Regulation of voice amplitude by the monkey.

Old World monkeys (Macaca) were trained to vocalize at a steady rate in the laboratory by the use of operant conditioning techniques with food as a reinforcer. While the animals were vocalizing, they were subjected to one of two noise bands (200–500 Hz or 8–16 kHz) at different sound pressure levels (70, 80, and 90 dB SPL). Vocal amplitude was measured as a function of the SPL of the noise bands. The monkeys increased voice amplitude to the band of low‐frequency noise but not to the high‐frequency band. These results suggest that in monkey, as in man, a relation exists between speaking and hearing: both man and monkey will increase voice amplitude in the presence of masking noise of the appropriate spectral composition. Subject Classification: 65.22; 70.20.