CATEGORICAL PERCEPTION

The identification of phonemes by reference to sharply demarcated categories which are consistent across listeners. The example usually given involves distinguishing plosives such as /b/ and /p/. For the listener, the chief difference between the sequences /ba/ and /pa/is that, in the first, the onset of voicing for the /a/ begins very close to the ‘burst’ of the plosive, whereas in the second there is a short delay before voicing occurs. By manipulating this Voice Onset Time (VOT) using a computer, one can produce recordings where it increases in increments of 5 milliseconds. These might range from an exemplar with immediate voice onset which is clearly /ba/ to one with a 60 milliseconds VOT delay which is clearly /pa/.

 Tests have shown that English listeners are consistent in the point at which they switch their interpretation from ba to pa; it is always around the 25–30 millisecond mark. Presented with two exemplars which fall within the /ba/ range (from 0 to 25 milliseconds), they find it difficult to say whether they are the same or different. They experience similar difficulty in distinguishing two exemplars within the /pa/ range with VOTs of more than 30 milliseconds. But they have no problem in distinguishing an exemplar which falls on one side of the 25 millisecond divide from one which falls on the other– even when the VOT difference between the two is quite small. Hence the conclusion that the distinction between /b/ and /p/ is dependent upon sharply delimited category boundaries. Perception along the ba pa continuum is said to be discontinuous. (This finding is not unchallenged. Some experimenters have suggested that listeners are able to rate a stimulus as a good or bad example within a phoneme category.)

The VOT results with ba and pa have been replicated with other plosives in both initial and final position and with fricatives in final position. Similar tests have shown categorical perception in distinguishing place of articulation for plosives, fricatives, nasals and liquids. As for vowels, there is some evidence that those of short duration may be perceived categorically, but that longer ones are not. It would appear that perception along a vowel-vowel continuum is generally continuous, with one value shading into another. This is unsurprising, given the considerable acoustic diversity between realisations of the same vowel, from speaker to speaker and from context to context.

Using the high-amplitude sucking procedure, researchers obtained evidence of categorical perception in infants as young as 1–4 months old. The infants were shown to apply categorical distinctions in relation to both VOT and place of articulation. This provided possible evidence of some kind of innate capacity for discriminating the sounds of speech. However, it was then discovered that chinchillas apply the same kind of categorical distinctions as human beings. One conclusion is that the categorical boundaries identified in these experiments represent points in the speech range to which the mammalian ear is particularly sensitive. The implication would be that language has shaped itself to take advantage of these sensitivities.

Evidence supporting this hypothesis comes from Thai, which divides the b-p continuum three ways, between /b/, /p/ and /ph/. The Thai /b/ is identifiable by an onset of voicing which occurs at least 20 milliseconds before the plosive ‘burst’ while the boundary between /p/ and /ph/ falls around 25–30 milliseconds after it. Tests have shown that three-year-old children acquiring Thai are much more able to make the distinction between /p/ and /ph/ than that between /b/ and /p/. They appear to be born with a sensitivity to the 25–30 millisecond boundary of other languages, but to acquire at a later stage the boundary which is specific to Thai.

The phonemic categories of an adult are narrowly related to native language: hence, for example, the difficulties of Japanese speakers in distinguishing auditorily between English /l/ and /r/, which are variants of the same sound in Japanese. Studies have attempted to establish the age at which children become so attuned to the categories appropriate to their first language that they can no longer distinguish those of other languages. The findings suggest that the ability to make certain categorical distinctions may be lost as early as 10–12 months of age.

Though categorical perception is sometimes treated as if it were absolute, there is evidence that it is subject to external influences.

Phonetic context. The position of the category boundary between two consonants may shift according to the following vowel. The same synthesised sound may be perceived as [ Ð ] when followed by [a] and as [s] when followed by [u]. A similar effect has even been recorded across word boundaries: along the /t-k/ continuum, there is a shift in favour of perceiving an untypical exemplar as a /t/ after /s/(Christmas t/capes) and as a /k/ after /Ð/(foolish t/capes). 

Lexical constraints. There is some evidence (not universally accepted) that lexical knowledge may affect perceptual boundaries, with subjects more likely to accept a poor exemplar from the /g/–/k/ continuum as a/k/ if it contributes to forming an actual word (KISS) than as a /g/if it entails a non-word (GISS).

Desensitisation. If a listener is exposed repeatedly to the same sound, their acoustic feature detectors tire, and perceptual boundaries become shifted in a way that disfavours the sound being heard. For example, extended repetition of the syllable /ba/ desensitises a listener to the VOT features which characterise /b/, with the result that they manifest a perceptual bias in favour of /p/. This phenomenon is known as selective adaptation.

See also: Phonological development, Speech perception: phoneme variation

Further reading: Jusczyk (1997); Miller (1990); Pickett (1999: Chap. 12); Yeni Komshian (1998: 130–6)

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Dictionaries versus encyclopaedias

The encyclopaedic view of meaning

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Schematic systems

Semantic structure

Conceptual structure

Image schemas and abstract thought

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Image schemas