Context independence and phonetic mediation in categorical perception.

Categorical perception, as an ideal situation, is rarely if ever observed in the laboratory. Two separate requirements must be met for categorical perception: (a) Discrimination performance must be phonetically mediated (i.e., it should be predictable from labeling performance), (b) labeling responses must be independent of the stimulus context. To determine the extent to which departures from the ideal are due to failures to meet one or both of these requirements, we used four stimulus continua in same-different (AX) discrimination and labeling tasks: stop-consonant-vowel (CV) syllables, isolated vowels, isolated fricative noises, and nonspeech sounds varying in timbre. We found that perception of CV syllables fell short of the ideal only because of contextual influences on labeling. Neither criterion of categorical perception was met by vowels or fricative noises, but subjects were more prone to phonetic mediation with vowels than with fricative noises, and they showed more context independence with fricative noises than with vowels. Surprisingly, the nonspeech timbre stimuli satisfied both requirements better than either vowels or fricative noises. This findings was attributed to the short duration of our timbre stimuli, which prevented them from sounding vowel-like but at the same time may have prevented stable auditory memory traces.