Adaptation to auditory motion in the horizontal plane: effect of prior exposure to motion on motion detectability.

Thresholds for auditory motion detectability were measured in a darkened anechoic chamber while subjects were adapted to horizontally moving sound sources of various velocities. All stimuli were 500-Hz lowpass noises presented at a level of 55 dBA. The threshold measure employed was the minimum audible movement angle (MAMA)--that is, the minimum angle a horizontally moving sound must traverse to be just discriminable from a stationary sound. In an adaptive, two-interval forced-choice procedure, trials occurred every 2-5 sec (Experiment 1) or every 10-12 sec (Experiment 2). Intertrial time was "filled" with exposure to the adaptor--a stimulus that repeatedly traversed the subject's front hemifield at ear level (distance: 1.7 m) at a constant velocity (-150 degrees/sec to +150 degrees/sec) during a run. Average MAMAs in the control condition, in which the adaptor was stationary (0 degrees/sec,) were 2.4 degrees (Experiment 1) and 3.0 degrees (Experiment 2). Three out of 4 subjects in each experiment showed significantly elevated MAMAs (by up to 60%), with some adaptors relative to the control condition. However, there were large intersubject differences in the shape of the MAMA versus adaptor velocity functions. This loss of sensitivity to motion that most subjects show after exposure to moving signals is probably one component underlying the auditory motion aftereffect (Grantham, 1989), in which judgments of the direction of moving sounds are biased in the direction opposite to that of a previously presented adaptor.