Auditory Motion Aftereffects in the Horizontal Plane: The Effects of Spectral Region, Spatial Sector, and Spatial Richness

When an observer is exposed for some seconds to a sound that repeatedly traverses the horizontal plane in one direction, he may experience a motion aftereffect (MAE); that is, a subsequently presented stationary target may be judged to be moving in the opposite direction to that of the original sound [1]. The current series of experiments was designed to measure the spectral and spatial characteristics of the auditory MAE. A virtual mode of stimulus presentation was employed: moving and stationary sounds were initially recorded through the KEMAR manikin's two ears as he was positioned in an anechoic chamber, and these recordings were subsequently presented to subjects through insert earphones as they sat in a darkened sound booth. Five subjects were tested. For each condition of study, the subject was maintained in an adapted state by prolonged presentation of a moving adaptor stimulus, whose velocity was held constant at one of nine values between ±180°/s. Every three seconds, the adaptor was interrupted, and a 1000 ms probe stimulus was presented that was either stationary or slowly moving in either direction. The subject indicated with a button press whether the probe appeared to be moving left or right. Results from experiment 1 showed that the MAE was frequency-specific: a larger MAE resulted when the adaptor and probe had the same spectrum than when their spectra did not overlap. Results from experiment 2 demonstrated that a "multi-contour" adaptor (consisting of three spatially distinct auditory objects moving together) produced a consistently larger MAE than a single-contour adaptor, although this enhancement depended on adaptor and probe having the same spatial characteristics. Results from experiment 3 indicated that the MAE is spatially selective: adaptor and probe must be presented from the same spatial quadrant in order to obtain a significant positive MAE. The spatial and spectral specificity of the auditory MAE as revealed in these experiments suggest that an auditory horizontal-plane motion analysis system may exist as a processing stage following the binaural cross-correlator, and that such a system preserves motion information in independent channels associated with frequency and spatial sector.