Detection and discrimination of simulated motion of auditory targets in the horizontal plane

Abstract

Three experiments investigated subjects’ ability to detect and discriminate the simulated horizontal motion of auditory targets in an anechoic environment. ‘‘Moving’’ stimuli were produced by dynamic application of stereophonic balancing algorithms to a two‐loudspeaker system with a 30° separation. All stimuli were 500‐Hz tones. In experiment 1, subjects had to discriminate a left‐to‐right moving stimulus from a stationary stimulus pulsed for the same duration (300 or 600 ms). For both durations, minimum audible ‘‘movement’’ angles (‘‘MAMA’s’’) were on the order of 5° for stimuli presented at 0° azimuth (straight ahead), and increased to greater than 30° for stimuli presented at ±90° azimuth. Experiment 2 further investigated MAMA’s at 0° azimuth, employing two different procedures to track threshold: holding stimulus duration constant (at 100–600 ms) while varying velocity; or holding the velocity constant (at 22°–360°/s) while varying duration. Results from the two procedures agreed with each other and with the MAMA’s determined by Perrott and Musicant for actually moving sound sources [J. Acoust. Soc. Am. 6 2, 1463–1466 (1977b)]: As stimulus duration decreased below 100–150 ms, the MAMA’s increased sharply from 5°–20° or more, indicating that there is some minimum integration time required for subjects to perform optimally in an auditory spatial resolution task. Experiment 3 determined differential ‘‘velocity’’ thresholds employing simulated reference velocities of 0°–150°/s and stimulus durations of 150–600 ms. As with experiments 1 and 2, the data are more easily summarized by considering angular distance than velocity: For a given ‘‘extent of movement’’ of a reference target, about 4°–10° a d d i t i o n a l extent is required for threshold discrimination between two ‘‘moving’’ targets, more or less independently of stimulus duration or reference velocity. These data suggest that for the range of simulated velocities employed in these experiments, subjects respond to spatial changes—not velocity p e r s e—when presented with a ‘‘motion’’ detection or discrimination task.