- Email: [email protected]
prior to speaking inhibits stuttering in adults in the English language
Neuroscience Letters 306 (2001) 111±115
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Producing the vowel/a/ prior to speaking inhibits stuttering in adults in the English language Vikram N. Dayalu, Tim Saltuklaroglu, Joseph Kalinowski*, Andrew Stuart, Michael P. Rastatter Stuttering Research Lab, Department of Communication Sciences and Disorders, East Carolina University, USA Received 20 February 2001; received in revised form 16 March 2001; accepted 22 April 2001
Abstract This study investigated the effects of producing and listening to the vowel /a/ on the frequency of overt stuttering moments in eight people who stuttered. Stuttering frequency counts were made for the speech produced in the control condition, and after each of these four experimental conditions: (a) producing a vowel /a/ for 4 s; (b) producing a vowel /a/ for 4 s and waiting for 4 s; (c) listening to a recording of the vowel /a/ for 4 s; and (d) listening to a recording of the vowel / a/ for 4 s and waiting for 4 s. A signi®cant reduction in the stuttering frequency was only observed following production of the vowel /a/ without a 4 s delay (P 0:02), suggesting that the vowel production prior to speech, serves as a temporary ¯uency enhancer. Its similarity to the occurrence of overt stuttering moments (e.g. discrete part-word repetitions and prolongation's) and its relationship to the fundamental nature of the pathology are discussed. q 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Stuttering, Sensory motor integration, Overt manifestations, Second speech signal
Stuttering is de®ned as involuntary part-word repetitions and prolongation's that interrupt the forward ¯ow of speech, impairing communication intent and understanding [3,20]. The causal factor of stuttering is considered to exist at a central level [6], yet the observable behaviors are manifested in the periphery as a speech disruption. If, as de®ned, the disorder is involuntary, then it may be stated that the overt part-word repetitions, prolongation's, and secondary behaviors are peripheral markers of a central pathology, or central involuntary block [11]. Numerous theories have attempted to explain why people who stutter generate the overt manifestations of stuttering that impede the forward ¯ow of speech. Possible explanations are derived from a speech motor control approach [19,23], a psycholinguistic approach [21], a neurolinguistic approach [16], and a disordered feedback approach [7]. The underlying idea common to all these theories is that the core behaviors of stuttering are the problematic markers of a proposed causal factor, depending on the perspective taken. The aim of the present study is to investigate a possible use for the overt markers of * Corresponding author. CSDI, School of Allied Health, Oglesby Drive, Greenville, NC-27858, USA. Tel.: 11-252-3281986; fax: 11-252-328-4469. E-mail address: [email protected] (J. Kalinowski).
stuttering. It is proposed, that the production of core stuttering behaviors represents an attempt by the person who stutters to overcome the involuntary block, and regain forward ¯owing speech. This study aims to provide empirical support for this hypothesis. Irrespective of stuttering severity, part-word repetitions appear to be the most prevalent form of core stuttering behaviors in children who stutter [1]. The presence of part-word repetitions is also a fundamental core marker in adults who stutter and has been widely used as a diagnostic criterion in the assessment of stuttering [3,19]. The possibility exists that the signi®cant proportion of part-word repetitions in the overt manifestations of stuttering may be indicative of a relationship with the underlying pathology. A means of examining this possible relationship may be found in looking at the general nature of part-word repetitions. Part-word repetitions are syllabic, with a commonly observed signature event being the presence of a vowel, that is often neutralized to become a schwa [3,8,19]. For example, a person who stutters may produce `tuh-tuh-tuh table', producing a schwa (unstressed) vowel three times prior to saying the word. The acoustic features of vocalic sounds, as compared to non-vocalic sounds, are low frequency, periodicity and high intensity. As stated above, syllabic repeti-
0304-3940/01/$ - see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 0 1) 01 86 9- 9
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tions are voiced in nature and this vocalic or voiced characteristic may be representative of a symbiotic relationship between the part-word repetitions and the pathology. A recent study stated that one of the common elements in a second speech signal that enhances ¯uency is its voiced component [11]. Further support for the role of the voiced signal is found in ¯uency enhancement brought about by pseudostuttering (i.e. voluntary stuttering or stuttering on purpose) [3,19]. Pseudostuttering is generally considered to bring about increased levels of ¯uency in the ensuing speech, but pseudostuttering characterized by the use of repetitions is considered to be more effective in producing ¯uency enhancement than pseudostuttering characterized by prolongation's [3,9,19]. In addition, the occurrence of an overt stuttering moment, or a concentrated cluster of stuttering moments, may induce short periods of ¯uency in the ensuing speech [18]. The implications from the above reports converge on the signi®cance of part-word repetitions and their voiced feature in the stuttering pathology. They may present a new avenue towards understanding the symbiotic relationship between the pathology and its core markers. If a voiced signal were produced before the occurrence of speech, it would be interesting to observe its effect on the resultant speech output. A reduction in the overt manifestations of stuttering in the ensuing speech would be indicative of an inhibitory effect derived from the voiced signal. As vowels are by nature voiced and continuous speech sounds, a decrease in stuttering frequency following production of a sustained vowel would support the hypothesis that partword repetitions, which are also voiced, are generated in an attempt to overcome the involuntary block and regain forward ¯owing speech. This study aims to investigate this hypothesis by proposing a simple test. A non-stuttered, sustained vowel /a/ will be produced by the person who stutters, prior to the production of an utterance to test its block inhibiting effects on the ensuing speech. The carryover effect of this signal may also be evaluated by inserting a silent period between the sustained /a/ and the speech act. This could shed light on the temporal nature of this proposed release mechanism. It would also be interesting to note the effect of listening to the vowel prior to speech initiation, as second speech signals have been known to produce signi®cant ¯uency enhancement when provided concomitantly with the speech act [11]. Eight normal-hearing people who stutter (seven males, one female, Mean 19.2 years, SD 4.8) participated in this study. Participants did not present with any other speech or language disorders. All participants had a history of therapy, but were currently not receiving any formal therapeutic intervention. Informed consent (approved by the University and Medical Center Institutional Review Board, East Carolina University) was obtained from all the participants. For each condition (one control and four experimental), the participants read a different series of six to nine syllable sentences. The order of presentation of the conditions was
randomized between subjects. The sentences were selected from different junior-high level passages, with similar theme and syntactic complexity. Based on informal clinical measures (stuttering frequency in the control condition), severity of stuttering for the participants in this study ranged from moderate to severe. They were instructed throughout the experiment to read using a normal speaking voice at their normal rate and loudness, and not to use any controls to reduce or inhibit stuttering. Prior to data collection, the participants were trained to produce an /a/ for 4 s using their normal voice and loudness, cease phonation, and then commence speech on a new breath cycle. They were also trained to produce an /a/ for 4 s using their normal voice and loudness, cease phonation, wait for 4 s (during which the participants were instructed to breathe normally and refrain from holding their breath), and then commence speech on a new breath cycle. The participants were also instructed to not use exaggerated breathing patterns at any point during the experimental conditions. The conditions were as follows: (a) the participants read the sentence without any modi®cation (control); (b) the participants produced /a/ for 4 s, waited for 4 s, and then initiated speech; (c) the participants produced /a/ for 4 s, stopped phonating, and initiated speech; (d) the participants listened to /a/ for 4 s, waited for 4 s, and initiated speech; and (e) the participants listened to /a/ for 4 s, then immediately initiated speech. As the listening conditions required no phonation prior to speech, the participants initiated speech on a normal speech breathing cycle, similar to the production tasks. The experimenter used a stopwatch to time 4 s of /a/ in conditions b and c, and used a hand signal to indicated to the participant when to cease /a/ production and initiate speech. For the two conditions involving listening to the /a/, participants listened to the auditory speech signals via supra-aural earphones (headphones that completely cover the external ear) at a most comfortable listening level and were provided with a 150 ms, 4 kHz pure tone cue to initiate speech. Recording of the 4-s /a/ for conditions d and e was made in a sound-treated room with a digital tape-recorder (Sony model 8819). A normal ¯uent American English-speaking adult male produced the vowel. The samples were produced at a normal vocal effort. The recorded signals were then fed into a personal computer (Apple Power Macintosh 9600/ 300) via an Apple sound input port. Sampling was at 44 kHz. Sound analysis software (Sound Edit version 2) was used to introduce the silence. These were then recorded onto a compact disk that would be used to deliver the signal via a compact disk player (Sony model CFD-S28). The signals were delivered binaurally via headphones (Optimus model PRO-50MX) at the participants' most comfortable level. All participants spoke into a lapel microphone (Radio Shack model 33±3003) af®xed no more than 15 cm from their mouths with an approximate orientation of 08 azimuth and -1208 altitude. The microphone output was fed into a video camera (Sony model CCD-TVR 75).
V.N. Dayalu et al. / Neuroscience Letters 306 (2001) 111±115
Stuttering was de®ned as part-word repetitions, prolongations, and postural ®xations. Frequency counts of stuttered syllables were made for each participant under each condition. The ®rst 300 syllables of data were used in the analysis for each condition. The data was analyzed using a one-factor repeated measure analysis of variance. The means and standard deviations for the frequency of stuttered syllables for each condition are as follows: (a) Control condition (Mean 47.5, SE 9.94); (b) participant producing /a/ for 4 s, waiting for 4 s, and then initiating speech, (Mean 41.5, SE 11.61); (c) participant producing /a/ for 4 s, stopping phonation, and initiating speech (Mean 32.1, SE 8.97); (d) participant listening to /a/ for 4 s, waiting for 4 s, and initiating speech, (Mean 49.6, SE 11.02); and (e) participant listening to /a/ for 4 s, then immediately initiating speech (Mean 46.3, SE 10.46). The stuttering frequency on the control condition was compared to that of the four experimental conditions. A one factor repeated measure analysis of variance revealed a signi®cant main effect of the experimental conditions on stuttering frequency (F 4;28 6:384, P 0:001, h2 0:477). A post-hoc single-d.f. comparison revealed a signi®cant reduction in stuttering frequency for the produced /a/ without a 4 s silence before speech initiation, when compared to the control condition (P 0:02). The other three experimental conditions did not reveal a signi®cant difference in stuttering frequency when compared to the control condition. Signi®cant differences were also obtained when the stuttering frequency of the condition in which the /a/ was produced without a 4 s silence was compared with the two conditions in which the subject listened to an /a/ (P , 0:05). An inhibitory effect on the core stuttering markers occurs when a steady state vowel /a/ is produced immediately prior to the initiation of the speech act. This inhibition is approximately 30% effective in reducing the overt manifestation of stuttering symptoms in the ensuing utterance. Thus, when a person who stutters produces a vowel for 4 s before beginning to speak, there is a signi®cant decrease in frequency of overt stuttering behaviors in the ensuing speech. However, when a 4 s silence was inserted between the production of the /a/, and the ensuing speech act, no signi®cant decrease in the overt stuttering behaviors was observed. Further, neither of the conditions in which the participant listened to an /a/ prior to speech showed signi®cant reductions in the overt manifestations of stuttering. These data are by far, the most compelling evidence to support the notion that the production of a steady state vowel prior to speech appears to serve as a temporary buffer or prophylaxis from the involuntary block. This inhibitory effect allows speech to temporarily proceed forward, with diminished opportunity for the block to occur, resulting in a relative decrease in the appearance of the discrete core markers. However, the vowel is produced prior to the speech act and its buffering effect appears to be shortlived. This is evident on comparing the ¯uency enhance-
113
ment effects of the two /a/ production conditions (with and without the silence), before the ensuing speech act (conditions b and c). 4 s is suf®cient latency for the steady state vowel to lose its ¯uency enhancing effects. The results of the listening conditions (conditions d and e) in this study contrast with the results obtained in a previous study [11], whereby /a/ was provided as a temporally congruent second speech signal (either continuously or intermittently). In this previous study [11], the reduction in the overt manifestations of stuttering was considerably greater in comparison to the control condition (approximately 80%). It is proposed that this contrast was observed because the /a/ provided in this study was not provided simultaneously with the speech act. The lack of effect when listening to an /a/ prior to speech conditions is not surprising, considering that people who stutter appear to receive no ameliorative effect from conversational partners, during conversational speech. What then, is occurring in the speech system when a person who stutters produces a steady state /a/ vowel before speaking the intended utterance, and how is this similar in nature to producing the overt manifestations of stuttering? A person's own speech, provides feedback information that modulates auditory input. It also supplies information on the exact timing and pattern of motor activity that helps sensory motor integration [15]. Thus, we may state that sensory motor integration provides the link between the perception and production of speech and helps in the `feed-forward' mechanism required for speech production. It is proposed that, in people who stutter, sensory motor integration may be susceptible to breakdown, possibly due to interhemispheric imbalance [17], compromised cortico± cortical [6], or cortico±subcortical [22] connections. Thus, appropriate feedback from one's own voice is not fed forward due to the compromise in sensory motor integration. Therefore, a mismatch in this domain may contribute to a breakdown of forward ¯owing speech, and hence result in an involuntary block. It is postulated that, in a person who stutters, the involuntary block is primarily overcome by the generation of the overt manifestations of stuttering. The manifestations that are vocalic in nature (syllabic repetitions) appear to act as the best compensatory mechanism at the system's disposal. However, people who stutter are a heterogeneous population and produce different frequencies and proportions of repetitions, prolongations and postural ®xations. As earlier stated, part-word repetitions may be the true core markers of stuttering and hence, the direct overt manifestation of the involuntary block. All other overt symptoms of stuttering (e.g. consonantal repetitions, prolongations, and postural ®xations) may be inef®cient compensatory mechanisms, that are motoric reactions of the individual to producing repetitions, possibly an indicator of the developmental course that the pathology has taken within the individual. When the /a/ is produced prior to initiating speech, its effect is similar to producing vocalic overt manifestations during speech, in that they both impede the involuntary
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block, although temporally at different levels. Part-word repetitions help overcome the block after it has occurred, whereas production of the /a/ prior to speech induces a temporary inhibitory effect, that reduces the possibility of the block occurring. This is supported by the results obtained in previous studies [5,13,14] that were aimed at investigating the role of the normal speaker's own speech in producing forward-¯owing speech. These studies indicated that a person's own speech primes the auditory cortex, temporarily enhancing sensory motor integration. However, as the appropriate feedback is short-lived (as evident when / a/ was produced and a 4-s silence followed), the system returns to its natural compensatory mechanism of generating part-word repetitions and prolongations. An alternative viewpoint may be that the observed effects of producing an /a/ for 4 s prior to speech may be a result of continuous phonation that primes the peripheral speech anatomy by reducing the complexity of the speech task, thereby facilitating the production of forward ¯owing speech. The participants were asked to cease phonation and initiate speech on a new breath cycle in both of the experimental conditions that the participants were required to produce an /a/. By commencing each utterance on a new breath cycle, any peripheral priming effects that may induce motoric facilitation of the initiatory speech act were potentially negated. This suggests that any resultant priming in the speech system while producing the /a/ to help generate forward ¯owing speech occurred at a central rather than a peripheral level. While overcoming a block, the generation of vocalic partword repetitions may not be the only means of producing the necessary feedback, but it appears to be among the most ef®cient forms of compensation available to the system. Similar accounts for the signi®cant reduction of overt symptoms can be provided when second speech signals (Choral speech, Shadow speech, Delayed Auditory Feedback, Frequency Altered feedback, Visual Choral Speech) are used [2±4,10,12]. Second speech signals provide a continuous vocalic feature that results in a more continuous inhibition of the involuntary block. This allows the system to produce forward-¯owing speech without the production of vocalic part-word repetitions that are the natural compensatory mechanisms. Based on the model presented in this study, the focus of treatment for people who stutter, would best be directed at inhibiting the occurrence of the involuntary block, rather than attempting to behaviorally correct the overt manifestations, that are actually distal symptoms of the pathology. It also suggested that the use of vocalic /a/ production prior to speech initiation may be bene®cial when applied to expressive aphasia. Expressive aphasia has been compared to stuttering in terms of its overt symptomatology and the manner in which auditory cueing may provide ameliorative effects [11]. Further investigation in this area appears to be warranted. Wendell Johnson originally stated that stuttering is what the person who stutters does to prevent stuttering [9]. This statement was made in order to de®ne stuttering as a reac-
tion to an anticipatory struggle. The implications of this study, although comparable to Johnsonian theory, are different and may shed more light on the ®eld of stuttering. This study helps to distinguish between the central pathology of stuttering and its external core markers, and suggests that these peripheral markers may actually be produced to overcome a central, involuntary block. [1] Ambrose, N.G. and Yairi, E., Normative dis¯uency data for early childhood stuttering, J. Speech Lang. Hear. Res., 42 (1999) 895±909. [2] Barber, V., Studies in the psychology of stuttering: XV Chorus reading as a distraction in stuttering, J. Speech Disord., 4 (1939) 371±383. [3] Bloodstein, O., A Handbook on Stuttering, 5th Edition, Singular, San Diego, CA, 1995. [4] Cherry, E. and Sayers, B., Experiments upon total inhibition of stammering by external control and some clinical results, J. Psychosom. Res., 1 (1956) 233±246. [5] Curio, G., Neuloh, G., Numminen, J., Jousmaki, V. and Hari, R., Speaking modi®es voice-evoked activity in the human auditory cortex, Hum. Brain Mapping, 9 (2000) 183±191. [6] Fox, P.T., Ingham, R.J., Ingham, J.C., Hirsch, T.B., Downs, J.H., Martin, C., Jerabek, P., Glass, T. and Lancaster, J.L., A PET study of the neural systems of stuttering, Nature, 382 (1996) 158±161. [7] Harrington, J., Stuttering: delayed auditory feedback and linguistic rhythm, J. Speech Hear. Res., 31 (1988) 36±47. [8] Howell, P. and Williams, M., Acoustic analysis and perception of vowels in children's and teenager's stuttered speech, J. Acoust. Soc. Am., 91 (1992) 1697±1706. [9] Johnson, W. and Associates, The Onset of Stuttering: Research Findings and Implications, University of Minnesota Press, Minneapolis, MN, 1959. [10] Kalinowski, J., Armson, J., Roland-Mieszkowski, M., Stuart, A. and Gracco, V.L., Effects of alterations in auditory feedback and speech rate on stuttering frequency, Lang. Speech, 36 (1993) 1±16. [11] Kalinowski, J., Dayalu, V.N., Stuart, A., Rastatter, M.P. and Rami, M.K., Stutter-free and stutter-®lled speech signals and their role in stuttering amelioration for English speaking adults, Neurosci. Lett., 293 (2000) 115±118. [12] Kalinowski, J., Stuart, A., Rastatter, M.P., Snyder, G. and Dayalu, V., Inducement of ¯uent speech in persons who stutter via visual choral speech, Neurosci. Lett., 281 (2000) 198±200. [13] Numminen, J. and Curio, G., Differential effects of overt, covert and replayed speech on vowel-evoked responses of the human auditory cortex, Neurosci. Lett., 272 (1999) 29±32. [14] Numminen, J., Salmelin, R. and Hari, R., Subject's own speech reduces reactivity of the human auditory cortex, Neurosci. Lett., 265 (1999) 119±122. [15] Paus, T., Perry, D., Zatorre, R., Worsely, K. and Evans, A., Modulation of cerebral blood ¯ow in the human auditory cortex during speech: Role of motor-to-sensory discharges, Eur. J. Neurosci., 8 (1996) 2236±2246. [16] Perkins, W.H., Kent, R.D. and Curlee, R.F., A theory of neuropsycholinguistic function in stuttering, J. Speech Hear. Res., 34 (1991) 734±752. [17] Salmelin, R., Schnitzler, A., Schmitz, F., Jancke, L., Witte, O.W. and Freund, H.J., Functional organization of the auditory cortex is different in stutterers and ¯uent speakers, NeuroReport, 9 (1998) 2225±2229. [18] Still, A.W. and Griggs, S., Changes in the probability of
V.N. Dayalu et al. / Neuroscience Letters 306 (2001) 111±115 stuttering following a stutter: a test of some recent models, J. Speech Hear. Res., 22 (1979) 565±571. [19] Van Riper, C., The Nature of Stuttering, 2nd Edition, Prentice- Hall Inc., Englewood Cliffs, NJ, 1982. [20] Wingate, M.E., A standard de®nition of stuttering, J. Speech Hear. Disord., 29 (1964) 484±489. [21] Wingate, M.E., The Structure of Stuttering, Springer, New York, 1988.
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[22] Wu, J.C., Maguire, G., Riley, G., Fallon, J., LaCasse, L., Chin, S., Klein, E., Tang, C., Cadwell, S. and Lottenberg, S., A positron emission tomography [18 F] deoxyglucose study of developmental stuttering, NeuroReport, 6 (1995) 501± 505. [23] Zimmerman, G., Stuttering: a disorder of movement, J. Speech Hear. Res., 23 (1980) 122±136.
www.elsevier.com/locate/neulet
Producing the vowel/a/ prior to speaking inhibits stuttering in adults in the English language Vikram N. Dayalu, Tim Saltuklaroglu, Joseph Kalinowski*, Andrew Stuart, Michael P. Rastatter Stuttering Research Lab, Department of Communication Sciences and Disorders, East Carolina University, USA Received 20 February 2001; received in revised form 16 March 2001; accepted 22 April 2001
Abstract This study investigated the effects of producing and listening to the vowel /a/ on the frequency of overt stuttering moments in eight people who stuttered. Stuttering frequency counts were made for the speech produced in the control condition, and after each of these four experimental conditions: (a) producing a vowel /a/ for 4 s; (b) producing a vowel /a/ for 4 s and waiting for 4 s; (c) listening to a recording of the vowel /a/ for 4 s; and (d) listening to a recording of the vowel / a/ for 4 s and waiting for 4 s. A signi®cant reduction in the stuttering frequency was only observed following production of the vowel /a/ without a 4 s delay (P 0:02), suggesting that the vowel production prior to speech, serves as a temporary ¯uency enhancer. Its similarity to the occurrence of overt stuttering moments (e.g. discrete part-word repetitions and prolongation's) and its relationship to the fundamental nature of the pathology are discussed. q 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Stuttering, Sensory motor integration, Overt manifestations, Second speech signal
Stuttering is de®ned as involuntary part-word repetitions and prolongation's that interrupt the forward ¯ow of speech, impairing communication intent and understanding [3,20]. The causal factor of stuttering is considered to exist at a central level [6], yet the observable behaviors are manifested in the periphery as a speech disruption. If, as de®ned, the disorder is involuntary, then it may be stated that the overt part-word repetitions, prolongation's, and secondary behaviors are peripheral markers of a central pathology, or central involuntary block [11]. Numerous theories have attempted to explain why people who stutter generate the overt manifestations of stuttering that impede the forward ¯ow of speech. Possible explanations are derived from a speech motor control approach [19,23], a psycholinguistic approach [21], a neurolinguistic approach [16], and a disordered feedback approach [7]. The underlying idea common to all these theories is that the core behaviors of stuttering are the problematic markers of a proposed causal factor, depending on the perspective taken. The aim of the present study is to investigate a possible use for the overt markers of * Corresponding author. CSDI, School of Allied Health, Oglesby Drive, Greenville, NC-27858, USA. Tel.: 11-252-3281986; fax: 11-252-328-4469. E-mail address: [email protected] (J. Kalinowski).
stuttering. It is proposed, that the production of core stuttering behaviors represents an attempt by the person who stutters to overcome the involuntary block, and regain forward ¯owing speech. This study aims to provide empirical support for this hypothesis. Irrespective of stuttering severity, part-word repetitions appear to be the most prevalent form of core stuttering behaviors in children who stutter [1]. The presence of part-word repetitions is also a fundamental core marker in adults who stutter and has been widely used as a diagnostic criterion in the assessment of stuttering [3,19]. The possibility exists that the signi®cant proportion of part-word repetitions in the overt manifestations of stuttering may be indicative of a relationship with the underlying pathology. A means of examining this possible relationship may be found in looking at the general nature of part-word repetitions. Part-word repetitions are syllabic, with a commonly observed signature event being the presence of a vowel, that is often neutralized to become a schwa [3,8,19]. For example, a person who stutters may produce `tuh-tuh-tuh table', producing a schwa (unstressed) vowel three times prior to saying the word. The acoustic features of vocalic sounds, as compared to non-vocalic sounds, are low frequency, periodicity and high intensity. As stated above, syllabic repeti-
0304-3940/01/$ - see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 0 1) 01 86 9- 9
112
V.N. Dayalu et al. / Neuroscience Letters 306 (2001) 111±115
tions are voiced in nature and this vocalic or voiced characteristic may be representative of a symbiotic relationship between the part-word repetitions and the pathology. A recent study stated that one of the common elements in a second speech signal that enhances ¯uency is its voiced component [11]. Further support for the role of the voiced signal is found in ¯uency enhancement brought about by pseudostuttering (i.e. voluntary stuttering or stuttering on purpose) [3,19]. Pseudostuttering is generally considered to bring about increased levels of ¯uency in the ensuing speech, but pseudostuttering characterized by the use of repetitions is considered to be more effective in producing ¯uency enhancement than pseudostuttering characterized by prolongation's [3,9,19]. In addition, the occurrence of an overt stuttering moment, or a concentrated cluster of stuttering moments, may induce short periods of ¯uency in the ensuing speech [18]. The implications from the above reports converge on the signi®cance of part-word repetitions and their voiced feature in the stuttering pathology. They may present a new avenue towards understanding the symbiotic relationship between the pathology and its core markers. If a voiced signal were produced before the occurrence of speech, it would be interesting to observe its effect on the resultant speech output. A reduction in the overt manifestations of stuttering in the ensuing speech would be indicative of an inhibitory effect derived from the voiced signal. As vowels are by nature voiced and continuous speech sounds, a decrease in stuttering frequency following production of a sustained vowel would support the hypothesis that partword repetitions, which are also voiced, are generated in an attempt to overcome the involuntary block and regain forward ¯owing speech. This study aims to investigate this hypothesis by proposing a simple test. A non-stuttered, sustained vowel /a/ will be produced by the person who stutters, prior to the production of an utterance to test its block inhibiting effects on the ensuing speech. The carryover effect of this signal may also be evaluated by inserting a silent period between the sustained /a/ and the speech act. This could shed light on the temporal nature of this proposed release mechanism. It would also be interesting to note the effect of listening to the vowel prior to speech initiation, as second speech signals have been known to produce signi®cant ¯uency enhancement when provided concomitantly with the speech act [11]. Eight normal-hearing people who stutter (seven males, one female, Mean 19.2 years, SD 4.8) participated in this study. Participants did not present with any other speech or language disorders. All participants had a history of therapy, but were currently not receiving any formal therapeutic intervention. Informed consent (approved by the University and Medical Center Institutional Review Board, East Carolina University) was obtained from all the participants. For each condition (one control and four experimental), the participants read a different series of six to nine syllable sentences. The order of presentation of the conditions was
randomized between subjects. The sentences were selected from different junior-high level passages, with similar theme and syntactic complexity. Based on informal clinical measures (stuttering frequency in the control condition), severity of stuttering for the participants in this study ranged from moderate to severe. They were instructed throughout the experiment to read using a normal speaking voice at their normal rate and loudness, and not to use any controls to reduce or inhibit stuttering. Prior to data collection, the participants were trained to produce an /a/ for 4 s using their normal voice and loudness, cease phonation, and then commence speech on a new breath cycle. They were also trained to produce an /a/ for 4 s using their normal voice and loudness, cease phonation, wait for 4 s (during which the participants were instructed to breathe normally and refrain from holding their breath), and then commence speech on a new breath cycle. The participants were also instructed to not use exaggerated breathing patterns at any point during the experimental conditions. The conditions were as follows: (a) the participants read the sentence without any modi®cation (control); (b) the participants produced /a/ for 4 s, waited for 4 s, and then initiated speech; (c) the participants produced /a/ for 4 s, stopped phonating, and initiated speech; (d) the participants listened to /a/ for 4 s, waited for 4 s, and initiated speech; and (e) the participants listened to /a/ for 4 s, then immediately initiated speech. As the listening conditions required no phonation prior to speech, the participants initiated speech on a normal speech breathing cycle, similar to the production tasks. The experimenter used a stopwatch to time 4 s of /a/ in conditions b and c, and used a hand signal to indicated to the participant when to cease /a/ production and initiate speech. For the two conditions involving listening to the /a/, participants listened to the auditory speech signals via supra-aural earphones (headphones that completely cover the external ear) at a most comfortable listening level and were provided with a 150 ms, 4 kHz pure tone cue to initiate speech. Recording of the 4-s /a/ for conditions d and e was made in a sound-treated room with a digital tape-recorder (Sony model 8819). A normal ¯uent American English-speaking adult male produced the vowel. The samples were produced at a normal vocal effort. The recorded signals were then fed into a personal computer (Apple Power Macintosh 9600/ 300) via an Apple sound input port. Sampling was at 44 kHz. Sound analysis software (Sound Edit version 2) was used to introduce the silence. These were then recorded onto a compact disk that would be used to deliver the signal via a compact disk player (Sony model CFD-S28). The signals were delivered binaurally via headphones (Optimus model PRO-50MX) at the participants' most comfortable level. All participants spoke into a lapel microphone (Radio Shack model 33±3003) af®xed no more than 15 cm from their mouths with an approximate orientation of 08 azimuth and -1208 altitude. The microphone output was fed into a video camera (Sony model CCD-TVR 75).
V.N. Dayalu et al. / Neuroscience Letters 306 (2001) 111±115
Stuttering was de®ned as part-word repetitions, prolongations, and postural ®xations. Frequency counts of stuttered syllables were made for each participant under each condition. The ®rst 300 syllables of data were used in the analysis for each condition. The data was analyzed using a one-factor repeated measure analysis of variance. The means and standard deviations for the frequency of stuttered syllables for each condition are as follows: (a) Control condition (Mean 47.5, SE 9.94); (b) participant producing /a/ for 4 s, waiting for 4 s, and then initiating speech, (Mean 41.5, SE 11.61); (c) participant producing /a/ for 4 s, stopping phonation, and initiating speech (Mean 32.1, SE 8.97); (d) participant listening to /a/ for 4 s, waiting for 4 s, and initiating speech, (Mean 49.6, SE 11.02); and (e) participant listening to /a/ for 4 s, then immediately initiating speech (Mean 46.3, SE 10.46). The stuttering frequency on the control condition was compared to that of the four experimental conditions. A one factor repeated measure analysis of variance revealed a signi®cant main effect of the experimental conditions on stuttering frequency (F 4;28 6:384, P 0:001, h2 0:477). A post-hoc single-d.f. comparison revealed a signi®cant reduction in stuttering frequency for the produced /a/ without a 4 s silence before speech initiation, when compared to the control condition (P 0:02). The other three experimental conditions did not reveal a signi®cant difference in stuttering frequency when compared to the control condition. Signi®cant differences were also obtained when the stuttering frequency of the condition in which the /a/ was produced without a 4 s silence was compared with the two conditions in which the subject listened to an /a/ (P , 0:05). An inhibitory effect on the core stuttering markers occurs when a steady state vowel /a/ is produced immediately prior to the initiation of the speech act. This inhibition is approximately 30% effective in reducing the overt manifestation of stuttering symptoms in the ensuing utterance. Thus, when a person who stutters produces a vowel for 4 s before beginning to speak, there is a signi®cant decrease in frequency of overt stuttering behaviors in the ensuing speech. However, when a 4 s silence was inserted between the production of the /a/, and the ensuing speech act, no signi®cant decrease in the overt stuttering behaviors was observed. Further, neither of the conditions in which the participant listened to an /a/ prior to speech showed signi®cant reductions in the overt manifestations of stuttering. These data are by far, the most compelling evidence to support the notion that the production of a steady state vowel prior to speech appears to serve as a temporary buffer or prophylaxis from the involuntary block. This inhibitory effect allows speech to temporarily proceed forward, with diminished opportunity for the block to occur, resulting in a relative decrease in the appearance of the discrete core markers. However, the vowel is produced prior to the speech act and its buffering effect appears to be shortlived. This is evident on comparing the ¯uency enhance-
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ment effects of the two /a/ production conditions (with and without the silence), before the ensuing speech act (conditions b and c). 4 s is suf®cient latency for the steady state vowel to lose its ¯uency enhancing effects. The results of the listening conditions (conditions d and e) in this study contrast with the results obtained in a previous study [11], whereby /a/ was provided as a temporally congruent second speech signal (either continuously or intermittently). In this previous study [11], the reduction in the overt manifestations of stuttering was considerably greater in comparison to the control condition (approximately 80%). It is proposed that this contrast was observed because the /a/ provided in this study was not provided simultaneously with the speech act. The lack of effect when listening to an /a/ prior to speech conditions is not surprising, considering that people who stutter appear to receive no ameliorative effect from conversational partners, during conversational speech. What then, is occurring in the speech system when a person who stutters produces a steady state /a/ vowel before speaking the intended utterance, and how is this similar in nature to producing the overt manifestations of stuttering? A person's own speech, provides feedback information that modulates auditory input. It also supplies information on the exact timing and pattern of motor activity that helps sensory motor integration [15]. Thus, we may state that sensory motor integration provides the link between the perception and production of speech and helps in the `feed-forward' mechanism required for speech production. It is proposed that, in people who stutter, sensory motor integration may be susceptible to breakdown, possibly due to interhemispheric imbalance [17], compromised cortico± cortical [6], or cortico±subcortical [22] connections. Thus, appropriate feedback from one's own voice is not fed forward due to the compromise in sensory motor integration. Therefore, a mismatch in this domain may contribute to a breakdown of forward ¯owing speech, and hence result in an involuntary block. It is postulated that, in a person who stutters, the involuntary block is primarily overcome by the generation of the overt manifestations of stuttering. The manifestations that are vocalic in nature (syllabic repetitions) appear to act as the best compensatory mechanism at the system's disposal. However, people who stutter are a heterogeneous population and produce different frequencies and proportions of repetitions, prolongations and postural ®xations. As earlier stated, part-word repetitions may be the true core markers of stuttering and hence, the direct overt manifestation of the involuntary block. All other overt symptoms of stuttering (e.g. consonantal repetitions, prolongations, and postural ®xations) may be inef®cient compensatory mechanisms, that are motoric reactions of the individual to producing repetitions, possibly an indicator of the developmental course that the pathology has taken within the individual. When the /a/ is produced prior to initiating speech, its effect is similar to producing vocalic overt manifestations during speech, in that they both impede the involuntary
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block, although temporally at different levels. Part-word repetitions help overcome the block after it has occurred, whereas production of the /a/ prior to speech induces a temporary inhibitory effect, that reduces the possibility of the block occurring. This is supported by the results obtained in previous studies [5,13,14] that were aimed at investigating the role of the normal speaker's own speech in producing forward-¯owing speech. These studies indicated that a person's own speech primes the auditory cortex, temporarily enhancing sensory motor integration. However, as the appropriate feedback is short-lived (as evident when / a/ was produced and a 4-s silence followed), the system returns to its natural compensatory mechanism of generating part-word repetitions and prolongations. An alternative viewpoint may be that the observed effects of producing an /a/ for 4 s prior to speech may be a result of continuous phonation that primes the peripheral speech anatomy by reducing the complexity of the speech task, thereby facilitating the production of forward ¯owing speech. The participants were asked to cease phonation and initiate speech on a new breath cycle in both of the experimental conditions that the participants were required to produce an /a/. By commencing each utterance on a new breath cycle, any peripheral priming effects that may induce motoric facilitation of the initiatory speech act were potentially negated. This suggests that any resultant priming in the speech system while producing the /a/ to help generate forward ¯owing speech occurred at a central rather than a peripheral level. While overcoming a block, the generation of vocalic partword repetitions may not be the only means of producing the necessary feedback, but it appears to be among the most ef®cient forms of compensation available to the system. Similar accounts for the signi®cant reduction of overt symptoms can be provided when second speech signals (Choral speech, Shadow speech, Delayed Auditory Feedback, Frequency Altered feedback, Visual Choral Speech) are used [2±4,10,12]. Second speech signals provide a continuous vocalic feature that results in a more continuous inhibition of the involuntary block. This allows the system to produce forward-¯owing speech without the production of vocalic part-word repetitions that are the natural compensatory mechanisms. Based on the model presented in this study, the focus of treatment for people who stutter, would best be directed at inhibiting the occurrence of the involuntary block, rather than attempting to behaviorally correct the overt manifestations, that are actually distal symptoms of the pathology. It also suggested that the use of vocalic /a/ production prior to speech initiation may be bene®cial when applied to expressive aphasia. Expressive aphasia has been compared to stuttering in terms of its overt symptomatology and the manner in which auditory cueing may provide ameliorative effects [11]. Further investigation in this area appears to be warranted. Wendell Johnson originally stated that stuttering is what the person who stutters does to prevent stuttering [9]. This statement was made in order to de®ne stuttering as a reac-
tion to an anticipatory struggle. The implications of this study, although comparable to Johnsonian theory, are different and may shed more light on the ®eld of stuttering. This study helps to distinguish between the central pathology of stuttering and its external core markers, and suggests that these peripheral markers may actually be produced to overcome a central, involuntary block. [1] Ambrose, N.G. and Yairi, E., Normative dis¯uency data for early childhood stuttering, J. Speech Lang. Hear. Res., 42 (1999) 895±909. [2] Barber, V., Studies in the psychology of stuttering: XV Chorus reading as a distraction in stuttering, J. Speech Disord., 4 (1939) 371±383. [3] Bloodstein, O., A Handbook on Stuttering, 5th Edition, Singular, San Diego, CA, 1995. [4] Cherry, E. and Sayers, B., Experiments upon total inhibition of stammering by external control and some clinical results, J. Psychosom. Res., 1 (1956) 233±246. [5] Curio, G., Neuloh, G., Numminen, J., Jousmaki, V. and Hari, R., Speaking modi®es voice-evoked activity in the human auditory cortex, Hum. Brain Mapping, 9 (2000) 183±191. [6] Fox, P.T., Ingham, R.J., Ingham, J.C., Hirsch, T.B., Downs, J.H., Martin, C., Jerabek, P., Glass, T. and Lancaster, J.L., A PET study of the neural systems of stuttering, Nature, 382 (1996) 158±161. [7] Harrington, J., Stuttering: delayed auditory feedback and linguistic rhythm, J. Speech Hear. Res., 31 (1988) 36±47. [8] Howell, P. and Williams, M., Acoustic analysis and perception of vowels in children's and teenager's stuttered speech, J. Acoust. Soc. Am., 91 (1992) 1697±1706. [9] Johnson, W. and Associates, The Onset of Stuttering: Research Findings and Implications, University of Minnesota Press, Minneapolis, MN, 1959. [10] Kalinowski, J., Armson, J., Roland-Mieszkowski, M., Stuart, A. and Gracco, V.L., Effects of alterations in auditory feedback and speech rate on stuttering frequency, Lang. Speech, 36 (1993) 1±16. [11] Kalinowski, J., Dayalu, V.N., Stuart, A., Rastatter, M.P. and Rami, M.K., Stutter-free and stutter-®lled speech signals and their role in stuttering amelioration for English speaking adults, Neurosci. Lett., 293 (2000) 115±118. [12] Kalinowski, J., Stuart, A., Rastatter, M.P., Snyder, G. and Dayalu, V., Inducement of ¯uent speech in persons who stutter via visual choral speech, Neurosci. Lett., 281 (2000) 198±200. [13] Numminen, J. and Curio, G., Differential effects of overt, covert and replayed speech on vowel-evoked responses of the human auditory cortex, Neurosci. Lett., 272 (1999) 29±32. [14] Numminen, J., Salmelin, R. and Hari, R., Subject's own speech reduces reactivity of the human auditory cortex, Neurosci. Lett., 265 (1999) 119±122. [15] Paus, T., Perry, D., Zatorre, R., Worsely, K. and Evans, A., Modulation of cerebral blood ¯ow in the human auditory cortex during speech: Role of motor-to-sensory discharges, Eur. J. Neurosci., 8 (1996) 2236±2246. [16] Perkins, W.H., Kent, R.D. and Curlee, R.F., A theory of neuropsycholinguistic function in stuttering, J. Speech Hear. Res., 34 (1991) 734±752. [17] Salmelin, R., Schnitzler, A., Schmitz, F., Jancke, L., Witte, O.W. and Freund, H.J., Functional organization of the auditory cortex is different in stutterers and ¯uent speakers, NeuroReport, 9 (1998) 2225±2229. [18] Still, A.W. and Griggs, S., Changes in the probability of
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[22] Wu, J.C., Maguire, G., Riley, G., Fallon, J., LaCasse, L., Chin, S., Klein, E., Tang, C., Cadwell, S. and Lottenberg, S., A positron emission tomography [18 F] deoxyglucose study of developmental stuttering, NeuroReport, 6 (1995) 501± 505. [23] Zimmerman, G., Stuttering: a disorder of movement, J. Speech Hear. Res., 23 (1980) 122±136.