A phenomenon of particular interest for attentional accounts of perceptual adaptation is categorical perception. According to this phenomenon, people are better able to distinguish between physically different stimuli when the stimuli come from different categories than when they come from the same category (Calder et al, 1996; see Harnad, 1987 for several reviews of research). The effect has been best documented for speech phoneme categories. For example, Liberman, Harris, Hoffman, and Griffith (1957) generated a continuum of equally spaced consonant-vowel syllables going from /be/ to /de/. Observers listened to three sounds -- A followed by B followed by X - and indicated whether X was identical to A or B. Subjects performed the task more accurately when syllables A and B belonged to different phonemic categories than when they were variants of the same phoneme, even when physical differences were equated.
There is evidence that some categorical perception effects are not learned, but are either innate or a property of the acoustical signal itself. Infants as young as 4 months showed categorical perception for speech sounds (Eimas, Siqueland, Jusczyk, & Vigorito, 1971), and even chinchillas (Kuhl & Miller, 1987) and crickets (Wyttenbach, May, & Hoy, 1996) show categorical perception effects for sound.
Still, recent evidence has indicated that sound categories, and categorical perception more generally, is subject to learning (Lively, Logan, & Pisoni, 1993). Whether categorical perception effects are found at particular physical boundaries depends on the listener's language. In general, a sound difference that crosses the boundary between phonemes in a language will be more discriminable to speakers of that language than to speakers of a language in which the sound difference does not cross a phonemic boundary (Repp & Liberman, 1987; Strange & Jenkins, 1978). Laboratory training on the sound categories of a language can produce categorical perception among speakers of a language that does not have these categories (Pisoni, Aslin, Perey, & Hennessy, 1982). Expert musicians, but not novices, show a categorical perception effect for relative pitch differences, suggesting that training was instrumental in sensitizing boundaries between semitones (Burns & Ward, 1978; Zatorre & Halpern, 1979). A visual analog exists; faces for which subjects are "experts," familiar faces, show categorical perception (increased sensitivity to differences at the half-way point between the faces) as one familiar faces is transformed into another familiar face; however, no categorical perception is found for unfamiliar faces (Beale & Keil, 1995).
There are several ways that physical differences between categories might become emphasized relative to within-category differences. In support of the possibility that people lose their ability to make within-category discriminations, very young infants (2 months old) show sensitivity to differences between speech sounds that they lose by the age of 10 mo. (Werker & Lalonde, 1988; Werker & Tees, 1984). This desensitization only occurs if the different sounds come from the same phonetic category of their native language. However, given the difficulty in explicitly instructing infants to respond to physical rather than phonetic differences between sounds, these results should be conservatively interpreted as showing that physical differences that do not make a functional difference to children become perceptually or judgmentally de-emphasized. Laboratory experiments by Goldstone (1994) have suggested that physical differences between categories become emphasized with training. After learning a categorization in which one dimension was relevant and a second dimension was irrelevant, subjects were transferred to same/different judgments ("Are these two squares physically identical?"). Ability to discriminate between stimuli in the same/different judgment task was greater when they varied along dimensions that were relevant during categorization training, and was particularly elevated at the boundary between the categories. Further research showed that category learning systematically distorts the perception of category members by shifting their perceived dimension values away from members of opposing categories (Goldstone, 1995). In sum, there is evidence for three influences of categories on perception: 1) category-relevant dimensions are sensitized, 2) irrelevant variation is de-emphasized, and 3) relevant dimensions are selectively sensitized at the category boundary.
Computational efforts at explaining categorical perception have mainly centered on neural networks. In two such models, equally spaced stimuli along a continuum are associated with category labels, and the networks adapt their input-to-category connections so that the stimuli come to evoke their correct category assignment (Anderson, Silverstein, Ritz, & Jones, 1977; Harnad, Hanson, & Lubin, 1995). In effect, the category feedback establishes attractor states that pull the different members of a category to a common point, thereby reducing their distinctiveness.