Barbara L. Davis

Developmental apraxia of speech : determiners of differential diagnosis

Developmental apraxia of speech (DAS) is a neurologically based disorder in the programming of sequential articulatory movements. This definition, based purely on motoric limitations, is fraught with controversy concerning aetiology, clinical manifestations, treatment, and even identification of the disorder as a separate clinical entity. An understanding of developmental apraxia depends on consistent utilization of a group of symptoms for diagnosis so that data-based results can be used to generate inferences about the disorder. Results from studies of children who are diagnosed with developmental apraxia, but who may not be apraxic, complicates application to theories attempting to account for observed symptoms. A longitudinal study of children with DAS has been under way at the University of Texas at Austin since 1985. Of 22 children referred as apraxic, a diagnosis has been confirmed in only four. Phonological and language evaluation data for five clients evaluated during this project are presented. O...

Acquisition of serial complexity in speech production: a comparison of phonetic and phonological approaches to first word production.

Comparison was made between performance-based and competence-based approaches to the understanding of first word production. The performance-related frame/content approach is representative of the biological/functional perspective of phonetics in seeking explanations based on motor, perceptual and cognitive aspects of speech actions. From this perspective, intrasyllabic consonant-vowel (CV) co-occurrence patterns and intersyllabic sequence patterns are viewed as reflective of biomechanical constraints emerging from mandibular oscillation cycles. A labial-coronal sequence effect involved, in addition, the problem of interfacing the lexicon with the motor system, as well as the additional problem of initiation of movement complexes. Competence-based approaches to acquisition are within the generative phonological tradition; involving an initial assumption of innate, speech-specific mental structures. While various current phonological approaches to acquisition involve consideration of sequence effects and intrasyllabic patterns, they do not adequately establish the proposed mental entities in infants of this age, and are nonexplanatory in the sense of not considering the causes of the structures and constraints that they posit.

Token‐to‐token variability in developmental apraxia of speech: three longitudinal case studies

Variability in the speech production patterns of children with developmental apraxia of speech (DAS) was investigated in a three‐year longitudinal study of three children with DAS. A metric was developed to measure token‐to‐token variability in repeated word productions from connected speech samples. Results suggest that high levels of total token and error token variability and low levels of word target stability and token accuracy characterize the disorder. Overall levels of variability and patterns of change over time differed between participants. Longitudinal patterns were indicative of decreasing total token variability and increasing token accuracy. However, change was not consistently unidirectional for two of the three children in the study, suggesting day‐to‐day performance differences in addition to within‐session variability.

Acquisition of Speech Production: The Achievement of Segmental Independence

As a primate communicative event, the repetitive, rhythmic, open-close alternation of the mandible, accompanied by phonation, is observable in three forms; some variants of the lipsmack, which is widespread in other primates, the initial babbling of human infants, and the production of the syllables of adult speech. In the first two of these, successive cycles tend to be uniform but in the third there is a highly variegated pattern in successive cycles. Adult segmental serial ordering errors, (e.g. spoonerisms) the effects of which are strongly constrained in terms of syllable structure, suggest that variegation is achieved by placement of independently controlled “Content” elements in syllable “Frames”. This paper considers implications of the view that development of infant communicative vocalizations from initial reduplicated babbling to variegated babbling and then to speech, primarily involves a gradual functional differentiation of segmental and subsegmental content elements from a phylogenetically prior basis consisting of ‘pure’ syllable frames; that is, mandibular oscillations without internal articulatory modulation.

Motor Explanations of Babbling and Early Speech Patterns

From the onset of canonical babbling, human vocal output is dominated by the cyclical open-close alternation of the mandible. Mandibular cyclicity has a long evolutionary history in sucking, licking and chewing in mammals, and also appears communicatively in lipsmacks, tonguesmacks and teeth chatters in other primates. It is argued that many of the articulatory regularities in the sound patterns of babbling, and early speech, which closely resembles babbling, (including consonants, vowels, syllables, and many of their detailed attributes) can be attributed directly to properties of this basic mandibular cycle. In addition, some interarticulator synergies evolving with the cycle, plus developmental limitations in changing locus of control between and within articulators during utterances, seem responsible for most other regularities in babbling and early speech.

Prosodic Correlates of Stress in Babbling: An Acoustical Study

Prelinguistic babbling often seems remarkably speech-like, not because it has recognizable words but because it seems to have adult-like prosody. To quantify this impression, we compared disyllabic sequences from five infants and five adults in terms of the use of frequency, intensity, and duration to mark stress. Significantly larger values for the three acoustic variables were observed on stressed than on unstressed syllables independent of syllable position for both groups. Adults showed the correlates of utterance final syllables--lower f0, lower intensity, and longer duration; infants showed only decrease in intensity. Ratios for stressed to unstressed syllables and participation of the three variables in stress production in individual disyllables were highly similar in both groups. No bias toward the English lexical trochaic stress pattern was observed. We conclude that infants in English environments produce adult-like stress patterns before they produce lexical items, which specify stress. Acoustic and perceptual analyses are used to explore stress marking by prelinguistic infants in an English language environment. Results show that infants employ the three acoustic correlates of stress in individual syllables in a manner largely similar to that of adult speakers, although they do not show second-syllable declination effects or an English language trochaic stress bias.

An Embodiment Perspective on the Acquisition of Speech Perception

Understanding the potential relationships between perception and production is crucial to explanation of the nature of early speech acquisition. The ‘embodiment’ perspective suggests that mental activity in general cannot be understood outside of the context of body activities. Indeed, universal motor factors seem to be more responsible for the distribution of early production preferences regarding consonant place and manner, and use of the vowel space than the often considerable cross-language differences in input available to the perceptual system. However, there is evidence for a perceptual basis to the establishment of a language-appropriate balance of oral-to-nasal output by the beginning of babbling, illustrating the necessary contribution of ‘extrinsic’ perceptual information to acquisition. In terms of representations, at least one assumption that segmental units underlying either perception or production in early phases of acquisition may be inappropriate. Our work on production has shown that the dominant early organizational structure is a relatively unitary open-close ‘frame’ produced by mandibular oscillation. Consideration of the role of ‘intrinsic’ (self-produced) perceptual information suggests that this frame may be an important basis for perceptual as well as production organization.

Phonological Process Occurrence in Phonologically Disordered Children.

Recent research in phonological disorders has focused on phonological processes, to determine the number of children using particular processes. This study investigated individual patterns of phonological process occurrence in nine phonologically disordered children. The purpose of the study was to determine frequency of occurrence of each process for each child and frequency of occurrence of unusual processes. Results indicated that a small, basic subset of phonological processes seemed to account for the majority of errors made by these children. Unusual processes were primarily changes in word structure. Within the common framework of basic and unusual processes, the children realized the processes in unique ways.

Organization of babbling: a case study.

Speech is probably the most complex serially ordered behavior in living forms. However, no systematic investigation of the organization of speech-related output when it is presumably simplest, namely during the babbling stage, has been attempted. Transcriptions of 423 babbled utterances (1145 syllables) were obtained from one subject 7–12 months of age. Most results could be interpreted in terms of a basic mouth opening-closing alternation, responsible not only for the typical vowel-consonant alternation of babbling, but also for many prominent details including within-utterance variation in vowel height (often stress-related) and in degree of closure for consonants. The results suggest that a “frame” for babbling is provided by mandibular oscillation, perhaps reflected, when operating alone, in the common alternation between labial consonants and central vowels. Variation in the amplitude of this oscillation may be responsible for the within-utterance vowel height and consonant manner variation and much of the perceived stress variation. Further variation is attributed to fronting movements of the tongue, the effects of which often spread beyond single vowels and consonants.

Origin of Serial-Output Complexity in Speech

During the babbling and early speech stages (7–18 months), infants strongly prefer to repeat the same consonant and, less often, the same vowel in multisyllabic utterances (e.g., /baba/), even when the target word has two different consonants. These utterances apparently have a simple structure, typically consisting only of repeated cycles of mandibular oscillation (called frames), with other speech articulators (lips, tongue, soft palate) in either a resting or nonresting position, but not moving independently (MacNeilage, Davis, & Matyear, l997). How, then, does an infant increase serial-output complexity by reducing this consonant harmony to the statistically low levels required by adult languages? The most well-known relatively discrete step toward this goal during the first-50-word stage (12–18 months) is to begin the first syllable with a labial consonant (involving the lips) but then follow it with a coronal consonant (involving the tongue front: e.g., “man,” “bad,” “pat”). In a review of seven reports involving five different language communities (MacNeilage & Davis, 1998), this pattern was observed in 21 of 22 infants, including 2 who produced this pattern when attempting words with the opposite sequence (e.g., “soup” → “pooch”). Table 1 is a summary of results from a study of the words of 10 infants during the 50-word stage (MacNeilage et al., 1997). Consonant-vowel-consonant (CVC) and consonant-vowel-consonant-vowel (CVCV) words were studied. The labial consonants were /b/, /p/, and /m/, and the coronal consonants were /d/, /t/, and /n/. Nine of the infants showed the trend toward more labial-coronal (LC) than coronal-labial (CL) sequences, and the 10th infant showed no trend. The overall ratio of LC to CL sequences was 2.55. This preference is not only characteristic of infants, but also present in languages. Table 2 shows ratios of LC to CL sequences in the first and second consonants (with an intervening vowel) in words in 10 diverse languages representing several different major language families.1 The preferences in all these languages are statistically significant beyond the .001 level in chi-square tests, except for Swahili and Japanese, which alone shows a countertrend. The results suggest that languages perpetuate a strong infant preference. The common occurrence of this preference in infants and adults may be of fundamental significance. Infants are unlikely to be simply copying adults, as their preferences tend to be stronger than adult preferences (especially in the 2 infants who reversed adult CL sequences), even though they have as yet learned only a few words. Some considerations suggest that a syllable beginning with a labial consonant is simpler to produce than one beginning with a coronal consonant. First, although the former may be made simply with a cycle of mandibular oscillation, the latter requires an additional tongue-positioning movement (MacNeilage & Davis, 1998). Second, there is an increase in labial consonants relative to coronal consonants when infants go from the babbling stage to the firstword stage (Boysson-Bardies et al., 1992), a trend that can be interpreted as a regression toward easier production forms in the face of the new functional demand to interface the motor system with the lexicon. Steger and Werker (1997) have recently reported an analogous example of apparent simplification of operation at the signalprocessing level in the presence of demands associated with concurrent building of a mental lexicon for speech perception. Infants show less discrimination of fine phonetic detail when required to pair words with objects than they show in syllable discrimination tasks. The usual excess of coronals over labials in babbling seems to be counterevidence to this labial-ease hypothesis. However, it may be due in part to the fact that there are a great deal more coronals than labials in the typical adult language being assimilated. It has been shown that babbling infants have some sensitivity to major differences in consonant frequencies between languages (Boysson-Bardies & Vihman, 1991). The third consideration favoring the labial-ease hypothesis is that infants who have been prevented from vocalization during the babbling and early speech periods by early tracheostomies show a very strong preference for labials over coronals in their initial posttracheostomy speech efforts (e.g., Locke & Pearson, 1992). The achievement of increased serial-output complexity by beginning a word with a simple cycle of mandibular oscillation and adding a tongue movement to the next cycle is not presently explicable in terms of the phonological component of a genetically specified universal grammar (Chomsky, 1986). The pattern is more likely to be a result of self-organization. Just as infants may more easily simulate serial complexity of a word by beginning with a mandibular cycle not including the tongue and then adding tongue action, earlier hominids, under pressures for increases in the size of their linguistic message set,