MINIMUM AUDIBLE MOVEMENT ANGLE IN THE HORIZONTAL PLANE AS A FUNCTION OF STIMULUS FREQUENCY AND BANDWIDTH, SOURCE AZIMUTH, AND VELOCITY

Minimum audible movement angles (MAMAs) were measured in the horizontal plane for four normal-hearing adult subjects in a darkened anechoic chamber. On each trial, a single stimulus was presented, and the subject had to say whether it came from a stationary loudspeaker or from a loudspeaker that was moving at a constant angular velocity around him. Thresholds were established by adaptively varying stimulus duration. In experiment 1, MAMAs were measured as a function of center frequency (500-5000 Hz), velocity (10-degrees-180-degrees/s), and direction of motion (left versus right). There was no effect of direction of motion. MAMAs increased with velocity, from an average of 8.8-degrees of arc for a target moving at 10-degrees/s to an average of 20.2-degrees of arc for a target moving at 180-degrees/s. MAMAs were higher for a 3000-Hz tone than for tones of lower or higher frequencies, as has been previously reported [D. R. Perrott and J. Tucker, J. Acoust. Soc. Am. 83, 1522-1527 (1988)]. In experiment 2, minimum audible angles (MAAs) were measured with sequentially presented stationary tone pulses (500-5000 Hz), and were shown to exhibit the same dependence on signal frequency that the MAMAs showed (average MAA at 3000 Hz: 8.4-degrees; average MAA at the other frequencies: 3.4-degrees). In experiment 3, MAMAs and MAAs were measured as a function of stimulus bandwidth (centered at 3000 Hz) and listening azimuth (0-degrees vs 60-degrees). Average MAAs decreased monotonically as stimulus bandwidth increased from 0 Hz to wideband (from 8.4-degrees to 1.2-degrees at 0-degrees azimuth; from 11.3-degrees to 1.5-degrees at 60-degrees azimuth). As in experiment 1, MAMAs increased with stimulus velocity, from values comparable to the MAAs for the slowest-velocity (10-degrees/s) targets to 70-degrees of arc or more in the poorest condition (third-octave band of noise presented at a velocity of 180-degrees/s and an azimuth of 60-degrees). MAMAs obtained in the slower-velocity conditions depended in the same way on stimulus bandwidth and listening azimuth that MAAs depended on these variables. In no case was the MAMA ever smaller than the MAA. It is hypothesized that a minimum integration time is required to achieve optimal performance in a dynamic spatial resolution task. Average estimates of this minimum time based on the current data vary from 336 ms (for targets presented at midline) to 1116 ms (for narrow-band targets presented at 60-degrees azimuth). The relatively large magnitudes of these minimum integration times are consistent with the view of the binaural processor as a sluggish system.