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.
The acuity of auditory localization in Old World monkeys (Macaca) was determined psychophysically for 44 noise bands graded in bandwidth and center frequency. The acuity of localization was assessed through the method of constant stimuli under free-field conditions in an anechoic chamber. Monkeys were trained through positive-reinforcement operant-conditioning procedures to report a change in azimuth of the signal by releasing a response disk. The results show that localization thresholds are dependent upon the bandwidth of the signal over much of the macaque's range of audibility. Thresholds for the detection of a change in location varied from 18 degrees (for a signal 250 Hz in bandwidth centered at 11 200 Hz) to 4 degrees (for a signal 8000 Hz in bandwidth centered at 8000 Hz). The results suggest that the localization of spectrally complex signals is determined by a mechanism sensitive to periodicity (time-domain) information across the monkey's range of audibility.
Monkeys (Macaca) were trained by operant conditioning techniques to report the minimum detectable change in location of a sound in space, and were tested with a series of recorded coo or clear call vocalizations. Acuity of localization varied from approximately 4 degrees to 15 degrees and was a function of the magnitude of the change in pitch (frequency modulation) of the different clear calls.
Monkeys (Macaca) were trained by operant conditioning techniques to report the minimum discriminable change in the locus of a sound in space. When a monkey made contact with the response key, a pulsed acoustic stimulus was presented from a standard location (zero degrees azimuth). After a variable number of pulses the stimulus changed position from the standard to one of several comparison locations. If the monkey reported this change by releasing the key, it received food as reinforcement. Thresholds for the minimum discriminable change in locus of a sound were determined by the method of constant stimuli. Testing was conducted under free-field conditions in an anechoic chamber. Monkeys were tested with noise bands of various bandwidths. The results indicate an inverse relation between threshold and bandwidth; as bandwidth is decreased the threshold for detection of a change in acoustic locus is increased. [This research was supported by a grant from NSF.]
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