‟s
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response was not consistent with the letter to which they pointed indicating the first letter of the response (ex. The child said “wake” for “rake”, but pointed to the letter “r” as the first letter), the item was scored based on the child‟s pointing response.To ensure any obtained group differences were not attributable to knowledge of specific vocabulary used in this study, after the completion
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of the word recognition task, all participants were administered a vocabulary probe comprising words used in the tasks. Children were presented with picture plates with four pictures, one of which represented the target word. The other three pictures represented phonological, semantic,
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and unrelated foils. A one-way ANOVA revealed no difference between groups in performance on this probe, F(2,35) = 1.872 (p=.170). Correlation analysis further verified there were no
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statistically significant relationships between vocabulary probe accuracy and any of the word recognition dependent variables for any of the groups (all ps > 0.10). The data were analyzed
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using a series of repeated-measures ANOVAs to address our research questions. Throughout the
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analyses, post-hoc comparisons were used to decompose interactions. In all cases, an alpha level of .05 was used to determine statistical significance.
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3. Results
The purpose of this study was to determine how children with different phonological
deficits (i.e., dyslexia or persistent speech delay) perceive amplitude comodulation structure of words compared to novel sine-wave words during a word recognition task. Our first research question asked whether children with dyslexia or persistent speech delay exhibited more
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difficulty perceiving sine-wave speech and amplitude-comodulated sine-wave speech as compared to their typically-developing peers. Because individuals with persistent speech delay have demonstrated difficulty perceiving fine temporal structure in speech signals as compared to their peers with dyslexia, we predicted that children with dyslexia would have less difficulty
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perceiving sine-wave speech and amplitude-comodulated sine-wave speech. Our second
research question asked whether children with persistent speech delay have more difficulty
perceiving a phoneme erred in their own production (Locke, 1980; Shuster, 1998). We predicted that children with persistent speech delay would have more difficulty perceiving the /r/ phoneme
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in both sine-wave and amplitude-comodulated words, as compared to their peers with dyslexia and their typically-developing peers. 3.1 Descriptive Measures
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All children were administered a battery of articulation, language and reading assessments for assignment of participants to the appropriate group. Descriptive statistics
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regarding word reading, articulation, nonverbal intelligence, vocabulary and phonological awareness skills are displayed in Table 2. As selected, the groups differed in word reading such
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that children with dyslexia exhibited significantly lower word reading scores than those in the
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persistent speech delay and typically-developing groups, F (2, 35)=29.026, p<.001. Likewise, as selected the groups differed in speech articulation such that children in the persistent speech
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delay group exhibited significantly lower articulation skills than those in the dyslexia and typically-developing groups F(2,35)=51.839, p<.001. Furthermore, children in the persisent speech delay group performed significantly worse than children in the typically-developing and dyslexia groups on the R-probe, F(2,35)=990.398, p<.001. Of the 12 children with persistent speech delay, 11 failed to produce /r/ in any position of words in isolation or connected speech.
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One child with speech sound disorder exhibited a correct /r/ production in pre-vocalic positions of words (ex. rat, green), but failed to produce /r/ post-vocalically (ex. deer, fire). There were no group differences in receptive vocabulary, F(2,35) = .650, p=.528, expressive vocabulary, F(2,35) =2.805, p=.075, or nonverbal intelligence, F(2,35) = 1.927, p=.162. Although not
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statistically different, there were marginal differences in phonological awareness (F(2,35) = 3.003, p=.063) with the children with dyslexia performing numerically poorer than the other children.
SSD (n=12) Mean (SD) 100.25 (5.79)a 112.58 (8.69)a 77.83 (10.88)b 112.08 (9.61)a 105.42 (12.03)a
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DYS (n=12) Mean (SD) 106.26 (9.29)a 89.41 (3.96)b 102.42 (5.93)a 107.58 (12.23)a 96.25 (12.70)a
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Table 2. Descriptive Statistics. TD (n=12) Mean (SD) Age (in months) 103.3 (8.53)a WRMT-R/NU 111.75 (11.08)a 105.17 (1.95)a GFTA-2 ROWPVT 112.58 (13.33)a EOWPVT 107.92 (13.34)a
106.00 (14.41)a 94.1667 (7.22)a
101.5 (12.95)a
RIAS R-Probe (out of 46)
116.5 (13.37)a 46.00 (0.0)a
113.25 (16.45)a 1.42 (4.91)b
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CTOPP
106.00 (9.52)a 46.00 (0.0)a
Note. Means in the same row that do not share subscripts differ at p<.05.
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(TD = Typically-developing; DYS = Dyslexia; SSD = Speech sound disorder; WRMT-R/NU=Woodcock Reading Mastery Test-Revised/Normative Update, Basic Skills cluster; GFTA-2=Goldman-Fristoe Test of Articulation-2nd Ed., standard score (SS) reported; ROWPVT=Receptive One-Word Picture Vocabulary Test, SS reported; EOWPVT=Expressive One-Word Picture Vocabulary Test, SS reported; CTOPP=Comprehensive Test of Phonological Processing, SS reported; RIAS=Reynolds Intellectual Assessment Scales, SS reported; R-probe, raw score reported)
3.2 Experimental Measures The purpose of the word recognition task was to determine whether children differed in their recognition of impoverished speech signals in two acoustic conditions. One condition, sine-wave speech, presented words as four independent time-varying sinusoids designed to
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mimic center frequency values of the first four formants of a naturally-spoken word. The other condition, amplitude-comodulated sine-wave speech, applied a modulating waveform of 80 Hz to the sine-wave speech words. Previous research has demonstrated that both adults and children typically recognize more amplitude-comodulated sine-wave speech words than sine-wave speech
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words, when all other acoustic and linguistic structure is held constant (Carrell & Opie, 1992; Lewis & Carrell, 2007).
To analyze these data, we used a 2 (modulation: sine-wave speech, amplitude-
comodulated sine-wave speech) x 2 (phoneme: /r/, /m) x 3 (group: dyslexia, speech sound
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disorder, typically-developing) mixed factorial design repeated-measures ANOVA. Results revealed a main effect of modulation, F(1,33) = 27.920, p < .001, ηp2 = .458 such that on average, as expected, children recognized initial phonemes in the sine-wave speech condition
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with less accuracy than words in the amplitude-comodulated sine-wave speech condition. Figure 2 shows average performance on /m/ and /r/ words across groups.
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Figure 2. Initial phoneme accuracy for /m/ and /r/ collapsed across all participants. The plain white bar represents sine-wave speech (SWS) and the striped bar represents amplitudecomodulated sine-wave speech (AMSWS). Error bars reflect standard error of the mean.
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The main effect of group was not significant, F(2,33) = 2.268, p=0.119, ηp2 = .121, but the modulation by group interaction was significant, F(1,33) = 3.581, p=.039, ηp2 = .178, suggesting that the groups differed in their recognition accuracy across the two modulation conditions. Planned comparisons across the different phoneme conditions for each group
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revealed that the children with dyslexia recognized more amplitude-comodulated sine-wave speech words than sine-wave speech words for the /m/ phoneme, t(11) = -5.014, p<.001, and the /r/ phoneme, t(11) = -3.544, p=.005. By contrast, children with persistent speech delay
recognized more amplitude-comodulated sine-wave speech words than sine-wave speech words
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for the /m/ phoneme, t(11) = -2.785, p=.018, and there was no difference in accuracy between amplitude-comodulated sine-wave speech and sine-wave speech for the /r/ phoneme, t(11) = .951, p = .362. Children in the typically-developing group did not differ in accuracy between
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amplitude-comodulated sine-wave speech and sine-wave speech for the /m/ phoneme, t(11) 2.057, p = .064 nor the /r/ phoneme, t(11) -1.358, p = .202. Nonsignificant differences for the
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typically-developing group may be attributable to ceiling effects, as the typically-developing children recognized a high percentage of phonemes in the sine-wave speech condition and thus
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did not have the opportunity for substantial gains in phoneme recognition in the amplitude-
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comodulated sine-wave speech condition. Additionally, using word stimuli comprising simple CVC structures may have contributed to the high rates of accuracy in these children.
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There were no group differences in accuracy for sine-wave speech without comodulation for both the /m/ and /r/ phonemes (all ps > .05). All groups achieved similar levels of accuracy in the amplitude-comodulated sine-wave speech condition for the /m/ phoneme (all ps > .05), but children with persistent speech delay recognized significantly fewer /r/ phonemes in the amplitude-comodulated sine-wave speech condition, as compared to their peers with dyslexia
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(t(11) = 2.098, p = .048) and those who were typically-developing (t(-2.721, p=.012). The threeway interaction between modulation, phoneme and group was not significant, F(1,33) = .254, p=.777, ηp2 = .015, nor was the two-way interaction of modulation and phoneme significant, F(1,33) = 2.857, p=.100, ηp2 = .080. Figures 3 and 4 show performance level for each group by
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each phoneme and condition, and include individual data points for all participants (N=36) for the reader to more fully conceptualize performance across each group.
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Figure 3. Initial phoneme accuracy for /m/ for each participant and participant group. Each dot represents a child and connecting lines delineate performance in sine-wave speech (SWS) versus amplitude-comodulated sine-wave speech (AMSWS). Bolded lines indicate more than one child achieved this pattern of scores. Please see Appendix A for individual performance scores for each child in each group. The first panel shows children in the dyslexia (DYS) group with SWS performance on the left and AMSWS performance on the right. The second panel shows word recognition performance for children with persistent speech delay (PSD) and the third panel shows performance for typically-developing (TD) children. Hashed lines represent the group mean for each group for each condition. Asterisks indicate a statistically significant difference (p < .05) between SWS and AMSWS.
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Figure 4. Initial phoneme accuracy for /r/ for each participant and participant group. Each dot represents a child and connecting lines delineate performance in sine-wave speech (SWS) versus amplitude-comodulated sine-wave speech (AMSWS). Bolded lines indicate more than one child achieved this pattern of scores (e.g. in the TD group, 6 children achieved 100% accuracy for both SWS and AMSWS conditions). Please see Appendix A for individual performance scores for each child in each group. The first panel shows children in the dyslexia group with SWS performance on the left and AMSWS performance on the right. The second panel shows word recognition performance for children with persistent speech delay (PSD) and the third panel shows performance for typically-developing (TD) children. Hashed lines represent the group mean for each group for each condition. Asterisks indicate a statistically significant difference (p < .05) between SWS and AMSWS.
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As these figures show, for the /m/ phoneme, the majority of children across all three groups showed either improved word recognition from sine-wave speech to amplitude-comodulated sine-wave speech or exhibited a very similar performance between the two conditions. Visual inspection of individual performance for the /r/ phoneme, however, across the three groups of children shows a greater variability in performance for children with persistent speech delay
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versus children in the dyslexia and typically-developing group. Notably, several children in this group performed poorer in the amplitude-comodulated sine-wave speech condition. 4. Discussion The purpose of the present study was to determine the degree to which children with
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differing phonological deficit profiles, namely children with dyslexia and children with persistent speech delay, perceived impoverished speech signals containing varying amounts of acoustic information. Thirty-six children aged 7;6 – 9;6, based on word reading and speech articulation skills, were classified into three groups: 1) dyslexia, 2) persistent speech delay, and 3) typically-
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developing. All children listened to two types of impoverished speech: sine-wave speech and amplitude-comodulated sine-wave speech. We had two main hypotheses: 1) Children with dyslexia would exhibit superior word recognition for sine-wave speech and amplitude-
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comodulated sine-wave speech, as compared to their peers with persistent speech delay; and 2) children with persistent speech delay would exhibit particularly poor word recognition for
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stimuli containing a phoneme erred in their own production. We discuss two key findings here. 4.1 Amplitude comodulation improved speech perception in children with phonological
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impairments, particularly for children with dyslexia
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Regardless of the presence or absence of a phonological impairment, all children in this study, on average, performed better for the amplitude-comodulated stimuli as compared to the
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traditional sine-wave stimuli. While this response pattern has been reported for typical adults and children (Cabbage, Carrell, & Hogan, 2011; Carrell & Opie, 1992; Lewis & Carrell, 2007), it was unknown whether children with deficient phonological representations would perform similarly. As compared to typically-developing children, children with dyslexia exhibited substantial difficulty perceiving unmodulated sine-wave speech. When the stimuli were
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amplitude -comodulated, however, children with dyslexia achieved a performance at the same level as their typically-developing peers. That is, amplitude comodulation yielded substantial gains in perception for children with dyslexia. It is unsurprising that children with dyslexia showed poor perception of phonemes in the unmodulated sine-wave speech as this is consistent
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with previous work showing that adults with dyslexia struggle to perceive sine-wave speech as compared to typical adult listeners (Rosner, et al., 2003). Modifying the stimuli via amplitude comodulation, however, yielded substantial gains in perception for the children with dyslexia suggesting amplitude comodulation provided a particularly necessary cue for children with
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dyslexia.
Given the benefits in perception for sine-wave speech versus amplitude-comodulated sine-wave speech across all groups in this study, we hypothesize these stimuli are processed at an
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acoustic, or sensory, level as opposed to a more linguistic, or phonological, level. The two types of stimuli presented here contained identical phonetic parameters including formant frequency
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values, transitions, and duration. The only difference between the stimuli was the acoustic manipulation of amplitude comodulation suggesting that it is the only factor that is contributing
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to the improved intelligibility for these children. Some have argued the perceptual deficit in
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children with dyslexia and persistent speech delay is attributable to a deficit at a more phonetic, or linguistic, speech level of processing (Manis et al., 1997; Studdert-Kennedy & Mody, 1995),
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rather than a non-speech sensory auditory level of processing (Tallal, 1980; Vandermosten et al., 2011). While there has been much debate in the literature on this issue (see Rosen, 2003, for a review), it seems that while a subgroup of children with dyslexia may exhibit some low-level sensory processing deficiencies, it is not likely the basis of the phonological deficit seen in most
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children with dyslexia. From these results, it appears that children with dyslexia are indeed sensitive to acoustic manipulations that do not depend on linguistic, or phonetic, processing. The mechanism by which amplitude-comodulated sine-wave speech is usually more intelligible than sine-wave speech, despite the absence of any additional phonetic or linguistic
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structure is not well understood. One possibility for the better perception for amplitude-
comodulated sine-wave speech over unmodulated sine-wave speech is that in typical speech perception, neural fibers of the auditory nerve respond, in synchrony, to the harmonic elements of a spoken utterance. According to Koch et al. (1999), it is precisely the timing pattern of the
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neural impulses and the synchronization of neural firing that are critical for speech perception in compromised situations, such as a noisy background. Such timing and synchronization may also be important when listening to impoverished speech signals. Because sine-wave speech
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comprises independent sinusoids, it lacks any harmonic structure and thus, synchronous neural firing in the brain during speech perception is unlikely. It is possible that amplitude
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comodulation, which applies a simultaneous modulating signal to the independent sinusoids comprising the sine-wave speech signal, mimics the harmonic structure found in natural spoken
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utterances. The brain may process the sinusoids together for synchronous neural firing during
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processing, which results in more “speech-like” perception of amplitude-comodulated sine-wave speech as compared to sine-wave speech. While this was not specifically tested in this study, it
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would be an important future direction to address whether amplitude-comodulated sine-wave speech induces a level of neural synchrony that boosts intelligibility as compared to sine-wave speech.
Studies investigating amplitude modulation detection at very slow rates (i.e., 4 Hz, 10 Hz) have demonstrated that individuals with dyslexia show different patterns of response, both
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behaviorally (Goswami et al., 2002) and at the level of neuronal processing (Goswami, 2011; Hamalainen et al., 2012). These slow rates of amplitude modulation have been thought to be relevant to the extraction of syllable-level and prosodic elements, whereas faster rates of modulation are related more to the extraction of acoustic-phonetic information (Hamalainen et
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al., 2012). If neural synchrony plays a significant role in the improved intelligibility of
amplitude-comodulated sine-wave speech, which provides a faster level of amplitude modulation (i.e., 80 Hz, in the present study), the present findings would lend support to the notion that
dyslexic individuals have specific deficits in realms of slower, as opposed to rapid, temporal
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processing (Goswami, 2002; Goswami, 2011; Hamalainen et al., 2012). This follows from tasks that emphasize the salience of amplitude structure, such as amplitude envelope detection and band-limited speech tasks, for which dyslexic listeners show difficulty (Goswami, 2002; Lorenzi
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et al., 2000; Pasquini, Corriveau, & Goswami, 2007; Rocheron et al., 2002). The temporal sampling framework for developmental dyslexia posits that children with
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dyslexia exhibit specific difficulty for the perception of amplitude, or „global‟, structure in complex signals such as speech (Goswami, 2011). Although the current study sought to
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determine the degree to which these children responded to stimuli constrained to include
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primarily spectral detail, it would be important to understand how these same children respond to stimuli constrained by the removal of most spectral detail, such as vocoded speech. Vocoded
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speech is composed of an utterance that has been band-pass filtered into a selected number of channels, which are then processed with a designated type of noise (e.g. speech-shaped noise, white noise, etc), in order to remove temporal fine structure detail. In a study using vocoded speech sentences with 10- to 11-year-old children, Johnson et al. (2011) showed marginal effects of a perceptual deficit for children with dyslexia-only and statistically significant deficits for
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children with a history of both speech sound disorder and dyslexia. Children with a history of speech sound disorder-only (with no dyslexia) performed more similarly to their typical peers when listening to band-limited speech (Johnson et al., 2011). Because listeners with dyslexia have reportedly exhibited particular difficulty with band-
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limited speech (Ziegler et al., 2009; Johnson et al., 2011), it suggests they may more heavily rely on spectral structure for speech perception accuracy. The results of the current study suggest this indeed may be the case. In contrast to band-limited speech, sine-wave speech comprises
reasonably preserved spectral structure and so it is plausible that children with dyslexia may
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show a particular aptitude for sine-wave speech and amplitude-comodulated sine-wave speech as compared to their peers with persistent speech delay, because of the preserved temporal fine structure present in the signal. Further evidence suggests individuals with dyslexia show
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particular impairments for perception tasks relying on cues associated with amplitude structure such as amplitude envelope detection (Goswami, 2011), rise time discrimination (Goswami et
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al., 2002; Goswami et al., 2011; Muneaux et al., 2004), and amplitude modulation detection (Lorenzi et al., 2000; Rocheron et al., 2002). All of these tasks preserve amplitude structure,
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which primarily transmits information regarding energy variation over time cueing syllable and
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word-level features, as opposed to fine spectral detail that cues specific phonetic information, such as formant transitions.
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4.2 Children with persistent speech delay have difficulty perceiving words containing their erred sound
Previous evidence has suggested children with speech sound disorder show particular
perceptual vulnerability for phonemes which are erred in their own speech production (Hoffman et al., 1985; Rvachew & Jamieson, 1989). Children were presented with words beginning with
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the /m/ phoneme, a phoneme not produced in error by children with persistent speech delay, and were also presented with words beginning with the /r/ phoneme, a phoneme which was produced incorrectly by all children with speech sound disorder in our study. Children in the speech sound disorder group exhibited comparable recognition of /m/ words for sine-wave speech and
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amplitude-comodulated sine-wave speech, as compared to their peers. For words beginning with the /r/ phoneme, however, children with persistent speech delay did not show a gain in accuracy between sine-wave speech and amplitude-comodulated sine-wave speech, while their peers did, thus supporting our second hypothesis. While children with persistent speech delay did not show
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an overall difficulty perceiving phonemes presented in the context of impoverished speech, they showed specific difficulty perceiving a phoneme that was produced in error. This is consistent with previous research that has demonstrated some children with speech sound disorder have
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difficulty perceiving words containing phonemes that are distorted in their own speech production (Hoffman et al., 1985; Rvachew & Jamieson, 1989). Some have argued that
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imprecise articulation skills may be attributable to a weakness in the development of robust phonological representations (Carroll, Snowling, Hulme, & Stevenson, 2003; Raitano et al.,
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2004) for adequate speech production. Deficits in speech perception suggest that children with
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speech sound disorder may exhibit weak phonological representations, particularly for erred sounds in their own speech.
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By definition, sine-wave speech and amplitude-comodulated sine-wave speech comprise primarily spectral structure. Notably, the children in the persistent speech delay group only exhibited difficulty perceiving the /r/ phoneme, but did not show the same difficulty for the correctly produced /m/ phoneme. While this may lead to the conclusion that these children do not have difficulty perceiving fine temporal structure, it is also possible that these children
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35
capitalized on the fact that rudimentary amplitude structure was mimicked and provided in the creation of the /m/ stimuli in this study. The /m/ stimuli were created by reducing the overall intensity of the independent sinusoids by 5 dB to mimic reduced amplitude caused by antiresonance in the natural production of nasal phonemes. To further test this hypothesis,
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children with persistent speech delay should be given sine-wave speech and amplitude-
comodulated sine-wave speech containing phonemes that are not produced in error, but contain acoustic cues similarly constrained by fine temporal structure as the /r/ phoneme such as /l/. This study, like all studies, is not without limitations that should fuel future work. The
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present study investigated speech perception skills in a small subset of children with dyslexia or persistent speech delay. Importantly, the children included here met stringent criteria in both word reading and articulation skills to isolate phonological deficits specific to dyslexia or
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persistent speech delay. To date, only one known study of speech perception in children has accounted for both reading and speech production skill (Johnson et al., 2011) and the results of
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the present study likewise noted that children with dyslexia and children with speech sound disorder exhibit distinct profiles of speech perception skill. In our study, children provided
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verbal responses as well as pointing responses in an attempt to circumvent ambiguous speech
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production second to speech articulation errors in the group of children with persistent speech delay. Arguably, having children provide a verbal response during a perceptual task may have
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inadvertently interfered with the speech perception process. Future research should seek to mitigate this interference by designing tasks that require no verbal response via spelling responses or alternate paradigms that do not require generative responses. Additionally, in this study we investigated speech perception performance on a limited set of phonemes (i.e., /m/, /r/) in sine-wave speech and amplitude-comodulated sine-wave speech. Future work should include
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additional phonemes as well as additional acoustic manipulations to further describe the nature of the perceptual deficits in these children. 5. Conclusion The nature of the underlying perceptual deficits in children with phonological
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impairment, whether dyslexia or speech sound disorder, is not well understood. The present study sought to disentangle the commonalities and differences in these children by specifically addressing whether children with dyslexia and children with speech sound disorder responded to spectral structure in speech signals with the same degree of sensitivity. Although all children
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benefitted from added structure (i.e., sine-wave speech vs. amplitude-comodulated sine-wave speech) to combine acoustic cues into meaning, the present study lends support to the notion that children with dyslexia benefit particularly from this additional acoustic structure prior to the
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engagement of higher-level linguistic processes, as this gain was seen without the addition of phonetic, syntactic, or semantic context. Importantly, when subjected to a perceptual task that
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involved an erred phoneme in their own speech production, children with speech sound disorder failed to show significant improvements in perception from sine-wave speech to amplitude-
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comodulated sine-wave speech. The present results suggest that children with dyslexia and
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speech sound disorder exhibit distinct perceptual strengths and weaknesses when subjected to
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impoverished speech stimuli that preserves spectral structure.
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Acknowledgements This research was supported by funds from the Barkley Memorial Trust at the University of Nebraska-Lincoln (Cabbage), the American Speech-Language-Hearing Foundation (Cabbage),
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and the National Institutes of Health, R03 DC9667 (Hogan).
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Efficiency. Austin, TX: PRO-ED.
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Ghesquiere, P. (2010). Adults with dyslexia are impaired in categorizing speech and
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nonspeech sounds on the basis of temporal cues. Psychological and Cognitive Sciences, 107(23), 10389-10394.
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Wagner, R. K., Torgesen, J. K., & Rashotte, C. A. (1999). Comprehensive Test of Phonological Processing. Austin, TX: PRO-ED, Inc.
Woodcock, R. (1998). The Woodcock reading mastery tests-Revised-Normative Update. Circle Pines, MN: American Guidance Service. Ziegler, J.C., Pech-Georgel, C., George, F., & Lorenzi, C. (2009). Speech-perception-in- noise
ACCEPTED MANUSCRIPT SPEECH PERCEPTION DIFFERENCES
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AN US
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deficits in dyslexia. Developmental Science, 12(5), 732-745.
ACCEPTED MANUSCRIPT SPEECH PERCEPTION DIFFERENCES
46 Appendix A
Table 1. Word recognition accuracy for each child in each group (dyslexia, persistent speech delay, typically-developing) for sine-wave speech (SWS) and amplitude co-modulated sine-wave speech (AMSWS) for /m/ and /r/ phonemes. Scores are reported as percent correct (out of 12 words in each condition). /m/
CE
AC
/r/ AMSWS 100.00% 83.33% 91.67% 100.00% 100.00% 100.00% 83.33% 66.67% 100.00% 75.00% 91.67% 100.00%
SWS 100.00% 83.33% 58.33% 75.00% 16.67% 41.67% 66.67% 66.67% 91.67% 16.67% 41.67% 83.33%
AMSWS 100.00% 100.00% 100.00% 100.00% 100.00% 100.00% 50.00% 100.00% 100.00% 75.00% 100.00% 100.00%
100.00% 66.67% 25.00% 91.67% 100.00% 41.67% 91.67% 0.00% 100.00% 16.67% 41.67% 91.67%
100.00% 91.67% 66.67% 91.67% 100.00% 75.00% 100.00% 100.00% 100.00% 25.00% 83.00% 91.67%
46.17% 33.33% 25.00% 66.67% 66.67% 91.67% 83.33% 0.00% 100.00% 91.67% 100.00% 100.00%
33.33% 75.00% 58.33% 100.00% 75.00% 58.33% 100.00% 58.33% 83.33% 100.00% 83.33% 58.33%
100.00% 83.33% 100.00% 91.67% 91.67% 100.00% 83.33% 91.67% 100.00% 66.67% 83.33% 50.00%
83.33% 100.00% 50.00% 100.00% 91.67% 91.67% 100.00% 100.00% 100.00% 100.00% 50.00% 83.33%
83.33% 100.00% 91.67% 100.00% 100.00% 75.00% 100.00% 100.00% 100.00% 100.00% 83.33% 91.67%
AN US
M
ED 66.67% 100.00% 58.33% 100.00% 75.00% 100.00% 75.00% 75.00% 100.00% 83.33% 41.67% 0.00%
CR IP T
SWS 100.00% 41.67% 66.67% 66.67% 83.33% 41.67% 0.00% 41.67% 33.33% 50.00% 16.67% 75.00%
PT
Child ID Dyslexia D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 Persistent Speech Delay SD1 SD2 SD3 SD4 SD5 SD6 SD7 SD8 SD9 SD10 SD11 SD12 Typically-Developing TD1 TD2 TD3 TD4 TD5 TD6 TD7 TD8 TD9 TD10 TD11 TD12