Relationship between the summing-localization behavior and perceived width of sound image

Abstract

We examined the relationship between the summing-localization behavior and perceived width of a sound image via two hearing experiments based on relative comparisons of the perceived directions to two sound stimuli. Two types of sound stimuli were employed. One stimulus, the “composite stimulus,” was generated by two sound sources located in horizontal plane with time lag (−3.0 to 3.0 ms) between them. The composite stimulus produced a perception of the summing localization or precedence effect. The other stimulus was generated by a single sound source as the reference for the perceived sound image. The first experiment (Exp. 1) investigated the relationship between the perceived direction of the composite stimulus and the time lag between the sound sources. The second experiment (Exp. 2) was carried out in order to compare the perceived width of the composite stimulus to that of the reference stimulus. The results of Exp. 1 demonstrated that the perceived direction of the composite stimulus smoothly shifted from the middle of the sound sources to that generating the preceding sound, these findings roughly corresponded to the results of the past studies. However, the tendency of the shift was different among the subjects, and could be classified into two types according to whether the shift reached the direction of the preceding source. The results of Exp. 2 showed that the “included range,” defined as the range for which the direction of the reference stimulus is perceived as included within the fused image of the composite stimulus, differed for each subject. The maximum width of the included range was over 30 degrees. From the results of Exps. 1 and 2 taken together, it is clear that there are individual differences with respect to how direction of a stimulus is perceived, within the included range of the fused sound image. This implies that the difference in the answer policy of each subject makes this difference.