Localization of pure tones by Old World monkeys

Abstract

The acuity of auditory localization in Old World monkeys (Macaca) was psychophysically determined for nine sinusoidal stimuli ranging from 250 Hz to 16 kHz. Monkeys were trained through positive reinforcement operant conditioning procedures to contact a response disk (observing response) initiating a repetitive series of 300-ms pure tones presented from 0° azimuth (standard location). At random intervals the stimulus changed from the standard to one of four comparison locations. The monkey reported the change in azimuth by releasing the response disk. The acuity of localization was assessed through the method of constant stimuli under free-field conditions in an anechoic chamber. The results were consistent with the classic duplex theory of auditory localization. Thresholds for the detection of a change in space decreased from 21.8° at 250 Hz to 3.5° at 1000 Hz, and corresponded to a constant 11° interaural phase difference. The localization of high-frequency tones (above 1000 Hz) was highly variable across frequencies and between subjects, ranging from less than 3° to greater than 20°, and displayed the basic attributes associated with the detection of interaural differences in sound pressure level.