How children learn to organize their speech gestures: further evidence from fricative-vowel syllables.

Previous studies with fricative-vowel (FV) syllables have shown that the difference in overall spectrum between fricatives is less in children's speech than in that of adults, but that fricative noises show greater differences in the region of the second formant (F2) as a function of the upcoming vowel than those of adults at corresponding points in the fricative. These results have been interpreted as evidence that children produce fricatives that are not spatially differentiated as those of adults and that children initiate vowel gestures earlier during syllable production than adults do (Nittrouer, Studdert-Kennedy, & McGowan, 1989). The goals of the present study were (a) to replicate the previous age-related difference for F2 with FV syllables; (b) to test the alternative interpretation that age-related differences in fricative f2 reflect age-related differences in vocal-tract geometry; (c) to determine whether age-related differences in F2 (and so, by inference, in articulatory organization) might extend beyond the syllable boundaries, perhaps into the schwa of a preceding unstressed syllable; and (d) determine if gestures other than fricative gestures show less spatial differentiation in children's than in adults' speech. To these ends, F2 frequencies were measured in schwa-fricative-vowel utterances (consisting of the fricatives /s/ and [symbol:see text] and of the vowels /i/ and /a/) from 40 speakers (10 each of the ages of 3, 5, 7 years, and adults) at three locations (for the entire schwa, for 10 ms of fricative noise centered at 30 ms before voicing onset, and 10 pitch periods from vocalic center). Results of several analyses supported four conclusions: (a) the earlier finding was replicated; (b) age-related differences in vocal-tract geometry could not explain the age-related difference in vowel effects on fricative noise; (c) children master intersyllabic gestural organization prior to intrasyllabic gestural organization; and (d) unlike fricative gestures, children's vowel gestures are more spatially distinct than those of adults.