Recent models of developmental changes in speech perception suggest that the weights assigned to acoustic properties change as children gain experience with a native language. Empirical evidence supports this position, but few suggestions have been offered as to what guides this shift. These three experiments were designed to improve our ability to predict how perceptual weighting schemes change with development. The specific hypothesis explored was twofold: (1) the weight assigned by adults to any one acoustic property differs across phonetic environments according to how informative that property is in each environment; and (2) the weight assigned by children to any one acoustic property differs less across phonetic environments because children have not fully learned the patterns of covariation between phonetic informativeness and environment for each property. Experiment 1 replicated previous findings of age-related differences in the weights assigned to noise spectra and formant transitions in labeling of syllable-initial fricatives (/s/ or /[symbol: see text]/). In experiment 2 the variation in F3-onset frequency associated with place of fricative constriction was eliminated. This property differs more (i.e., is more informative) in /u/ than in /a/. Accordingly adults' transition effect was reduced more for /u/ than for /a/ from experiment 1. Children's transition effect was similarly reduced across vowel environments. In experiment 3, F3-onset frequency was appropriately manipulated for both vowels, and adults transition effect increased more for /u/ than for /a/ from experiment 2. The increase in children's transition effect was more similar across vowels. We conclude that the children had not fully learned how information provided by F3 transitions varies across /a/ and /u/ environments, and suggest that developmental weighting shifts may be guided by children learning the relation between phonetic informativeness and environment.
Previous studies have convincingly shown that the weight assigned to vocalic formant transitions in decisions of fricative identity for fricative-vowel syllables decreases with development. Although these same studies suggested a developmental increase in the weight assigned to the noise spectrum, the role of the aperiodic-noise portions of the signals in these fricative decisions have not been as well-studied. The purpose of these experiments was to examine more closely developmental shifts in the weight assigned to the aperiodic-noise components of the signals in decisions of syllable-initial fricative identity. Two experiments used noises varying along continua from a clear /s/ percept to a clear /[symbol: see text]/ percept. In experiment 1, these noises were created by combining /s/ and /[symbol: see text]/ noises produced by a human vocal tract at different amplitude ratios, a process that resulted in stimuli differing primarily in the amplitude of a relatively low-frequency (roughly 2.2-kHz) peak. In experiment 2, noises that varied only in the amplitude of a similar low-frequency peak were created with a software synthesizer. Both experiments used synthetic /a/ and /u/ portions, and efforts were made to minimize possible contributions of vocalic formant transitions to fricative labeling. Children and adults labeled the resulting stimuli as /s/ vowel or /[symbol: see text]/ vowel. Combined results of the two experiments showed that children's responses were less influenced than those of adults by the amplitude of the low-frequency peak of fricative noises.
This study examined differences between adults and children and between normal and poor readers in the use of phonemic coding strategies for storing words in working memory. In the first experiment, adults, 11-year-olds, and 8-year-olds (categorized as normal or poor readers) recalled eight-item strings of rhyming and nonrhyming words. A developmental decrease in errors was observed for adults, 11-year-olds, and normal-reading 8-year-olds that reflected an improvement in the phonemic coding of items in working memory, but no difference was found between normal- and poor-reading 8-year-olds in the use of phonemic coding strategies. A second experiment with shorter lists and more children supported the latter finding. The results were interpreted as demonstrating that the ability to access syllable-internal phonemic structure is a necessary precursor to the development of phonemic coding strategies for working memory, but that the use of that structure for storing words in working memory is a skill that develops independently and later than the ability to access phonemic structure.
We examined whether children modify their perceptual weighting strategies for speech on the basis of the order of segments within a syllable, as adults do. To this end, fricative-vowel (FV) and vowel-fricative (VF) syllables were constructed with synthetic noises from an If I-to-/s I continuum combined with natural lal and lui portions with transitions appropriate for a preceding or a following IfI or Is/. Stimuli were played in their original order to adults and children (ages of 7 and 5 years) in Experiment 1 and in reversed order in Experiment 2.The results for adults and, to a lesser extent, those for 7-year-olds replicated earlier results showing that adults assign different perceptual weights to acoustic properties, depending on segmental order. In contrast, results for 5-year-oldssuggested that these listeners applied the same strategies during fricative labeling, regardless of segmental order. Thus, the flexibility to modifyperceptual weighting strategies for speech according to segmental order apparently emerges with experience.
Weexamined the perceptual weighting by children and adults of the acoustic properties specifying complete closure of the vocal tract following a syllable-initial [s], Experiment 1 was a novel manipulation of previously examined acoustic properties (duration of a silent gap and first formant transition) and showed that children weight the first formant transition more than adults. Experiment 2, an acoustic analysis of naturally produced say and stay, revealed that, contrary to expectations, a burst can be present in stay and that first formant transitions do not necessarily distinguish say and stay in natural tokens. Experiment 3 manipulated natural speech portions to create stimuli that varied primarily in the duration of the silent gap and in the presence or absence of a stop burst, and showed that children weight these stop bursts less than adults. Taken together, the perception experiments support claims that children integrate multiple acoustic properties as adults do, but that they weight dynamic properties of the signal more than adults and weight static properties less.Phonetic perception requires the integration of multiple acoustic properties from across the spectral and temporal domains. Consequently, a change in the setting of one property alters the settings of other properties needed to elicit a specific phonetic decision. These reciprocal relations among acoustic properties, known as "trading relations," have been demonstrated by numerous labeling experiments (e.g., Bailey & Summerfield, 1980;Best, Morrongiello, & Robson, 1981; Dorman, StuddertKennedy, & Raphael, 1977;Fitch, Halwes, Erickson, & Liberman, 1980;Mann & Repp, 1980;Repp, 1982;Repp, Liberman, Eccardt, & Pesetsky, 1978;Summerfield & Haggard, 1977). Strictly speaking, demonstrating that a trading relation holds between acoustic properties for a phonetic category does not imply "perceptual equivalence" among (or between) those acoustic properties.' That is, stimuli that have different combinations of parameter settings across properties could elicit the same category response from listeners while remaining perceptually discriminable, especially given that typical labeling experiments employ only two category labels.However, several experiments have demonstrated perceptual equivalence among acoustic properties known to trade. Both Fitch et al. (1980) and Best et al. (1981) showed that longer durations of silence between an [s] noise and a vocalic portion were needed for adults to assign [s] +stop This work was supported by Research Grant 5 R01 DC 00633 to S.N. We thank Nancy Luke and John Schneider for help running subjects and Catherine Best, Joanne Miller, Donna Neff, Michael StuddertKennedy, and two anonymous reviewers for helpful comments on an earlier draft of this paper. Correspondence should be addressed to S. Nittrouer, Boys Town National Research Hospital, 555 North 30th St., Omaha, NE 68131 (e-mail: nittrouer@boystown.org). 51labels to stimuli when formant frequencies at voicing onset were high rather than low. That is, these two ...
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