Effects of speaking rate on the perceived internal structure of phonetic categories

Abstract

It is well known that when listeners identify consonants as voiced or voiceless on the basis of voice onset time (VOT), they do so in a rate-dependent manner: As rate of speech decreases such that overall syllable duration increases, the category boundary moves toward a longer VOT value. We have been examining whether this alteration in boundary location is accompanied by a change in the specific stimuli judged to be the best category exemplars. For our experiment we created two /bi/-/pi/ series with overall syllable durations of 125 and 325 ms, respectively. VOT varied from 10–120 ms for the 125-ms series and from 10–320 ms for the 325-ms series. We asked listeners to judge each syllable for its goodness as a member of the /p/ category, using a scale from 1–10; the higher the number, the better the exemplar. For each series, as VOT increased there was an initial increase in rating response, followed by a subsequent decline. Thus for both the short and long syllables, the /p/ category was perceived as having internal structure, with a limited range of stimuli serving as the best exemplars. However, the entire function for the longer syllables was shifted toward longer VOT values. That is, as syllable duration increased, so too did the VOT values of the stimuli judged to be the best category members. Moreover, in a subsequent control experiment we established that the shift in rating funtion was not due to range or frequency effects, but to the alteration in syllable duration, per se. These findings strongly suggest that the adjustment for speaking rate, whatever the underlying mechanism, involves a comprehensive modification in the mapping between acoustical signal and phonetic structure.