Lateralization of a moving auditory image: interrelation of interaural time and intensity differences.

Abstract

Lateralization of moving fused auditory images (FAIs) was studied under dichotic stimulation, with FAI movement from the right and left ears to midline. The movement was produced by the gradual change of interaural time delay (from +/- 630 to 0 microseconds) in a binaurally presented click train in which a constant interaural intensity difference (IID) between +/- 13 dB was also imposed. The task of the subjects was to show with her/his finger the point on the head surface where the FAI trajectory's ending or starting points were perceived. With IID change within +/- 13 dB, the FAI movement trajectory shifted toward the ear receiving the more intense stimulus. The length of the movement trajectory shortened with IID increase. Functions relating the value of perceived lateral position (Y) of the movement trajectory's ending and starting points to IID value (X) were nearly linear: Y = AX + B. These functions differed in their characteristics whether the movement was to the right versus to the left of midline. They also differed from analogous functions for stationary FAI. At IID = 0 the FAI movement trajectory's endpoint was shifted from midline in the direction of movement. Equivalence ratio for IID and ITD were estimated to be 51 and 29 microseconds/dB respectively for the trajectory's starting and ending points. The IID factor could be several times as effective in moving FAI lateralization as the ITD factor.