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Abstract
Understanding of the behavioural, cognitive and neural underpinnings of speech production is of interest theoretically, and is important for understanding disorders of speech production and how to assess and treat such disorders in the clinic. This paper addresses two claims about the neuromotor control of speech production: (1) speech is subserved by a distinct, specialised motor control system and (2) speech is holistic and cannot be decomposed into smaller primitives. Both claims have gained traction in recent literature, and are central to a task-dependent model of speech motor control. The purpose of this paper is to stimulate thinking about speech production, its disorders and the clinical implications of these claims. The paper poses several conceptual and empirical challenges for these claims – including the critical importance of defining speech. The emerging conclusion is that a task-dependent model is called into question as its two central claims are founded on ill-defined and inconsistently applied concepts. The paper concludes with discussion of methodological and clinical implications, including the potential utility of diadochokinetic (DDK) tasks in assessment of motor speech disorders and the contraindication of nonspeech oral motor exercises to improve speech function.
Keywords:
Notes
1 These are not the only possible views (Weismer, Citation2006), and each may represent a class of models. I focus on these two views, and these two specific claims, because they have been discussed relatively explicitly. Occasionally I will take liberties with stated positions to develop the broader discussion.
2 Similar debates about the existence of speech-specific systems versus a more general system occur in the speech perception literature (e.g. Holt & Lotto, Citation2008; Liberman & Whalen, Citation2000). The focus of the present paper is restricted to speech production however.
3 Moore and colleagues referred to variegated babbling as “prespeech” behaviour (Moore & Ruark, Citation1996, p. 1036) and considered such babble to have no communicative intent (vocalisations were generated during self-directed play and judged to be “neither meaningful nor referential”; Moore & Ruark, Citation1996, p. 1037). Although Moore and colleagues (Moore, Caulfield, & Green, Citation2001; Moore & Ruark, Citation1996) have convincingly demonstrated significant kinetic and kinematic differences between first words and oral motor behaviours such as chewing, their work also shows considerable similarities between variegated babbling and first words (in fact, Moore et al., Citation2001, grouped vocalisations, babbling, and “real” speech into a single category for analysis given lack of differences).
4 Typically, the task instructions are to say pa or pataka (etc.), not make this movement pattern. That is, typically DDK tasks are presented as a speaking task.
5 No quantified and independent measures of such third variables have been proposed, to my knowledge (severity as operationalised in terms of speaking rate or intelligibility is not independent from speech). As a result, such third-variable explanations tend to be untestable.
6 For example, Ziegler (Citation2003a, p. 29) refers to “integration of visual information with a subject’s body image” as a non-motoric task aspect that differs between speech and imitating oral movements.
7 Caviness et al. (Citation2006) explicitly define speech broadly as tasks involving simultaneous phonation and articulation, which includes sustained vowel production and reiterant speech, as well as two connected speech (reading) tasks. They reported differences between the two connected speech tasks.
8 Ziegler (Citation2003b) wrote: “Thus, macroscopically overlapping functions are, on closer examination, broken up into specialised and segregated functions which are optimally tuned to their behavioural goals.” (p. 101), and “At a low level of resolution the usual suspects, motor cortex, basal ganglia, cerebellum, and brainstem nuclei are implicated in most if not all of the behaviours at stake […]. Yet, at a higher level of resolution, the neural networks controlling motor functions turn out to be organised in a task-specific manner […]” (p. 102).
9 I assume that initial attempts at speech are in fact considered speech. If the system that supports initial attempts is the speech system, then the origin of this system cannot be based on experience-dependent plasticity.
10 In discussing adaptive trading relations in producing rounded back vowels, Ziegler (Citation2003b, p. 101) states: “Co-ordination here is clearly in the service of producing intelligible speech. […] the described organisational principle is speech-specific and is not useful for any other behaviour.” I argue that the organisational principle is not speech-specific, only its application to a specific-speech pattern (rounded back vowel).
11 Hickok (Citation2014, p. 53): “To a first approximation, what may primarily distinguish between domains then – what distinguishes a linguistic system from a manual control system – is the representational bits that are plugged into those computational architectures.”
12 Although practice on a nonspeech task is unlikely to produce changes in speech intelligibility (e.g. Bunton, Citation2008), for proof-of-concept of an integrative system, it would be sufficient to show a change in (for example) a particular kinematic or acoustic parameter observed in a speech task, following practice on that parameter embedded in a nonspeech task. For example, does practice of a particular rhythmic pattern in the context of a human beatbox task result in greater accuracy/stability of that same rhythmic pattern in a speech task (e.g. sentence repetition)?
13 “There is no other natural motor activity except speech and song which utilizes the specific layout of this neural circuitry, and it is also hard to imagine any artificially designed nonspeech assessment or training task in the clinic which would specifically engage this particular network.” (Ziegler & Ackermann, Citation2013, p. 59; italics mine).
14 To be clear, I am not advocating for relying exclusively on DDK-type tasks (or on any other single task) in assessment and diagnosis of speech disorders. See also Ballard et al. (Citation2000, p. 979–980): “Although it is necessary to consider the impairment of AOS in the context of speech production tasks, also studying nonspeech behaviours has the potential to disambiguate which characteristics are a result of the underlying motor impairment and which are related to the interaction between the motor and linguistic systems.” (italics mine)