Dynamic minimum audible angle: binaural spatial acuity with moving sound sources

This study examined in 4 normal-hearing young adults the effects of motion of the sound source upon the accuracy of auditory localization. S controlled the initiation of the energizing of a small loudspeaker on a rotating boom overhead such that the initiation of a brief tonal pulse at .5 kc/s was perceived to be at S's 0 degrees azimuth (a small light was a visual referent). Pulse duration was either a constant 80 msec or a constant arc (19 degrees). Minimum audible angle (m.a.a.) was taken as the standard deviation of a distribution of 10 judgments per set of conditions. A significant increase in m.a.a. occurred at the highest velocity (240 degrees/sec), but at slower velocities the m.a.a. was nearly identical (approximately 1.0 degrees) to that of earlier research using stationary sources. While motion appears to have a minimal effect on localization precision as defined here, large constant errors were observed in the apparent position of the source at onset as function of velocity. With moving sources, Ss experienced an apparent shift of the auditory image in the direction of motion. For constant pulse duration, shifts were 5.2, 5.6, 7.0, and 12.3 degrees, and for constant arc 8.7, 9.2, 9.9, and 11.2 degrees, at velocities of 45, 60, 120, and 240 degrees/sec, respectively. The present results indicated that the thresholds associated with the detection of motion (minimum audible movement angle, m.a.m.a.) and with binaural spatial resolution (m.a.a.) are probably independent. This, in turn, suggests that selectively tuned "motion detectors," analogous to the neurons described in the visual literature, may be present in the auditory system.