AUDITORY APPARENT MOTION IN THE FREE FIELD.

Abstract

The purpose of this investigation was to examine the illusion of auditory apparent motion (AM), and compare it to the visual AM function. Visual explanations of this phenomenon rely on a two-process theory, with the spatial separation between the two stimuli determining which process is involved. A pilot experiment examined the role of spatial separation on auditory AM. Subjects were required to listen to a pair of 50 msec. uncorrelated white noise sources, led through two speakers, and separated in time by interstimulus onset intervals (ISOI's) ranging from 0 to 500 msec. The speakers were positioned at one of eleven different locations which varied both in their separation (0-160° azimuth) and distance from the listener (17-34 inches). The subjects classified their experience of the stimulus presentation into one of five response categories. In addition, they were required to report the direction (left or right) of the first-occurring stimulus. Neither the angular separation between the sound sources nor the distance of the sources from the subject had any effect on the range or midpoint of the ISOI's which produce the illusion of motion. In addition, the percentage of correct direction judgements were not affected by the location of the sound sources. The main experiment examined the possibility of perceiving auditory AM in the absence of binaural cues. Six listeners were employed in this experiment, and only three separation (10, 40 and 160°). Each subject was tested at all speaker positions, both with one ear occluded and with both ears open. The results of this experiment indicated that AM can be perceived under monaural listening conditions. Spatial separation did effect the illusion in this condition. As the separation between the sound sources increased, the percentage of motion reports decreased. The detection of direction of the motion was more difficult as the separation decreased in the monaural condition. These results conflict with previous explanations of motion perception in the auditory modality, which rely exclusively on the presence of binaural spatial information. A two process theory of AM is also indicated, but the spatial separation does not determine which mechanism is being employed.