Academic journal article Psychonomic Bulletin & Review

The Development of Voicing Categories: A Quantitative Review of over 40 Years of Infant Speech Perception Research

Academic journal article Psychonomic Bulletin & Review

The Development of Voicing Categories: A Quantitative Review of over 40 Years of Infant Speech Perception Research

Article excerpt

Published online: 19 February 2014

© Psychonomic Society, Inc. 2014

Abstract Most research on infant speech categories has relied on measures of discrimination. Such work often employs categorical perception as a linking hypothesis to enable inferences about categorization on the basis of discrimination measures. However, a large number of studies with adults challenge the utility of categorical perception in describing adult speech perception, and this in turn calls into question how to interpret measures of infant speech discrimination. We propose here a parallel channels model of discrimination (built on Pisoni and Tash Perception & Psychophysics, 15(2), 285-290, 1974), which posits that both a noncategorical or veridical encoding of speech cues and category representations can simultaneously contribute to discrimination. This can thus produce categorical perception effects without positing any warping of the acoustic signal, but it also reframes how we think about infant discrimination and development. We test this model by conducting a quantitative review of 20 studies examining infants' discrimination of voice onset time contrasts. This review suggests that within-category discrimination is surprisingly prevalent even in classic studies and that, averaging across studies, discrimination is related to continuous acoustic distance. It also identifies several methodological factors that may mask our ability to see this. Finally, it suggests that infant discrimination may improve over development, contrary to commonly held notion of perceptual narrowing. These results are discussed in terms of theories of speech development that may require such continuous sensitivity.

Keywords Infant · Speech perception · Categorization · Phonological development · Voicing · Meta-analysis · Gradiency · Categorical perception · Discrimination · Development

Introduction

For over 4 decades, an important area of inquiry in language acquisition has been infants' ability to discriminate speech sounds. Researchers have asked how young infants perceive acoustic differences relevant to language (Eimas, Siqueland, Jusczyk, & Vigorito, 1971), whether this ability has an audi- tory basis (Jusczyk, Rosner, Reed, & Kennedy, 1989), when these abilities are tuned to their native language (Werker & Tees, 1984), and what learning mechanisms are involved (Maye, Werker, & Gerken, 2003). These are significant issues, and the field has reached something of a consensus on many of them. Research has shown that even very young infants are sensitive to a range of speech contrasts, including contrasts not used in their native language, and that a shift in this sensitivity occurs between 6 and 12 months as perceptual ability narrows toward the categories of the infant'snativelanguage(Aslin, Werker, & Morgan, 2002; Werker & Curtin, 2005). These findings have led to the view that phonological categories gradually emerge during this period of development (see Gottlieb et al., 1977, for a discussion).

Out of necessity, most empirical work has relied on mea- sures of discrimination, the ability to perceive the difference between sounds in an experimental task (not categorization, the ability to treat different sounds equivalently). Methods like habituation/dishabituation and conditioned head-turning form the bulk of the relevant data, and these are largely measures of how infants discriminate a target stimulus from a baseline stimulus. Yet, the broader interest of this work is phonological development-specifically, the development of speech cate- gories.1 Discrimination in these tasks is largely seen as a proxy for categorization. However, our growing understanding of how people (largely adults) discriminate speech sounds in experimental tasks raises concerns about whether the mapping between discrimination and categorization is isomorphic. The goal this article is to consider this link more carefully and, by doing so, begin to refine our understanding of the develop- ment of speech categories. …

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