Vertical and horizontal sound localization in primates.

Minimum audible angles for localization in the vertical and horizontal planes were psychophysically determined in Old World monkeys (Macaca). In the vertical condition the test stimuli consisted of primate vocalizations and bands of noise. Minimum audible angles ranged from 3 degrees to greater than 20 degrees for signals of various bandwidths. The acuity of vertical localization was dependent upon the high-frequency content of the signal. Two of the three monkeys were unable to vertically localize sounds if the high-frequency limit of the signal was below 2000 Hz. The acuity of horizontal localization was tested for pure tones 500, 2000, and 8000 Hz in frequency, positioned at several referent locations in the right frontal quadrant. The acuity of horizontal localization decreased as the test azimuth was displaced from the midline. Minimum audible angles ranged from approximately 4 degrees to greater than 20 degrees and varied as a function of test azimuth.