Discrimination of rise time in nonspeech signals: Is it categorical or noncategorical?

Abstract

Previous studies have reported that rise time at the onset of a sawtooth waveform may be discriminated in a categorical‐like manner under some experimental conditions or according to Weber's law under other conditions. In the present experiments, rise time discrimination was examined with both the traditional ABX paradigm and an adaptive tracking procedure. Rise time varied from 0 to 80 ms in 10‐ms intervals at the onset of a one second duration sawtooth signal. Discrimination functions were not categorical in the ABX task. This result was true even for subjects who practiced labeling the endpoint stimuli as “pluck” and “bow” before discrimination. In the adaptive tracking procedure, the Weber fraction obtained from the JND's of rise time was constant above 20‐ms rise time. The results from the two paradigms were then compared by predicting a JND for rise time from ABX discrimination by means of the underlying psychometric function. Using this analysis, the apparently categorical discrimination results from previous studies were shown to be quite similar to the results observed in this study. Our results demonstrate clearly that rise time discrimination follows Weber's law. [Supported by NINCDS grant NS‐12179.]