Auditory Aftereffects following Simulated Motion Produced by Varying Interaural Intensity or Time

Abstract

Simulated auditory motion, ie step-ramp modulated interaural intensity (delta I) or time (delta t) 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 delta I, but not when it was delta t. Further studies with delta t 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 microseconds for delta I-stimulated and delta t-stimulated motion, respectively). This displacement effect decayed with time after adaptation. The frequency dependence found for delta t 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.