Detection and discrimination of amplitude-modulated signals by macaque monkeys

Behavioral thresholds were determined from macaque monkeys for detection of amplitude modulation (AM) and for discrimination of increases in AM frequency. A positive-reinforcement, go/no-go behavioral paradigm was used with a two-down/one-up psychophysical procedure for all determinations. In the first set of experiments, temporal modulation transfer functions (TMTFs) were determined at two different stimulus levels, for both gated and continuous noise carriers. In the second set of experiments, difference limens for AM frequency were determined using modulated noise and pure-tone carrier signals. TMTFs for gated carriers exhibited a bandpasslike characteristic as has been previously shown. The high-frequency cutoff determined from the average of the gated-carrier TMTFs obtained at 58 dB SPL was 198 Hz, higher than that shown with wideband carriers for other species. With a continuous carrier, there was less of a low-frequency cutoff in the TMTF, again corresponding to previous results. Unlike previous results, however, the present TMTFs showed an effect of stimulus level. Difference limens for AM frequency increased as a function of standard modulation frequency and then leveled off or decreased slightly with further increases in AM frequency. Taken together, the AM discrimination data, coupled with the high cutoff frequency of the TMTF, suggest that detection and discrimination of rapid temporal events may play an important role in the acoustic world of primates.