AUDITORY AFTEREFFECTS FOLLOWING SIMULATED MOTION PRODUCED BY VARYING INTERAURAL INTENSITY OR TIME

Simulated auditory motion, ie step-ramp modulated interaural intensity (DELTAI) or time (DELTAt) was presented via headphones as an adapting stimulus (narrow-band signal of 1 kHz mean frequency). After adaptation, settings of a stationary test stimulus were systematically shifted in the opposite direction when the experimental parameter was DELTAI, but not when it was DELTAt. Further studies with DELTAt motion with the use of mean frequencies of 100 Hz or 6 kHz showed an aftereffect only at 6 kHz. Unlike visual motion aftereffects, no counter-motion was observed; rather the test stimulus appeared stationary, but settings of its interaural midline were displaced in a direction opposite to the direction of adaptation (on average by 1.2 dB or 30 mus for DELTAI-simulated and DELTAt-simulated motion, respectively). This displacement effect decayed with time after adaptation. The frequency dependence found for DELTAt motion suggests that the low-frequency mechanism of directional hearing that uses interaural ongoing-time (phase) differences is not able to adapt. The observed auditory aftereffects may be analogous to visual motion aftereffects since they are direction specific; however, because they lack apparent motion they also resemble disparity-specific stereoscopic aftereffects.