Detection of auditory "events" based on amplitude and frequency modulation.

Abstract

These experiments examined the ability of subjects to detect auditory "events" composed of a brief modulation in the temporal center of an otherwise steady sinusoid. In experiment 1, psychometric fluctuations were measured for detecting either amplitude modulation (AM) or frequency modulation (FM) composed of a single cycle of a raised-cosine function (either positive going or negative going); the modulation frequency was 10 Hz, so the event lasted for 100 ms. Then, psychometric functions were measured for stimuli with both AM and FM, using pairs of values of AM and FM that were equally detectable; pairs were always modulated in the same direction, so a positive amplitude excursion went together with a positive frequency excursion. Performance was compared with the "reference" detectability that would be predicted from the optimal combination of independent sources of information. When the AM and FM were synchronous, detectability was better than the reference detectability. When the FM was delayed by 100 ms relative to the AM, detectability was mostly equal to or less than the reference detectability. The better performance with synchronous AM and FM is consistent with an explanation based on an excitation-pattern model. Experiment 2 was similar to experiment 1, except that the event was a single cycle of sinusoidal modulation starting at 0 degree phase or 180 degrees phase. When the AM and FM were synchronous and in phase, detectability was better than the reference detectability. When the AM was delayed by 200 ms relative to the FM or had opposite starting phase, detectability was close to or below the reference detectability. However, for the case where the delay was 100 ms and the modulation had the same starting phase for AM and FM, detectability was better than the reference detectability. A control experiment using two successive cycles of either AM or FM showed a similar, but slightly smaller, effect. It appears that detectability can be enhanced when two successive events form a regular temporal pattern.