Discrimination of spectrotemporal ripple sweep direction near detection threshold

Abstract

There are single neurons in the auditory systems of some nonhuman animals that are sensitive to spectrotemporal ripples sweeping in one direction versus the other. We used a “2 × 2 forced-choice” simultaneous detection/discrimination paradigm [Nachmias and Weber, Vision Res. 15 217–223 (1975)] to search for behavioral evidence of similar neurons in the human auditory system (i.e., “labeled lines” for discriminating spectrotemporal ripple sweep direction). On each two-interval trial, observers were presented two successive sounds (one in each interval). One sound was an unmodulated noise and the other sound was a noise modulated by an upward-sweeping or downward-sweeping spectrotemporal ripple (±8 Hz/1 cycle/octave). Subsequently, observers registered two separate responses: a detection response (Which interval contained the ripple?) and a discrimination response (Was the ripple sweeping upward or downward?). Psychometric functions (proportion correct versus ripple-modulation depth) were constructed and compared for the two response categories. Labeled line theory predicts that the two functions should be identical: a correct detection response should always yield a correct discrimination response. Contrary to this prediction, observers required greater modulation depths to support discrimination than detection. This finding suggests that the detection of spectrotemporal ripples and the discrimination of spectrotemporal ripple sweep direction may depend on different mechanisms.