Auditory constraints on infant speech acquisition : a dynamic systems perspective
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This study examined the extent to which auditory sensitivity contributes to the emergence of vocalization patterns observed in the pre-linguistic canonical babbling period. Whereas mechanical characteristics of the speech output system have been suggested as contributing strongly to the emergent patterns of babbling vocalizations, the extent to which these vocalizations depend on auditory sensitivity to be present at the frequencies with which they typically occur is unknown. A dynamic systems perspective was adopted to explore the extent to which the emergence of vocalization patterns typically observed in infants depends on auditory sensitivity. Spontaneous vocalization samples were obtained from 15 infants with auditory sensitivity ranging from normal hearing (PTA 25 dB HL) to profound hearing impairment (PTA > 90 dB HL). Several vocalization inventories were obtained including, (a) vocalization types within utterance strings (e.g., singletons, marginal syllables, and syllables), (b) syllable alternations, (c) syllable shapes (e.g. CV, VC), (d) syllable onset patterns, (e) intra-syllabic CV co-occurrences, (f) inter-syllabic consonant and vowel variegation patterns, and (g) segmental patterns. Results from these analyses suggest that auditory sensitivity may not contribute significantly to the prominence of CV co-occurrence patterns, syllable alternations, and vowel variegation patterns. Syllable shapes, syllable onset, consonant variegation and segmental patterns were dependent on auditory sensitivity. Results show that auditory sensitivity is a significant control-parameter or variable contributing to the emergent patterns of vocalizations observed during the canonical babbling period, consistent with the dynamic systems perspective proposing that alternate arrangement of system variables (mechanical and perceptual) leads to differing patterns of output organization. The vocalization patterns of hearing infants were consistent with the prominent patterns reported in the literature during the pre-linguistic period. However, different vocalization patterns were observed in the infants with moderate and profound sensorineural hearing loss. Results suggest that the emerging patterns of vocalization organization are due to the contribution of multiple forces (bio-mechanical and auditory perceptual). Auditory sensitivity forces interact with and impinge on speech system propensities, ultimately contributing to the observed patterns of vocalization behaviors.