A Rational Account of the Perceptual Magnet Effect

A Rational Account of the Perceptual Magnet Effect Naomi H. Feldman (naomi feldman@brown.edu) Department of Cognitive and Linguistic Sciences, Brown University, Providence, RI 02912 USA Thomas L. Griffiths (tom griffiths@berkeley.edu) Department of Psychology, University of California at Berkeley, Berkeley, CA 94720 USA Abstract types of categorical effects: perceptual space is shrunk near the centers of categories and expanded near category bound- aries. An explanation for the perceptual magnet effect, then, should account for both of these components while still al- lowing for continuous vowel perception in which subjects can discriminate within-category contrasts. Previous computational models of the perceptual magnet effect have attributed various roles to phonetic categories. Guenther and Gjaja (1996) proposed a neural network model in which shrinkage of perceptual space near category cen- ters emerges through an unsupervised learning mechanism trained on specific distributions of speech sounds, with cat- egories playing no explicit role. Similarly, Damper and Har- nad (2000) have argued based on neural models that categor- ical perception is an emergent property of the stimulus con- tinuum. At the other extreme, Lacerda (1995) has proposed a model in which the perceptual magnet effect emerges as a side-effect of a classification problem. The goal of listen- ers is to classify sounds into phonetic categories; perception involves retrieving a set of numerical values that reflect the sound’s similarity to each phonetic category in a language. In this paper we take a novel approach to modeling the per- ceptual magnet effect. In the spirit of Marr (1982) and An- derson (1990), we consider the abstract computational prob- lem posed by speech perception and show that the percep- tual magnet effect emerges as part of the optimal solution to this problem. Specifically, we assume that listeners are op- timally solving the problem of perceiving speech sounds in the presence of noise. Listeners have knowledge of discrete phonetic categories, but their goal in speech perception is to extract phonetic detail in addition to category membership in order to reconstruct coarticulatory and non-linguistic infor- mation. This is a difficult problem for listeners because they cannot hear the speaker’s target production directly. Instead, they hear speech sounds that are similar to the speaker’s target production but that have been altered through articulatory and acoustic noise. We formalize this problem using Bayesian statistics and show that the optimal statistical solution to this problem produces the perceptual magnet effect. The paper is organized as follows. In the next section, we introduce our model of speech perception. The following sec- tion explores the relationship between category membership, perceptual bias, and perceptual distance, laying the back- ground for simulations that provide a quantitative comparison between the model’s behavior and empirical data. Finally, the discussion revisits the model’s assumptions and draws paral- lels to previous models of the perceptual magnet effect. The perceptual magnet effect involves reduced discriminabil- ity near prototypical vowel sounds in the native language. We present a Bayesian model to explain why this reduced discrim- inability might occur: it arises as a consequence of optimally solving the statistical problem of perceiving speech sounds in the presence of noise. In the optimal solution to this prob- lem, listeners’ perception of speech sounds is biased toward the means of phonetic categories because they use knowledge of these categories to guide their inferences about speakers’ target productions. Simulations show that the predictions of the model closely correspond to human data. Keywords: perceptual magnet effect; speech perception It has long been known that categories influence percep- tion, especially in the domain of speech sounds (Liberman, Harris, Hoffman, & Griffith, 1957). Similar categorical ef- fects have been described in other domains, including color perception (Davidoff, Davies, & Roberson, 1999) and ar- tificial categories of objects (Goldstone, Lippa, & Shiffrin, 2001). Despite widespread interest in this phenomenon, the reasons and mechanisms behind the connection between cat- egories and perception remain unclear. Categorical perception, the extreme case in which subjects can discriminate two speech sounds only when they belong to different phonetic categories, has been observed primarily for consonants. The role of phonetic categories in the percep- tion of vowels has been more controversial. Fry, Abramson, Eimas, and Liberman (1962) first noted that vowel perception was much more continuous than consonant perception. More recently, however, Kuhl and colleagues have found evidence of poor discrimination near phonetic category prototypes, a phenomenon they have called the perceptual magnet effect based on the idea that native language prototypes pull neigh- boring speech sounds toward them (Kuhl, Williams, Lacerda, Stevens, & Lindblom, 1992; but see Lotto, Kluender, & Holt, 1998, for an alternative point of view). The effect has been demonstrated in the English /i/ category (Iverson & Kuhl, 1995), the German /i/ category (Diesch, Iverson, Kettermann, & Siebert, 1999), and the Swedish /y/ category (Kuhl et al., 1992). Though the effect remains elusive in other English vowels (Thyer, Hickson, & Dodd, 2000), language-specific shrinking of perceptual space has also been shown near the English /r/ and /l/ prototypes in English but not Japanese speakers (Iverson & Kuhl, 1996; Iverson et al., 2003). While it has been argued that the perceptual magnet effect is a separate phenomenon from categorical perception (Iver- son & Kuhl, 2000), most evidence for the phenomenon has shown characteristics that are qualitatively similar to other