Positron emission tomography (PET) was used to investigate the functional anatomy of auditory and phonological processing. Stimulus sets were designed to determine areas of the brain significantly activated during speech and nonspeech acoustic processing for stimuli with or without rapidly changing acoustic cues. Performance of auditory target detection tasks using these stimulus sets produced increased activation in superior temporal, frontal opercular, and medial frontal (SMA) cortices, relative to a visual fixation control task. While the medial frontal and superior temporal changes are best explained by motor and sensory components of the task, respectively, the frontal opercular changes were dependent upon the task performed upon the auditory input (mere presentation of the stimuli did not result in significant activation). On the left, the frontal opercular increases were larger when subjects performed an auditory detection task upon stimuli that incorporated rapid temporal changes (words, syllables, and tone sequences) than steady-state vowels. A converging study involving performance of orthographic (ascending letter) and phonological (long vowel sound) word discrimination tasks supports anatomical and behavioral evidence suggesting the left frontal opercular region is important for certain types of auditory/temporal analysis, as well as high-level articulatory coding. In addition to the activation increases associated with performance of auditory target detection tasks, decreases in activation were observed bilaterally along the intraparietal sulcus and superior parietal cortex, in the Rolandic sulcus, and the posterior cingulate; these decreases may reflect an attentional shift away from areas involved in the fixation task during the performance of a difficult auditory task. These results demonstrate that focusing more closely on basic neural processing differences (such as temporal integration rates) may lead to a better understanding of the specific neural processes that underlie complex phonological tasks.
Three experiments investigated anticipatory lingual and labial coarticulation in the [sV] productions of children and adults. Acoustic, perceptual, and video data were used to trace the development of intrasyllabic coarticulation in the speech of adults and children (ages 3, 5, and 8 years). Although children show greater variability in their articulatory patterns than adults, the data do not support claims that young children produce a greater degree of intrasyllabic coarticulation than older children or adults. Rather, the acoustic and video data suggest that young children and adults produce similar patterns of anticipatory coarticulation, and the perceptual data indicate that coarticulatory cues in the speech of 3-year-old children are less perceptible than those of the other age groups.
Recent studies have shown that time-varying changes in formant pattern contribute to the phonetic specification of vowels. This variation could be especially important in children's vowels, because children have higher fundamental frequencies (f0's) than adults, and formant-frequency estimation is generally less reliable when f0 is high. To investigate the contribution of time-varying changes in formant pattern to the identification of children's vowels, three experiments were carried out with natural and synthesized versions of 12 American English vowels spoken by children (ages 7, 5, and 3 years) as well as adult males and females. Experiment 1 showed that (i) vowels generated with a cascade formant synthesizer (with hand-tracked formants) were less accurately identified than natural versions; and (ii) vowels synthesized with steady-state formant frequencies were harder to identify than those which preserved the natural variation in formant pattern over time. The decline in intelligibility was similar across talker groups, and there was no evidence that formant movement plays a greater role in children's vowels compared to adults. Experiment 2 replicated these findings using a semi-automatic formant-tracking algorithm. Experiment 3 showed that the effects of formant movement were the same for vowels synthesized with noise excitation (as in whispered speech) and pulsed excitation (as in voiced speech), although, on average, the whispered vowels were less accurately identified than their voiced counterparts. Taken together, the results indicate that the cues provided by changes in the formant frequencies over time contribute materially to the intelligibility of vowels produced by children and adults, but these time-varying formant frequency cues do not interact with properties of the voicing source.
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