A Unified Model of Categorical Effects in Consonant and Vowel Perception

A Unified Model of Categorical Effects in Consonant and Vowel Perception Yakov Kronrod (yakov@umd.edu) Emily Coppess (ercoppess@umd.edu) Naomi H. Feldman (nhf@umd.edu) Department of Linguistics, 1401 Marie Mount Hall University of Maryland, College Park, MD 20742 Abstract suggests that differences in the degree to which vowels and consonants are perceived categorically can be explained as parametric variations in a single underlying model. Our paper is organized as follows. First, we review evi- dence concerning categorical effects in consonants and vow- els, giving an overview of the types of explanations that have been proposed to account for these data. We then describe the model from Feldman et al. (2009) in detail, focusing on their results for vowel perception. In the subsequent section, we present simulations testing our extended model on two types of consonants, stop consonants and fricatives, to deter- mine whether a model built for vowels can also account for patterns in consonant perception. We conclude by summariz- ing our findings and discussing implications for theories of speech perception. Consonants and vowels differ in the extent to which they are perceived categorically. We use a Bayesian model of speech perception to explore factors that might cause this difference. Simulations show that perception of vowels, fricatives, and stop consonants can all be captured under a single model in which listeners use their knowledge of phonetic categories to infer the sound that a speaker intended. This suggests that the differences in the way we perceive vowels and consonants, when viewed at the computational level, can be explained as parametric variation within a single framework. Keywords: perceptual magnet effect; categorical perception; Bayesian modeling; computational linguistics Phonetic categories influence perception of speech sounds, with stimuli belonging to different categories being easier to discriminate than stimuli from a single category (Liberman, Harris, Hoffman, & Griffith, 1957; Kuhl, 1991). However, different types of sounds differ in the degree to which they are perceived categorically. At one end of the spectrum, perception of stop consonants is strongly categorical. Dis- crimination is little better than would be expected if listen- ers used only category assignments to distinguish sounds, and between-category differences are extremely pronounced (Liberman et al., 1957). At the other end of the spectrum, vowel perception is much more continuous, with some even arguing that vowels display no categorical effects at all (Fry, Abramson, Eimas, & Liberman, 1962). Researchers have used various mechanisms underlying speech perception to explain these differences. For example, differences have been proposed to stem from the way each type of sound is stored in memory (Pisoni, 1973) and to be re- lated to innate auditory discontinuities that seem to influence stop consonant perception (Pisoni, 1977; Eimas, Siqueland, Jusczyk, & Vigorito, 1971). However, the qualitative sim- ilarity of categorical effects in consonants and vowels sug- gests that in some ways these are also instances of the same phenomenon. This raises the possibility that perceptual dif- ferences among different classes of sounds are quantitative rather than qualitative. In this paper we explore these similarities and differences at Marr’s (1982) computational level, looking at the optimal solution to the problem of inferring speakers’ intended pro- ductions from the available acoustic information. We adapt a Bayesian model from Feldman, Griffiths, and Morgan (2009), in which listeners use their knowledge of phonetic categories to recover the sound a speaker intended. We show that an ex- tended version of this model can account for perceptual data from stop consonants and fricatives as well as vowels. This Categorical Effects in Speech Perception Categorical perception in stop consonants was first described by Liberman et al. (1957) as consisting of a sharp change in the identification function between different consonants, as well as a peak in the discrimination function at the location of the identification boundary. The authors proposed a model in which participants used only category assignments to deter- mine whether sounds were the same or different. If the sounds belonged to different categories, then participants would re- spond different; otherwise, they would respond same. By examining participants’ identification functions, Liberman et al. could use this model to predict the extent to which par- ticipants should be able to discriminate each pair of sounds. Participants’ actual discrimination performance exceeded the model’s predictions only by a small amount, and the authors took this as evidence of a strong categorical component in stop consonant perception. Liberman et al.’s experiment fo- cused on stop consonants that differed by place of articula- tion, but similar findings have been obtained along the voice onset time (VOT) dimension as well (Wood, 1976). Descriptions of categorical effects in vowels have focused primarily on the perceptual magnet effect (Kuhl, 1991). This effect was originally proposed as a within-category phe- nomenon, characterized by sounds near category centers be- ing more difficult to discriminate than sounds near cate- gory edges, with an accompanying correlation between good- ness ratings and discriminability. There is disagreement over whether categorical perception and the perceptual magnet ef- fect are separate phenomena or different variants of the same phenomenon (e.g. Lotto, Kluender, & Holt, 1998). Some characteristics of the perceptual magnet effect are similar to