RESULTADOS DE LA VARIABLE COMPETENCIAS

CAPÍTULO IV. RESULTADOS

4.4. RESULTADOS DE LA VARIABLE COMPETENCIAS

Motor speech involvement of unknown origin is a rela-tively new diagnostic category that is applied when children’s speech production deﬁcits are predominantly linked to sensorimotor planning, programming, or exe-cution (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 ei-ther phonological or DAS. However, recent empirical evidence suggests that a wider range of children (e.g., those with specific language impairment [SLI] or incon-sistent speech errors) may exhibit deficits that are influ-enced 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 inter-action of cognitive and motor development, most nota-bly in common brain mechanisms in the lateral perisylvian cortex, the neocerebellum, and the dorso-lateral prefrontal cortex (Diamond, 2000; Hill, 2001).

Apparently, speech motor and language domains co-develop 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 lan-guage and motor levels constrain one another. How-ever, 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 di¤erent 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. Sen-sory feedback is also a necessary component of motor systems (A. Smith, Go¤man, 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 plex linguistic structures are linked to increasingly com-plex 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 deﬁ-cit involves movement planning, imitation and repeti-tion may not aid performance (Bradford and Dodd, 1996). Although variability is observed in speech motor (A. Smith and Go¤man, 1998) and phonetic output of young children who are normally developing, it is ex-treme and persistent in disordered children. Usually, variability is discussed as a phonetic error type.

How-ever, 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 (Go¤man, 1999). Thus, both phonological and motor factors may contribute. Deﬁcits in planning and imple-menting spatially and temporally organized movements may inﬂuence the acquisition of stable phonological units (Hall, Jordan, and Robin, 1993).

Duration. Increased movement durations are a hall-mark 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 dia-dochokinetic 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 subsys-tems. 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 im-plement 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 di¤erenti-ated speech movements, both within and across articu-latory, laryngeal, and respiratory subsystems (Gibbon, 1999; Moore, 2001). A lack of di¤erentiated and co-ordinated movement leads to a collapsing of phonetic distinctions. It follows that segmental and syllabic in-ventories 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 (Pol-lock and Hall, 1991). Consonant sounds that are early-developing and that are most frequently seen in the phonetic inventories of children with motor speech defi-cits 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 re-gion, in their production of alveolar consonants. Such data indicate that motor control of di¤erentiated 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 pro-duction (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 struc-ture of speech, which is used to convey word and sen-tence meaning as well as a¤ect. 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 demon-strated errors characterized by even or misplaced stress in their spontaneous speech. In a study using direct measures of lip and jaw movement during the produc-tion of di¤erent stress patterns, Go¤man (1999) reported that children with a diagnosis of SLI, who also demon-strated speech production and morphological errors, were poor at producing large and small movements sequentially across di¤erent stress contexts. For exam-ple, in the problematic weak-strong prosodic sequence, these children had di‰culty producing small move-ments 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 lan-guage deficits (Morris and Klein, 1987). Empirical studies have provided evidence that aspects of gross and fine motor (e.g., peg moving, gesture imitation) perfor-mance are below expected levels in children with vari-able 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 e‰cacy 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 organiza-tion has also been emphasized, supporting the intimate Motor Speech Involvement in Children 143

links between linguistic and movement variables (Velle-man 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 re-cording respiratory, laryngeal, and articulatory behav-iors of infants and young children during the production of meaningful linguistic activity should provide crucial data for understanding how language and motor com-ponents of development interact across normal and dis-ordered 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.

—Lisa Go¤man

References

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Go¤man, L. (1999). Prosodic influences on speech produc-tion in children with specific language impairment and speech deficits. Journal of Speech, Language, and Hearing Research, 42, 1499–1517.

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Smith, A., Go¤man, L., and Stark, R. E. (1995). Speech motor development. Seminars in Speech and Language, 16, 87–99.

Smith, B. L. (1978). Temporal aspects of English speech pro-duction: A developmental perspective. Journal of Phonetics, 6, 37–67.

Square, P. A. (1994). Treatment approaches for developmental apraxia of speech. Journal of Communication Disorders, 4, 151–161.

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CAPÍTULO IV. RESULTADOS

4.4. RESULTADOS DE LA VARIABLE COMPETENCIAS

References

Further Readings