The development of tonal duration in Mandarin-speaking children.

Developmental changes in suprasegmental tonal duration were investigated in monolingual Mandarin-speaking children. Tone durations were acoustically measured in five- and eight-year-old children and adults. Children's tone duration and variability decreased with age. Five-year-olds produced significantly longer tone durations than adults. Adult-like duration patterns existed in all children: Tone 4 was the shortest and tone 3 the longest. Duration differences between tones 2 and 3 became larger between five- and eight-year-olds. Results suggest a prolonged process of tone development beyond establishing phonological contrasts, which can be viewed as a hybrid of physiological production capacities and perceptual learning for maximal contrastivity.

Testing the double phonemic boundary in bilinguals.

It is widely known that language influences the way speech sounds are categorized. However, categorization of speech sounds by bilinguals is not well understood. There is evidence that bilinguals have different category boundaries than monolinguals, and there is evidence suggesting that bilinguals' phonemic boundaries can shift with language context. This phenomenon has been referred as the double phonemic boundary. In this investigation, the double phonemic boundary is tested in Spanish-English bilinguals (N = 18) and English monolinguals (N = 16). Participants were asked to categorize speech stimuli from a continuum ranging from /ga/ to /ka/ in two language contexts. The results showed phonemic boundary shifts in bilinguals and monolinguals which did not differ across language contexts. However, the magnitude of the phoneme boundary shift was significantly correlated with the level of confidence in using English and Spanish (reading, writing, speaking, and comprehension) for bilinguals, but not for monolinguals. The challenges of testing the double phonemic boundary are discussed, along with the limitations of the methodology used in this study.

On the internal perceptual structure of distinctive features: The [voice] contrast.

Several fixed classification experiments test the hypothesis that F(1), f(0), and closure voicing covary between intervocalic stops contrasting for [voice] because they integrate perceptually. The perceptual property produced by the integration of these acoustic properties was at first predicted to be the presence of low frequency energy in the vicinity of the stop, which is considerable in [+voice] stops but slight in [-voice] stops. Both F(1) and f(0) at the edges of vowels flanking the stop were found to integrate perceptually with the continuation of voicing into the stop, but not to integrate with one another. These results indicate that the perceptually relevant property is instead the continuation of low frequency energy across the vowel-consonant border and not merely the amount of low frequency energy present near the stop. Other experiments establish that neither F(1) nor f(0) at vowel edge integrate perceptually with closure duration, which shows that only auditorily similar properties integrate and not any two properties that reliably covary. Finally, the experiments show that these acoustic properties integrate perceptually (or fail to) in the same way in non-speech analogues as in the original speech. This result indicates that integration arises from the auditory similarity of certain acoustic correlates of the [voice] contrast.

Acoustic and auditory phonetics: the adaptive design of speech sound systems.

Speech perception is remarkably robust. This paper examines how acoustic and auditory properties of vowels and consonants help to ensure intelligibility. First, the source-filter theory of speech production is briefly described, and the relationship between vocal-tract properties and formant patterns is demonstrated for some commonly occurring vowels. Next, two accounts of the structure of preferred sound inventories, quantal theory and dispersion theory, are described and some of their limitations are noted. Finally, it is suggested that certain aspects of quantal and dispersion theories can be unified in a principled way so as to achieve reasonable predictive accuracy.

Perception of rhythmic grouping: testing the iambic/trochaic law.

This study was designed to test the iambic/trochaic law, which claims that elements contrasting in duration naturally form rhythmic groupings with final prominence, whereas elements contrasting in intensity form groupings with initial prominence. It was also designed to evaluate whether the iambic/trochaic law describes general auditory biases, or whether rhythmic grouping is speech or language specific. In two experiments, listeners were presented with sequences of alternating /ga/ syllables or square wave segments that varied in either duration or intensity and were asked to indicate whether they heard a trochaic (i.e., strong-weak) or an iambic (i.e., weak-strong) rhythmic pattern. Experiment 1 provided a validation of the iambic/trochaic law in English-speaking listeners; for both speech and nonspeech stimuli, variations in duration resulted in iambic grouping, whereas variations in intensity resulted in trochaic grouping. In Experiment 2, no significant differences were found between the rhythmic-grouping performances of English- and French-speaking listeners. The speech/ nonspeech and cross-language parallels suggest that the perception of linguistic rhythm relies largely on general auditory mechanisms. The applicability of the iambic/trochaic law to speech segmentation is discussed.

Enhanced contrast for vowels in utterance focus: a cross-language study.

The present study examined several potential distinctiveness-enhancing correlates of vowels produced in utterance focus by talkers of American English, French, and German. These correlates included possible increases in vowel space size, in formant movement within individual vowels, and in duration variance among vowels. Each language group enhanced the distinctiveness of vowels in [+focus] context but used somewhat differing means to achieve this. All three groups used spectral differences, but only German talkers used durational differences, to enhance distinctiveness. The results suggest that the amount of distinctiveness enhancement of a vowel property in [+focus] context is positively related to the between-category variation of that property in [-focus] context. Thus, consistent with the theory of adaptive dispersion, utterance clarity appears to vary directly with information content.

The role of the insular cortex in pitch pattern perception: the effect of linguistic contexts.

Auditory pitch patterns are significant ecological features to which nervous systems have exquisitely adapted. Pitch patterns are found embedded in many contexts, enabling different information-processing goals. Do the psychological functions of pitch patterns determine the neural mechanisms supporting their perception, or do all pitch patterns, regardless of function, engage the same mechanisms? This issue is pursued in the present study by using 150-water positron emission tomography to study brain activations when two subject groups discriminate pitch patterns in their respective native languages, one of which is a tonal language and the other of which is not. In a tonal language, pitch patterns signal lexical meaning. Native Mandarin-speaking and English-speaking listeners discriminated pitch patterns embedded in Mandarin and English words and also passively listened to the same stimuli. When Mandarin listeners discriminated pitch embedded in Mandarin lexical tones, the left anterior insular cortex was the most active. When they discriminated pitch patterns embedded in English words, the homologous area in the right hemisphere activated as it did in English-speaking listeners discriminating pitch patterns embedded in either Mandarin or English words. These results support the view that neural responses to physical acoustic stimuli depend on the function of those stimuli and implicate anterior insular cortex in auditory processing, with the left insular cortex especially responsive to linguistic stimuli.

Auditory discontinuities interact with categorization: implications for speech perception.

Behavioral experiments with infants, adults, and nonhuman animals converge with neurophysiological findings to suggest that there is a discontinuity in auditory processing of stimulus components differing in onset time by about 20 ms. This discontinuity has been implicated as a basis for boundaries between speech categories distinguished by voice onset time (VOT). Here, it is investigated how this discontinuity interacts with the learning of novel perceptual categories. Adult listeners were trained to categorize nonspeech stimuli that mimicked certain temporal properties of VOT stimuli. One group of listeners learned categories with a boundary coincident with the perceptual discontinuity. Another group learned categories defined such that the perceptual discontinuity fell within a category. Listeners in the latter group required significantly more experience to reach criterion categorization performance. Evidence of interactions between the perceptual discontinuity and the learned categories extended to generalization tests as well. It has been hypothesized that languages make use of perceptual discontinuities to promote distinctiveness among sounds within a language inventory. The present data suggest that discontinuities interact with category learning. As such, "learnability" may play a predictive role in selection of language sound inventories.

Speech perception.

This chapter focuses on one of the first steps in comprehending spoken language: How do listeners extract the most fundamental linguistic elements-consonants and vowels, or the distinctive features which compose them-from the acoustic signal? We begin by describing three major theoretical perspectives on the perception of speech. Then we review several lines of research that are relevant to distinguishing these perspectives. The research topics surveyed include categorical perception, phonetic context effects, learning of speech and related nonspeech categories, and the relation between speech perception and production. Finally, we describe challenges facing each of the major theoretical perspectives on speech perception.

Differential brain activation patterns during perception of voice and tone onset time series: a MEG study.

Evoked magnetic fields were recorded from 18 adult volunteers using magnetoencephalography (MEG) during perception of speech stimuli (the endpoints of a voice onset time (VOT) series ranging from /ga/ to /ka/), analogous nonspeech stimuli (the endpoints of a two-tone series varying in relative tone onset time (TOT), and a set of harmonically complex tones varying in pitch. During the early time window (approximately 60 to approximately 130 ms post-stimulus onset), activation of the primary auditory cortex was bilaterally equal in strength for all three tasks. During the middle (approximately 130 to 800 ms) and late (800 to 1400 ms) time windows of the VOT task, activation of the posterior portion of the superior temporal gyrus (STGp) was greater in the left hemisphere than in the right hemisphere, in both group and individual data. These asymmetries were not evident in response to the nonspeech stimuli. Hemispheric asymmetries in a measure of neurophysiological activity in STGp, which includes the supratemporal plane and cortex inside the superior temporal sulcus, may reflect a specialization of association auditory cortex in the left hemisphere for processing speech sounds. Differences in late activation patterns potentially reflect the operation of a postperceptual process (e.g., rehearsal in working memory) that is restricted to speech stimuli.

Perceptual normalization for inter- and intratalker variation in Cantonese level tones.

Inter- and intratalker variation in the production of lexical tones may contribute to acoustic overlap among tone categories. The present study investigated whether such category overlap gives rise to perceptual ambiguity and, if so, whether listeners are able to reduce this ambiguity using contextual information. In the first experiment, native Cantonese-speaking listeners were asked to identify isolated Cantonese level tones produced by 7 talkers. Identification accuracy was significantly higher when the presentation of items was blocked by talker rather mixed across talkers. In the second experiment, listeners identified the final (target) tone of 6-syllable semantically neutral sentences with f0 patterns of the context (i.e., the first 5 syllables) altered. The same target tone was identified differently depending on the context. In the third experiment, the context portions of stimulus sentences from the second experiment were divided into 2 halves, and their f0 patterns were altered independently. In identifying the target tone, listeners relied more heavily on the f0 pattern of the second (last) half of the context. These results are discussed in relation to characteristic inter- and intratalker variations of lexical tones.

Generalizing a neuropsychological model of visual categorization to auditory categorization of vowels.

Twelve male listeners categorized 54 synthetic vowel stimuli that varied in second and third formant frequency on a Bark scale into the American English vowel categories [see text]. A neuropsychologically plausible model of categorization in the visual domain, the Striatal Pattern Classifier (SPC; Ashby & Waldron, 1999), is generalized to the auditory domain and applied separately to the data from each observer. Performance of the SPC is compared with that of the successful Normal A Posteriori Probability model (NAPP; Nearey, 1990; Nearey & Hogan, 1986) of auditory categorization. A version of the SPC that assumed piece-wise linear response region partitions provided a better account of the data than the SPC that assumed linear partitions, and was indistinguishable from a version that assumed quadratic response region partitions. A version of the NAPP model that assumed nonlinear response regions was superior to the NAPP model with linear partitions. The best fitting SPC provided a good account of each observer's data but was outperformed by the best fitting NAPP model. Implications for bridging the gap between the domains of visual and auditory categorization are discussed.

Bayesian natural selection and the evolution of perceptual systems.

In recent years, there has been much interest in characterizing statistical properties of natural stimuli in order to better understand the design of perceptual systems. A fruitful approach has been to compare the processing of natural stimuli in real perceptual systems with that of ideal observers derived within the framework of Bayesian statistical decision theory. While this form of optimization theory has provided a deeper understanding of the information contained in natural stimuli as well as of the computational principles employed in perceptual systems, it does not directly consider the process of natural selection, which is ultimately responsible for design. Here we propose a formal framework for analysing how the statistics of natural stimuli and the process of natural selection interact to determine the design of perceptual systems. The framework consists of two complementary components. The first is a maximum fitness ideal observer, a standard Bayesian ideal observer with a utility function appropriate for natural selection. The second component is a formal version of natural selection based upon Bayesian statistical decision theory. Maximum fitness ideal observers and Bayesian natural selection are demonstrated in several examples. We suggest that the Bayesian approach is appropriate not only for the study of perceptual systems but also for the study of many other systems in biology.

Vowels as Islands of Reliability.

Vowel nuclei of syllables appear to provide a relatively stable (although not stationary) frame of reference for judging consonant events. We offer evidence that reliable consonant identification demands prior or simultaneous evaluation of this "vocalic frame." Listeners were presented a list of /bVs/, /dVs/, and /gVs/ syllables and were instructed to press a response key immediately upon recognizing a particular initial consonant target. Three groups of subjects monitored for /b/, /d/, and /g/, respectively. The test syllables contained 10 English vowels varying substantially in intrinsic duration. Response times to the initial consonants correlated positively with the duration of the following vowels, even when the effect of consonant-vowel formant transition duration was partialed out. The results suggest that consonant recognition is vowel dependent and, specifically, that a certain amount or proportion of the vowel formant trajectory must be evaluated before consonants can be reliably identified.