Categorical Perception
Categorical perception is involved in processes of perceptual differentiation. People perceive speech sounds categorically, that is to say, they are more likely to notice the differences between categories (phonemes) than within categories. The perceptual space between categories is therefore warped, the centers of categories (or "prototypes") working like a sieve or like magnets for incoming speech sounds.
In an artificial continuum between a voiceless and a voiced bilabial plosive, each new step differs from the preceding one in the amount of VOT. The first sound is a pre-voiced, i.e. it has a negative VOT. Then, increasing the VOT, it reaches zero, i.e. the plosive is a plain unaspirated voiceless . Gradually, adding the same amount of VOT at a time, the plosive is eventually a strongly aspirated voiceless bilabial . (Such a continuum was used in an experiment by Lisker and Abramson in 1970. The sounds they used are available online.) In this continuum of, for example, seven sounds, native English listeners will identify the first three sounds as /b/ and the last three sounds as /p/ with a clear boundary between the two categories. A two-alternative identification (or categorization) test will yield a discontinuous categorization function (see red curve in Figure 4).
In tests of the ability to discriminate between two sounds with varying VOT values but having a constant VOT distance from each other (20 ms for instance), listeners are likely to perform at chance level if both sounds fall within the same category and at nearly 100% level if each sound falls in a different category (see the blue discrimination curve in Figure 4).
The conclusion to make from both the identification and the discrimination test is that listeners will have different sensitivity to the same relative increase in VOT depending on whether or not the boundary between categories was crossed. Similar perceptual adjustment is attested for other acoustic cues as well.
Read more about this topic: Speech Perception, Basics
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