Prosody and motor speech disorders: A retrospective review of a merger that is imminent

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Review

Prosody and motor speech disorders: A retrospective review of a merger that is imminent Frank Boutsen a,*, Justin Dvorak b a

Department of Communication Sciences and Disorders, College of Allied Health, University of Oklahoma Health Sciences Center, United States b Department of Biostatistics and Epidemiology, College of Public Health, University of Oklahoma Health Sciences Center, United States Received 13 June 2017; accepted 24 July 2017

[6_TD$IF]Abstract Even though prosody figures prominently in the classification and differential diagnosis of motor speech disorders, the theoretical frame works in which prosody and motor speech disorders have evolved have limited a full integration or merging of these domains. The Mayo Classification continues to be the gold standard and differentiates the motors speech disorders in terms of audible speech deviations including prosody. While briefly challenged by a prosodic classification of the motor speech disorders (Kent and Rosenbek, 1982), it was not until theoretical advances in normal prosody culminated in an objective measurement approach that shows promise to complement the audio-perceptual approach and map the prosodic landscape in motor speech disorders. Published by Elsevier B.V.

Keywords: Prosody; Motor speech disorders; Rhythm; Intonation

In an era when the terms speech and language were used interchangeably and Broca and Wernicke's constructs of ‘‘aphemia’’ and ‘‘aphasia symptom complex’’ seemingly found harmonious reconciliation in localizationist theories of language, Kusmaul (1877) broke with tradition as he argued the position that speech was not confined to a cerebral convolution. He also drew a clear distinction between the neurological disorders of speech and those of language. In what is conceivably the first classification of neurogenic communication disorders, he defined as separate from aphasia a group of articulation disorders that were due to organic or psychic disturbances of the central nervous system (CNS). He can be credited with having coined the term ‘‘dysarthria’’ to which he designated articulation disorders to be distinguished from the dyslalias that resulted from peripheral lesions and/or malformations of the articulators or the cranial nerves (Grewel, 1957). Kussmaul's classification, albeit provocative for its time, did not do much more than delimit the concept of dysarthria, confining it to the CNS apart from language and functional/organic speech disorders. Around the turn of the century localizations within the CNS that, if lesioned, were consequential for the (in) ability to articulate were proposed as was the notion that a graded recovery was associated with different clinical speech defects. For example Marie (1906) claimed that the anatomical substrate for ‘‘pure anarthria’’ lay in a ‘lenticular zone’ of frontal cortex and sub-cortex between the head of the caudate and the posterior end of the lenticular nucleus. Marie and Foix (1917) later made distinctions based on the degree of recovery of the speech impairment. They used the term ‘anarthric

* Corresponding author at: 1200 N. Stonewall, AHB 3079, Oklahoma City, OK 73117, United States. E-mail address: [email protected] (F. Boutsen). http://dx.doi.org/10.1016/j.lingua.2017.07.009 0024-3841/Published by Elsevier B.V.

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syndromes’ for recovered cases with speech impediments that in most severe examples presented with initial complete inability to utter words. They used the label ‘‘dysarthria’’ for instances where upon recovery words were mispronounced or phrases could not skillfully be produced. It is clear that this distinction foreshadows the modern classification of motor speech disorders as consisting of ‘‘dysarthria(s) and apraxia of speech’’. In the next two decades or so more refined neurologic classification schemes began to assert a coupling between the still fairly amorphous dysarthria symptom complex and etiologies that were bound within levels/components of the central nervous system. For example, Zentay (1937), Froeshels (1943) and Luchsinger and Arnold (1949) classified dysarthria as emanating from four levels within the central nervous system (cortico-bulbar, cortico-strio-pallido-rubro-bulbar, frontopontine, and cerebellar levels). Growing consideration of movement disorders as well speech processes other than ‘‘articulation’’, along with a broadening array of ‘‘dysarthric’’ symptoms needing theoretical cover, soon refined and even stretched the 4-level classification schemes to also include the peripheral nervous system (PNS). Interestingly, two major paradigmatic shifts occurred in the fifties and sixties. One was introduced by Monrad Krohn (1948) when he initiated the clinical study of prosody after caring for a native Norwegian woman during WWII who sustained shrapnel injury to the left frontal area causing a Broca's aphasia (Ross et al., 2013). Although the woman made an excellent recovery, her speech no longer sounded like that of a native Norwegian. While her speech had preserved melody, as evidenced by her ability to sing, intone and emote, she had inappropriate application of stresses and pauses, giving her speech a foreign accent quality (Ross et al., 2013). Based on this patient and others, Monrad-Krohn (1963) divided ‘‘prosody’’ into four major components: intrinsic, intellectual, emotional and inarticulate prosody. Intrinsic prosody enhances and clarifies the linguistic aspects of a language through judicious use of pitch, intonation, duration, pauses and stress. Examples include the pitch declination of the rise of pitch near the end of statements and questions. Prosodic stress along with pausing helps out with potentially ambiguous syntax by defining phrasal boundaries. Intellectual prosody signals the attitude of the speaker vis-a-vis the content that is imparted. For example, the speaker thus may convey a sentiment that is opposite to what is being stated as in ‘‘the lecture was really interesting’’ (boring). Emotional prosody injects primary types of emotions into speech, such as happiness, sadness, fear and anger. Inarticulate prosody is the use of certain paralinguistic nonverbal elements, such as grunts and sighs, to embellish discourse. Foundational for another major paradigm shift were two publications, one by Grewel (1957) and the other by Peacher (1950). Peacher, who was influenced by Monrad-Krohn's work, added rhythm (if not prosody) to the speech processes of articulation, phonation, resonation and respiration in a manuscript entitled ‘‘The etiology and differential classification of dysarthria’’. The title of this publication is telling as it introduced the (modern) era of the classified dysarthrias (plural). Peacher for the first time highlighted the role of speech processes in dysarthria and gave rhythm an overarching role in it. He argued the importance of rhythm in light of research showing its peripheral dependence on respiration, its dependence on auditory feed back (as demonstrated by failure of rhythm to develop in the deaf) but also its relation to ontogenetic factors such as language development and on an even higher level, emotional and intellectual factors. Grewel can be credited with defining praxis and execution in speech along with tying movement disorders to the dysarthrias. Execution was described as the release of a series of reflexes, was thought to be dependent on intact sensibility and required coordination of the successive stages of the speaking procedure (respiration, phonation, articulation). Along these lines, he defined, apraxia of speech as a subtype, albeit at the margins, of dysarthria. Peacher and Grewel both claimed that a neurological perspective on dysarthria lacked diagnostic power and therapeutic relevance if it was not also complemented by detailed speech analysis. In this regard, they proposed some procedural guidelines for the diagnosis of dysarthria and provided an initial broad description of the speech symptoms in the various dysarthria types. Even so, realizing that descriptions of dysarthric speech were subjective and at best impressionistic, Peacher (1950) called for a resurveying of the entire field using principles of experimental phonetics and speech pathology. Though not yet employing an experimental method, Darley et al. (1969a,b) attempted to do just that in the first systematic and comprehensive study of the audible characteristics of the dysarthrias. The raw data in this investigation comprised reading samples of 212 patients with dysarthria whose neurological diagnosis fell in one of seven categories: pseudobulbar palsy, bulbar palsy, amyotrophic lateral sclerosis, cerebellar lesions, parkinsonism, dystonia, and choreoathetosis. These neurological disorders were a likely choice, as they had been the subject of earlier dysarthria investigations in some form or another. Listening to the tapes, Darley and his colleagues tried to capture the distinct phenomenology of the dysarthrias, not within neuroanatomic levels as had been done before, but within the audible domain. Toward this goal, they conceptualized a series of speech and voice dimensions along which to rate the speech samples. Ultimately, Darley Aronson and Brown settled for 38 dimensions along seven major categories. They included pitch (level, breaks, tremor, or mono), loudness (level, decay, alternating, excessively varied, or mono), vocal quality (harsh/wet hoarseness, continuous/transient breathiness, strained voice, hypo/hypernasality, and nasal emission) audible aspects of respiration (forced inhalation or exhalation, audible inspiration, grunt at end of expiration), prosody (rate, short phrases, increased overall/segmental rate, variable rate, reduced/excessive stress, silences, short rushes of speech), and articulation (imprecise consonants, vowel distortion, prolonged/repeated phonemes, irregular articulatory Please cite this article in press as: Boutsen, F., Dvorak, J., Prosody and motor speech disorders: A retrospective review of a merger that is imminent. Lingua (2017), http://dx.doi.org/10.1016/j.lingua.2017.07.009

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Table 1 Clusters of dysarthric speech dimensions and proposed associated neuromuscular deficits (Darley et al., 1969b). Cluster

Deficit

Articulatory inacccuracy Prosodic excess Prosodic insufficiency Articulatory/resonatory incompetence Phonatory stenosis Phonatory/resonatory incompetence Phonatory--prosodic insufficiency

Inaccurate direction of movement No direct physiological implication Restricted range of movement Impaired force of muscle contraction and reduced range of movement Biased hypertonus Reduction in contraction force Hypotonia

breakdown). In addition, speech samples were rated along two overall dimensions: intelligibility and bizarreness. Overall, it can be seen that apart from the general impression dimensions, the perceptual categories rated by Darley et al. tapped dysfunction(s) along more or less the same major components/processes of speech, as were outlined within the neurological perspective on dysarthria. The results of Darley et al.’s (1969a) investigation revealed imprecise consonants, pitch level, mono-pitch, monoloudness, rate, and hypernasality to be prominent across the groups. The results further showed that for a given neurological disease, not only did a single dimension turn out to be unique, also the cooccurrence of several dimensions was distinct. The five basic ‘perceptual’ patterns that thus were found were given physiological labels and termed flaccid, spastic, ataxic, hypokinetic, and hyperkinetic dysarthria. This labeling, reminiscent of that fashioned by the neurological perspective on dysarthria, found further elaboration and justification in Darley et al. (1969b). In Darley et al. (1969b), they mapped the concurrent dimensions (clusters) on neuromuscular deficits. They are summarized in Table 1. As can be seen in Table 1, Darley et al. (1969b) asserted a movement deficit as the basis for each cluster except for prosodic excess. Later, in their now classic monograph, Darley et al. (1975) went on to define the dysarthrias along with apraxia of speech within the concept of motor speech disorders. In contrast with Peacher and Grewel, they drew a clear separation between apraxia of speech and dysarthria. Apraxia of speech in their view involved a deficit in one of three hierarchically organized conceptual motor stages (processes), including purpose formulation, movement planning and programming that were separate, physiologically and anatomically, from the processes that involve movement execution (and control) and considered within the response province of dysarthria. The former processes were, except for the global cortical process of purpose formulation, considered left lateralized and had no direct neuromuscular implication or a prosodic consequence. Dysarthric processes, on the other hand, involved motor execution to the exclusion of sensory processes and were relegated to upper motor neurons (bilaterally), as well as extrapyramidal, cerebellar, and lower motor neurons.

1. Pivotal views on prosody in the 1950s and 60s While in the early thirties ‘‘normal’’ prosody was generally viewed as involving secondary modifications of segmental features (Bloomfield, 1935), early forays in the realm of prosody by Jakobson (1931) and Trubetzkoy (1939) were debating the phonological status of prosody and its dimensions of length, pitch and accent (rhythm) and its place along the paradigmatic--syntagmatic axes in particular. The discussion surrounding length in this regard resulted in what has come to be known as the ‘‘quantity’’ problem (Fox, 2002). Trubetzkoy (1939) observed that all phonemes occupy time if they are to exist. Hence the possession of a basic unit of time by a phoneme could in his view in itself not be phonologically relevant and thus rendered time an irrelevant phonological property that cannot be used. Clearly this conclusion was problematic and the phonological versus prosodic status of length has never fully been resolved. It is in the fifties that Jakobson et al. (1951) fully distinguish the prosodic features of pitch, stress and duration from the distinctive features ‘‘inherent’’ (italics mine) in the segmental consonant and vowel sounds. They list three types of prosodic features: force, quantity, and tone, corresponding to the sensory attributes of voice-loudness, relative subjective duration, and voice-pitch. The closest physical correlates are, in the same order, intensity, time pattern, and vocal frequency (Kent and Rosenbek, [7_TD$IF]1982). For Jakobson et al. (1951) prosodic features are syntagmatic, i.e., encoding supra-segmental changes over position (or over time) within the utterance. Trubetzkoy (1939) introduced the view of prosody as the organizational structure of speech. Experimental phonetics also finds its prosodic footing in this era. Fry (1955) and Bolinger (1958) include fundamental frequency [F0] as an important correlate of word stress and phrasal prominence. Bolinger (1958) argues from artificial and natural speech data that the primary cue of what is termed stress is really pitch prominence and that intensity is negligible as a qualitative and a determinative factor in stress. Bolinger (1958) believes that it is better to speak of pitch accent and to Please cite this article in press as: Boutsen, F., Dvorak, J., Prosody and motor speech disorders: A retrospective review of a merger that is imminent. Lingua (2017), http://dx.doi.org/10.1016/j.lingua.2017.07.009

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leave the term stress to the domain of word stress. In the latter domain, he states that one possible kind of phonemic stress is potential for pitch accent. He further posits that pitch accents are not just one directional but rather but rather may be going up, down or not change direction and in doing so give accent to different words in the utterance. He gives the following examples to make this point. Perceived Stress 100 Wouldn’t it be easier to 120wait? Pitch movement (PM) up until to wait 100 Woudn’t it be 120 easier to wait? PM flat until easier easier 100 Woudn’t it be easier 120 to 100wait? PM down until wait wait Ladefoged and Mckinney (1963), Ladefoged (1967) and Lieberman (1967) can be credited with adding a physiologic perspective to the analysis of pitch across the utterance. Lieberman's work was foundational to the concept of breath groups (Lieberman, 1967). Ladefoged studied the contributions of subglottal pressure and laryngeal muscular tension to pitch. He investigated this in statements and questions in utterances that contained words that were contrastive in terms of linguistic stress. He used the phrases such as: /That's a digest/, /That's a digest? / and /He didn’t digest/, /He didn’t digest? /. (Underline denotes stress) Evidence showed that in statements sub-glottal pressure well predicts the pitch rise, plateau and fall, however, in questions there is a pitch rise while subglottal pressure goes down. The F0 rise near the end in questions therefore could not be attributed to subglottal pressure. He concluded that the F0 rise results from increased muscle tension produced by intrinsic laryngeal muscle activity. As for the linguistic stress, he noted increased pitch as an important cue, but also points to the importance of duration and intensity as others had done before him. Fry (1955) in an earlier investigation concluded that both duration and intensity were correlates of linguistic stress but that duration was the more important one. It is fair to say that by the end of the sixties there is a merging of acoustics with physiology that converges on the breath group, admittedly not without considerable debate (see Vanderslice, 1969). At this time, similar controversy also surrounds the concept of rhythm, its linguistic origins and whether it exists in speech, in speech perception or in both. This discussion spills over in a larger debate as to whether languages can be classified in terms of rhythm or prosody. For example, Abercrombie (1967) follows Pike (1945) when describing speech rhythm in terms of the intervals between the onsets of linguistic units, i.e., syllables, moras, or feet (Tilsen and Johnson, 2008). However he also elaborates the notion of rhythm as isochrony when he recognized two rhythmic classes, stress-timing and syllable-timing, and proposed that all languages belong to one or the other class. According to Abercrombie some linguistic constituent in the speech signal is planned to occur with temporal periodicity. Failure to observe or perceive this periodicity is attributed to ‘‘noise’’ factors one of which interestingly enough is the act of speech (Abercrombie, 1967). 2. Motor speech disorders post Darley-to the present: though briefly challenged, the Mayo Classification and the audio-perceptual method remain ‘‘The Gold Standard’’ In the early 1980s there was a brief challenge by Kent and Rosenbek ([7_TD$IF]1982) to the Mayo Classification approach espoused by Darley et al. (1975) which by then had well established its major tenets: (1) The dysarthrias and apraxia of speech define the motor speech disorders and can be distinguished from the neurological speech disorders, (2) apraxia of speech is a motor programming /planning disorder whereas the dysarthrias are disorders of movement execution (flaccidity, spasticity, ataxia, hyperkinesia, hypokinesia). Prosody could well be normal in apraxia of speech but is marked by inconsistent articulation errors including sequencing errors. (3) The differential diagnosis of the motor speech disorders can and should be achieved following the audio-perceptual method. Kent and Rosenbek ([7_TD$IF]1982) offered what they claimed to be the first acoustic analysis of two types of prosodic abnormality described by Monrad-Krohn (1963), i.e., dysprosody and aprosody. In so doing, they not only followed a prosodic classification but also proposed an acoustic method in their analysis and classification of motor speech disorders. As for their definition of prosody, while generally adhering to Jakobson et al.’s (1951) viewpoint, they deviate from them and follow Gibbon (1976) who states that supra-segmental and prosodic are not synonymous. They agree with Gibbon who notes that if on the one hand the term ‘‘suprasegmental’’ presupposes a segmental or phonemic analysis and suprasegmental features are described as a kind of residual, then it would become incorrect to define them at the same time without regard to a segmental organization. Kent and Rosenbek ([7_TD $DIF]1982) further borrow Gibbon's (1976) thinking when they state that the systems that might be included under prosodic analysis include intonation, tone accent, stress, juncture, rhythm, and relative duration (Gibbon, 1976). They propose that these systems can be approached acoustically by the measurement of temporal pattern, intensity, and F0 contour. In agreement with Kent and Netsell (1975) they note for example that the perceptual speech deviation of prosodic excess might a distortion of the time pattern of speech. This pattern is characterized by lengthening of normally reduced syllables Please cite this article in press as: Boutsen, F., Dvorak, J., Prosody and motor speech disorders: A retrospective review of a merger that is imminent. Lingua (2017), http://dx.doi.org/10.1016/j.lingua.2017.07.009

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and the appearance of atypical pauses or inter-syllable breaks. They describe ‘‘scanning’’ speech in acoustic terms, as they see it as characterized by limited variation in syllable duration, the presence of wide and nearly regular spacing between syllabic nuclei, and a generally flat F0 over the first five syllables. Overall, Kent and Rosenbek ([7_TD$IF]1982) further argue that the deviant perceptual dimensions associated with ataxic dysarthria, apraxia of speech, and hypokinetic dysarthria (as described by Darley et al., 1969a,b) and aprosodia (as described by Ross and Mesulam, 1979; Sackeim and Gur, 1978) more generally pattern into dysprosody (ataxic dysarthria, apraxia of speech) and aprosody (hypokinetic and right hemisphere dysarthria). In doing so, they deviate from Darley et al. as they point to acoustic overlap rather than what perceptually separates the motor speech disorders. Even so, the Mayo Classification has remained the gold standard in motor speech disorders for almost 50 years. Duffy's (1995) recent update of the theory added unilateral upper motor neuron dysarthria to the classification, but was otherwise faithful to Darley et al.’s (1969a,b, 1975) founding principles. Like Darley et al. (1969a,b, 1975), Duffy underscores the need for a strict adherence to a ‘perceptual’ roadmap guiding the classification of dysarthria and a pure motor interpretation of its neurophysiological underpinnings in accordance with a three-stage model of speech production, involving cognitive-linguistic, programming, and execution phases. Within the last two decades, Van der Merwe (1997), McNeil (1997) and Boutsen (Boutsen and Christman, 2002; Boutsen, 2003, 2004) have challenged some basic tenets of the Mayo Classification. In line with current research showing that both the cerebellum and the basal ganglia are involved with cognitive and linguistic, as well as motor aspects of speech, they argue that a sharp distinction between apraxia of speech and dysarthria is not tenable even in neurophysiological terms. In addition, underscoring the sensorimotor nature of speech, they assigned a pivotal role to audition and proprioception in speech disorders and argued that context specificity plays a role in motor speech disorders. Boutsen (2003, 2008) maintained in this regard that speech contexts variably tap into prosodic goals that are intrinsic or extrinsic to the language. Even so, while this theory returned the focus in motor speech disorders back on prosody an alternative acoustic--prosodic perspective to the perceptual auditory method could not be derived from it and was still lacking. 3. Developments in the realm of prosody: post pivotal shifts in the sixties to present It is fair to say that while the perceptual acoustic method has remained the status quo in motor speech disorders attempts to measure prosody or aspects thereof evolved significantly. Interestingly, impetus surrounding these measurement approaches evolved around frame works (speech science; phonology) or debates (stress or syllable-timed language types) that made their application in motor speech disorders not seem relevant. An additional impetus came from a challenge to overcome what can be termed the lack of reference problem that is especially pertinent in prosody (Xu, 2011). Due to the general absence of orthographic representations of prosody other than punctuations, there is little to fall back on when it comes to identifying the prosodic units, whether in terms of their temporal location, scope, phonetic property or communicative function (Xu, 2011).

[8_TD$IF]3.1. Phonology based approaches: units and the emergence of new metrices It is fair to say that the lack of reference problem has been addressed in so much that units have been defined in phonological approaches to prosody such as those proposed by Pierrehumbert (1980) and Ladd (1996) (intonational approach) on the one hand and by Selkirk (1981), Nespor and Vogel (1986) and Prince and Smolensky (1993) on the other (syntax based approach). Shattuck-Hufnagel and Turk (1996) and Jun (2007) summarized the prosodic units/ hierarchies from the literature that are mapped in both approaches in Table 2 shown. Table 2 Units in syntax and intonation based prosody approaches (Jun, 2007; Shattuck-Hufnagel and Turk, 1996). Syntax-based Nespor and Vogel (1986), Selkirk (1981) and Hayes (1989)

Intonation-based Pierrehumbert and Beckman (1988)

Utterance Intonation phrase Phonologic phrase

No equivalent Full intonation phrase (IP) Intermediate phrase (ip) Accentual phrase (Ap)

Prosodic word Foot Syllable

No equivalent Intonation phrase Major phrase Minor phrase Prosodic word Foot Syllable Mora

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As can be seen there, the proposed prosodic units/hierarchies overlap considerably while the underlying predictors, i. e., syntax or intonation are what distinguish these approaches (see Shattuck[9_TD$IF]-Hufnagel for discussion). It can further be said that the intonational approaches perhaps introduces the greater shift in phonology as they mark intonation as much a part of phonology as the phoneme once used to be. While articulatory and acoustic-perceptual features were certainly modeled in phonology, intonational phonology put F0 and intonation front and center in the realm of phonology. Specifically, the intonational approach differentiates pitch F0 from intonation and maintains that languages use F0 lexically or non-lexically (Hirst[1_TD$IF], 1998). Lexical uses of pitch are referred to as tone (change in F0 changes meaning in tone languages and pitch accent (word stress in English). Non-lexical uses include post (-or supra) lexical uses of pitch that is generally referred to as intonation while paralinguistic uses include changes in pitch to show emotion and even sarcasm (Hirst[1_TD$IF], [10_TD$IF]1998; Arvaniti and Fletcher, [1_TD$IF]2016). Within this frame work, intonation is conceived as the linguistically controlled and pragmatically meaningful use of F0 that spans entire utterances (Arvaniti and Fletcher, [12_TD$IF]2016; Xu, 2015). The primitives of intonation are tones or tonal targets: low (L) and high (H) while F0 contours are often considered categorical, gestalt like shapes that allow for variation in F0 detail (Arvaniti, 2011). In auto-segmental metrical (AM) models also known as the Pierrehumbert model (Pierrehumbert, 1980; Pierrehumbert and Beckman, 1988), and the IPO model ([13_TD$IF]’t Hart et al., 1990) sequenced tonal targets (pitch movements) are called auto-segments (e.g., LHL) that associate with structural positions in the metrical structure, hence the name autosegmental metrical. Tones are either prominence marking and contrastive as when linked to a stressed syllable (pitch accent) or boundary marking as when linked to the edge of a prosodic unit (phrase accent or boundary tone) (Jun, 2007). Metrical structure describes phrasing (or the grouping of words) that occurs over smaller or longer units (heads and boundaries of constituents). He of thought of / Ella enjoying the burrito/ {Ella = enjoying burrito} He of thought of Ella /enjoying the burrito/ {while enjoying the burrito} As is shown in above example, prosodic phrasing cues syntactic/semantic grouping. Items within the intonation phrase are interpreted together, but items across the phrase are not. Phrasing also affects attachment of relative clauses (RCs) to noun phrase (NPs) in an utterance. For example in NP1 + NP2 + RC, RC can attach high (modify NP1) or low (modify NP2) (Jun, 2003, 2007). Someone shot the servant (NP1) of the actress (NP2) who was on the balcony (RC) (Cuetos and Mitchell, 1988). Jun (2007) further points out that phrasing also determines scope as is evidenced in the following utterances: John didn’t hit Mary /because she was yelling (/ denotes pause) John didn’t hit Mary /w/ because she was yelling (/w/ denotes weak pause) Work on identifying and/or predicting the units in prosody evolved into the segmental anchoring hypothesis (SAH) originally conceived by Arvaniti et al. (1998). The idea behind the SAH is that both the beginning and the end of a rising pitch accent are anchored to specific landmarks in the segmental structure, regardless of segmental or syllable structure composition, and regardless of speaking rate (Prieto and Torreira, 2007). This aligned with and most likely promoted acoustic-gestural perspectives in the phonology of prosodic structure that were taking hold at that time. Within articulatory phonology (e.g., Browman and Goldstein, 1986, 1992), phonology and phonetics are isomorphic, and their units, called gestures, are phonologically relevant events of the vocal tract. Katsika et al. (2014) provide a description of the three types of gestures, namely constriction, tone and clock-slowing gestures that are defined in this approach. Constriction gestures form or release constrictions in the vocal tract. They are specified for abstract linguistic tasks (e.g., lip closure for /p/) and are realized by coordinated actions of specific articulators (e.g., lips and jaw for the labial closure in /p/). They extend in space and time, and are triggered by internal oscillators. The oscillator triggering the onset consonantal gesture (C gesture) is in-phase (synchronously) coordinated to the oscillator triggering the nucleus vocalic gesture (V gesture), and as a result the motion of the constrictor forming the onset consonant is initiated synchronously with motion of the constrictor forming the nucleus vowel. The oscillator triggering the coda C gesture, on the other hand, is anti-phase (sequentially) coordinated with the oscillator triggering the V gesture, and consequently, the motion of the constrictor forming the coda consonant is initiated as the motion of the constrictor forming the vowel reaches its target (Katsika et al., 2014). It can be seen that these two types of coupling can account for syllabic structure. Tone gestures are similar to constriction gestures, however, they involve a different set of articulators than the constriction gestures. The goal of tone gestures is to achieve linguistically relevant variations in the frequency of vibration of the vocal folds (McGowan and Saltzman, 1995; Fougeron and Jun, 1998). Finally, clock-slowing gestures are different from constriction and tone gestures in that they are not related to specific articulators. Their main effect is to modulate (lengthening and strengthening) the spatial and temporal properties of articulatory gestures that are active concurrently with them (e. g., Byrd and Saltzman, 2003). Katsika et al. (2014) and others have shown how articulatory phonology addresses Please cite this article in press as: Boutsen, F., Dvorak, J., Prosody and motor speech disorders: A retrospective review of a merger that is imminent. 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questions that clearly could have been asked in Auto-segmental Metrical models of phonology. For example these researchers posed the question as to how boundary tones are coordinated with constriction gestures and whether prominence (lexical (stress) and/or phrasal (pitch accent)) influences this coordination. In sum, it can be concluded that current phonological approaches to prosodic structure have made progress defining the locations where tone features are linked to i.e., the edge of a phonological word, a stressed (prominent) syllable within the word, or a phrase-final syllable. Likewise there is a body of evidence that prosodic structure influences the timing and magnitude of articulatory gestures for consonants and vowels. Generally speaking, it has been shown that gestures are lengthened and strengthened in certain prosodic positions (e.g., phrase-initially and in phrase-level prominent positions), while they are shortened and reduced in other positions (phrase-medially and in non-prominent positions) (Byrd and Saltzman, [10_TD$IF]1998; Cho, 2005; Byrd et al., 2006; Cho and Keating, 2009; Krivokapic´ and Byrd, 2012). These effects of prosodic context on articulation give rise to prosodically conditioned acoustic variation in consonants and vowels (i.e., segmental effects), as reported in numerous studies based on measures of acoustic duration, vowel formants, VOT, intensity and spectral measures of consonant place (Cho, 2005; Cole et al., [12_TD$IF]2007; Turk and Shattuck-Hufnagel, 2007). While intonational phonological approaches to prosody clearly flourished, (unsuccessful) attempts to classify language prosody in terms of rhythm (isochrony) and/or to consistently distinguish syllable, stress or mora timed languages left a number of rhythm metrices in its wake. Accepting the notion that neither the syllable nor stress were found to be timed at regular intervals, Dauer (1983) made the observation that various phonological properties distinguish the two groups of languages. He noted that ‘‘syllable-timed’’ languages have a smaller variety of syllable types than ‘‘stress-timed’’ languages, and that they do not display vowel reduction. These two characteristics are responsible for the fact that syllables in syllable-timed languages are more similar to each other in duration, helping to set up a perception of staccato or a machine gun effect. In Spanish and French, for example, more than half of the syllables (by type frequency) consist of a consonant followed by a vowel (CV) (Dauer, 1983; Nespor et al., 2011). This finding was tantamount to stating that different rhythms arise as a ‘‘consequence’’ of a series of independent phonological properties rather than that some unit is timed or controlled. The discovery that newborns are capable of discriminating stress-timed from syllable-timed languages (Mehler et al., 1988) gave a new underpinning to the reality that children respond to the difference, whatever it is, in these languages. Ramus et al. (1999) thought that first and foremost newborns may well hear speech as a sequence of vowels interrupted by noise (i.e., consonants); this hypothesis is known as the Time-Intensity Grid Representation (TIGRE; Mehler et al., 1996). Accordingly, they proposed that the perception of different rhythms is created by the way in which vowels alternate with consonants but also by the way this alternation is influenced by stress-related variability in vocalic intervals as determined by stressed and unstressed syllables, feet and words (Nespor et al., 2011). This idea was formalized in indexes including %V and DC. %V refers to amount of time occupied by vowels in the speech stream. A high %V implies that the repertoire of the possible syllable types is restricted and consequently the consonantal intervals do not vary a great deal. DC refers to the standard deviations of the duration of consonantal intervals. It can be seen that these indexes do not capture order in complexity of syllable alterations as is shown below:

DC½CCVCVCCCV ¼ DC½CCCVCCVCV ¼ DC½CCCVCVCCV Likewise it can also be seen that these measures are susceptible to (variations) speech rate. These considerations were taken into account by Grabe and Low (2002) who introduced the pairwise variability index (PVI) which looks at (local) variability in duration of all pairs of vocalic and intervocalic intervals. Grabe and Low (2002) assumed that consonants are less sensitive to changes in tempo than vowels and thus proposed that the raw measure rPVI be used to measure variability in consonantal intervals and the normalized measure nPVI be used to measure variability in vocalic intervals. Rathcke and Smith (2015) summarize the state of the art to date of the debate surrounding rhythm research and language typology quite well where they state that the extent to which timing processes and phonological structure go hand in hand is still rather poorly understood. They do agree, however, that there is a lot to say for Nolan and Asu (2009) suggestion that ‘stress-timing’ and ‘syllable-timing’ may function as orthogonal variables of linguistic rhythm and co-exist at different levels of prosodic structure in the same language. There also is consensus that rhythm metrics are very sensitive to the effects that methodological choices have on the durations of consonantal and vocalic intervals. In fact several studies have focused on documenting just that. Wiget et al. (2010) found that variability in the metric scores of individual sentences of English outweighed both differences due to segmentation practices among annotators and interspeaker variation. In addition, they found that spontaneous speech is more variable in its timing patterns compared to reading and that speakers differ considerably from each other, possibly because of choices such as speaking rate and the clarity of their speech (which could affect the degree of vowel reduction and the duration of consonant clusters). They conclude that comparing metric scores across studies, metrics and languages may be inadvisable. Please cite this article in press as: Boutsen, F., Dvorak, J., Prosody and motor speech disorders: A retrospective review of a merger that is imminent. Lingua (2017), http://dx.doi.org/10.1016/j.lingua.2017.07.009

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[14_TD$IF]4. Prosody metrices in motor speech disorders Within the last few years there has been growing evidence for the notion that temporal prosody in AOS is a distinctive feature for differential diagnosis (Haley et al., 2012; Murray et al., [15_TD$IF]2015; Virgis et al., 2014) and that the pairwise variability index of vowel duration in particular is very useful to differentiate individuals with AOS plus aphasia from those with aphasia alone and from healthy controls (Virgis et al., 2014). Automated software routines have been developed that identify vowel nuclei and facilitate data entry in the algorithm (de Jong and Wempe, 2009; Shahin and Ballard, 2012; Vogel et al., 2012). In addition, several studies have shown significant correlations between perceptual judgments and acoustic measures. For example, Haley et al. (2012) report that the measure of syllable duration in sentences correlated highly with judgments of overall prosody (0.78), slow speech rate (0.75), and restricted pitch range (0.73). Ballard et al. (2010) show that the pairwise variability index of vowel duration in multisyllabic words correlated highly with judgments of ‘‘goodness’’ of the stress pattern produced (0.80--0.91). The evidence for the dysarthrias is mixed. Liss et al. (2009) used several rhythm indices including DV, DC, %V, VarcoV, VarcoC, nPVI-V, and rPVI-C and found different combinations of them can reasonably well distinguish the dysarthrias from each other and from normally healthy speakers. Lowit (2014), however, reported that none of the metrices could do this despite clear perceptual differences. They mentioned that small sample size and the repetitive nature of the task in their investigation perhaps may have explained the negative findings in their study. That said, Lowit and Kuschmann (2012) were able to characterize intonation deficits in the spontaneous speech of persons with hypokinetic dysarthria (PD), ataxic dysarthria (AT) and foreign accent syndrome (FAS). Using Autosegmental-Metrical analysis they found that intonation profiles were different in the groups with speech disorders and the control groups (CON). In addition, they found that the groups with speech disorders differed from each other in terms of the prevalence of different pitch accents, frequency of pitch accentuation and phrasing patterns. Specifically, AT and FAS speakers used more rising and high pitch accents than PD and CON speakers. The AT group used the highest number of pitch accents per phrase, and all 3 MSD groups produced significantly shorter phrases than the CON group (Lowit and Kuschmann, 2012). As for the PD group these results confirm those of Mennen et al. (2008) who compared intonation profiles of 2 persons with PD with 2 normal control participants. This study also did not find differences in categorical elements of intonation per se between groups but rather found problems in the distribution and or phonetic realization of those representations. Said problems could occur either at the planning or the implementation level. [16_TD$IF]5. Conclusion Prosody and motor speech disorders tangentially intersected in the early 80s in Kent and Rosenbek ([7_TD$IF]1982). In more recent years, it appears as if there is perhaps a more serious merger taking place with rhythm and intonational analyses taking hold and complementing the differential classification of motor speech disorders that once was fully rooted in the auditory-perceptual method. Even so, methodological issues still remain and thwart application mostly in the realm of dysarthria classification. 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