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Motor speech involvement of unknown origin is a relatively new diagnostic category that is applied when children's speech production deficits are predominantly linked to sensorimotor planning, programming, or execution (Caruso and Strand, 1999). The disorder occurs in the absence of obvious neuromotor causes and often includes concomitant language deficits. This category is broader than and encompasses that of developmental apraxia of speech (DAS), which refers specifically to impaired planning, or praxis. Classically, developmental speech production deficits have been categorized as either phonological or DAS. However, recent empirical evidence suggests that a wider range of children (e.g., those with specific language impairment [SLI] or inconsistent speech errors) may exhibit deficits that are influenced by motor variables and, in these cases, may be classified as motor speech involved.
Although the underlying causes of motor speech involvement are unclear, there is general evidence that motor and cognitive deficits often co-occur (Diamond, 2000). Neurophysiological findings support the interaction of cognitive and motor development, most notably in common brain mechanisms in the lateral perisylvian cortex, the neocerebellum, and the dorsolateral prefrontal cortex (Diamond, 2000; Hill, 2001). Apparently, speech motor and language domains codevelop and mutually influence one another across development.
In late infancy, basic movement patterns observed in babbling are linked to emerging intents and words (de Boysson-Bardis and Vihman, 1991; Levelt, Roelofs, and Meyer, 1999). At this level, it is apparent how language and motor levels constrain one another. However, the relations between language and motor levels in later periods of development have not been specified. Language models include categories such as concepts, semantics, syntax, and phonology (Levelt, Roelofs, and Meyer, 1999). Motor systems are discussed in the very different terms of cortical inputs to pattern generators in the brainstem, which in turn provide inputs to motor neuron pools for the generation of muscle activity. Sensory feedback is also a necessary component of motor systems (A. Smith, Goffman, and Stark, 1995). Although it is established that motor and language domains both show a protracted developmental time course, speech production models are not explicit about the nature of the linkages. The general view is that increasingly complex linguistic structures are linked to increasingly complex movements in the course of development. Motor speech deficits occur when movement variables interfere with the acquisition of speech and language production.
A large range of speech and language characteristics have been reported in children diagnosed with motor speech disorders. In the following summary, emphasis is placed on those that are at least partially motor in origin.
Variability
Children with motor speech disorders have been reported to produce highly variable errors, even across multiple productions of the same word (Davis, Jakielski, and Marquardt, 1998). When the deficit involves movement planning, imitation and repetition may not aid performance (Bradford and Dodd, 1996). Although variability is observed in speech motor (A. Smith and Goffman, 1998) and phonetic output of young children who are normally developing, it is extreme and persistent in disordered children. Usually, variability is discussed as a phonetic error type. However, kinematic analysis of lip and jaw movement reveals that children with SLI show movement output that is less stable than that of their normally developing peers, even when producing an accurate phonetic segment (Goffman, 1999). Thus, both phonological and motor factors may contribute. Deficits in planning and implementing spatially and temporally organized movements may influence the acquisition of stable phonological units (Hall, Jordan, and Robin, 1993).
Duration
Increased movement durations are a hallmark of immature motor systems (B. L. Smith, 1978; Kent and Forner, 1980). In children with motor speech involvement, the slow implementation of movement may lead to decreased performance on a nonlinguistic diadochokinetic task (Crary, 1993) as well as increased error rates on longer and more complex utterances. An additional error type that may also be related to timing is poor movement coordination across speech subsystems. Such timing deficits in articulatory and laryngeal coordination may lead to voicing and nasality errors. Hence, these errors may have origins in movement planning and implementation. A decreased speech rate provides the child with time to process, plan, and implement movement (Hall, Jordan, and Robin, 1993), but it may also negatively influence speech motor performance.
Phonetic Movement Organization and Sequencing
As they develop, children produce increasingly differentiated speech movements, both within and across articulatory, laryngeal, and respiratory subsystems (Gibbon, 1999; Moore, 2001). A lack of differentiated and coordinated movement leads to a collapsing of phonetic distinctions. It follows that segmental and syllabic inventories are reduced for children with motor speech deficits (Davis, Jakielski, and Marquardt, 1998). Vowel and consonant errors may be considered in reference to articulatory complexity. Vowel production requires highly specified movements of the tongue and jaw (Pollock and Hall, 1991). Consonant sounds that are early-developing and that are most frequently seen in the phonetic inventories of children with motor speech deficits make relatively few demands on the motor system (Hall, Jordan, and Robin, 1993). Kent (1992) suggests that early-developing stop consonants such as [b] and [d] are produced with rapid, ballistic movements. Fricatives require fine force control and are acquired later. Liquids, which require highly controlled tongue movements, are learned quite late in the developmental process. Using electropalatography, Gibbon (1999) has provided direct evidence that children with speech deficits contact the entire palate with the tongue, not just the anterior region, in their production of alveolar consonants. Such data indicate that motor control of differentiated tongue movements has not developed in these children. Overall, as proposed by Kent (1992), motor variables account for many aspects of the developmental sequence frequently reported in speech-and language-impaired children.
Syllable shapes may also be influenced by motor factors. The earliest consonant-vowel structure seen in babbling is hypothesized to consist of jaw oscillation without independent control of the lips and tongue (MacNeilage and Davis, 2000). More complex syllable structures probably require increased movement control, such as the homing movement for final consonant production (Kent, 1992).
Prosodic Movement Organization and Sequencing
One major aspect of motor development that has been emphasized in motor speech disorders is rhythmicity. Rhythmicity is thought to have origins in prelinguistic babbling (and, perhaps, in early stereotypic movements, such as kicking and banging objects) (e.g., Thelen and Smith, 1994). Rhythmicity underlies the prosodic structure of speech, which is used to convey word and sentence meaning as well as affect. Children with motor speech disorders display particular deficits in prosodic aspects of speech. Shriberg and his colleagues (Shriberg, Aram, and Kwiatkowski, 1997) found that a significant proportion of children diagnosed with DAS demonstrated errors characterized by even or misplaced stress in their spontaneous speech. In a study using direct measures of lip and jaw movement during the production of different stress patterns, Goffman (1999) reported that children with a diagnosis of SLI, who also demonstrated speech production and morphological errors, were poor at producing large and small movements sequentially across different stress contexts. For example, in the problematic weak-strong prosodic sequence, these children had difficulty producing small movements corresponding to unstressed syllables. Overall, the control of movement for the production of stress is a frequently cited deficit in children with motor speech disorders.
General Motor Development
In the clinical literature, general neuromotor status has long been implicated as contributing to even relatively subtle speech and language deficits (Morris and Klein, 1987). Empirical studies have provided evidence that aspects of gross and fine motor (e.g., peg moving, gesture imitation) performance are below expected levels in children with variable speech errors, DAS (Bradford and Dodd, 1996), and many diagnosed with SLI (Bishop and Edmundson, 1987; Hill, 2001). Such findings suggest that many speech production disorders include a general motor component.
As is apparent, an understanding of speech motor contributions to the acquisition of speech and language is in its infancy. However, it is clear that intervention approaches for these children need to incorporate motor as well as language components. Although efficacy studies are scarce, several investigators have proposed techniques for the treatment of motor speech disorders in children. Although the emphasis has been on DAS, these approaches could be tailored to more general motor speech deficits. Major approaches to intervention have focused on motor programming (Hall, Jordan, and Robin, 1993) and tactile-kinesthetic and rhythmic (Square, 1994) deficits. Hierarchical language organization has also been emphasized, supporting the intimate links between linguistic and movement variables (Velleman and Strand, 1994).
New models of speech and language development are needed that integrate motor and language variables in a way that is consistent with recent neurophysiological and behavioral evidence. Further, new methods of recording respiratory, laryngeal, and articulatory behaviors of infants and young children during the production of meaningful linguistic activity should provide crucial data for understanding how language and motor components of development interact across normal and disordered development. Such tools should also help answer questions about appropriate interventions for children whose deficits are influenced by atypical motor control processes.
See also developmental apraxia of speech.
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