VOICE-ONSET TIME AND TONE-ONSET TIME - THE ROLE OF CRITERION-SETTING MECHANISMS IN CATEGORICAL PERCEPTION

Problems in modelling categorical perception (CP) and attempts to appl y signal detection theory (SDT) to CP are reviewed. An approach based on SDT supplemented by a theory of criterion setting is presented. Cri terion setting theory (CST) postulates mechanisms that reset the respo nse criterion on each trial, and it accounts for sequential dependenci es. A criterion setting model for discrimination is shown to fit data from the literature. The hypothesis that ''sharp'' category boundaries may arise from the suppression of noise caused by intertrial dependen cies was examined in an experiment on the identification of [ba] and [ pa] syllables, and tone combinations of varying tone-onset time. Howev er, it was shown that both positive and negative intertrial dependenci es were present. They could be fitted by the criterion-setting model; in this respect, CP resembles standard psychophysical judgements. Exam ination of the psychometric functions from the two CP tasks shows that they are not normal ogives, as in standard psychophysical tasks: thes e curves are steeper centrally and flatter at the extremes than a Gaus sian ogive; we describe them as ''hypersigmoid''. The description of C P identification functions as hypersigmoid provides a new, qualitative characterization of the ''sharp'' category boundaries traditionally c laimed for CP. Their causation remains to be determined.