Localization of pure tones by Old World monkeys

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Abstract
The acuity of auditory localization in Old World monkeys (Macaca mulatta) was psychophysically determined for 9 sinusoidal stimuli ranging at 250 Hz-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.degree. azimuth (standard location). At random intervals the stimulus changed from the standard to 1 of 4 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 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.degree. at 250 Hz to 3.5.degree. at 1000 Hz, and corresponded to a constant 11.degree. interaural phase difference. The localization of high-frequency tones (above 1000 Hz) was highly variable across frequencies and between subjects, ranging from < 3.degree. to > 20.degree., and displayed the basic attributes associated with the detection of interaural differences in sound pressure level.

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