The role of frequency modulation in the perception of complex stimuli by primates

Frequency modulation is a common feature of acoustic communication signals, including both human speech and many animal calls. In this study, linear frequency upsweeps were used as simple abstractions of the modulations found in communication signals. Macaque monkeys were trained using positive reinforcement operant conditioning procedures to respond when an ongoing repetitive acoustic signal changed from unmodulated (pure tone) to modulated (sweep). Thresholds for detecting modulation were determined using the psychophysical method of constant stimuli. In the first experiment, it was shown that the monkeys were most sensitive to modulation around a center frequency of 500 Hz. Subsequent experiments were carried out at that frequency, and varied stimulus duration and the frequency relationship between standard and comparison stimuli. The results of these studies indicated that subjects were responding primarily to discrete frequency cues rather than to the presence of modulation. When a premium was placed on attending to modulation by presenting discrete shifts between successive unmodulated stimuli thereby making such shifts an unreliable indicator of the presence of modulation, subjects continued to respond to the presence of the discrete shifts. These results are taken as evidence that the auditory system may deal with frequency modulation near threshold by recoding it as a discrete frequency percept.