Regulation of Voice Amplitude by the Monkey (Macaca)

Abstract

Increasing voice amplitude in the presence of noise has been referred to as the “Lombard reflex.” Lane [J. Speech Hearing Res. 14, 677–709 (1971)] believes that the response is not reflexive, but that speakers learn to take their audience into consideration by raising their voices in noise. In this experiment, we determined the effect of noise on monkey vocalizations, which have long been considered to be primarily affectire in nature and con- trolled by subcortical limbic structures [Robinson, Physiol. Behavior 2, 345–354 (1967)]. Monkeys were trained by oper- ant conditioning procedures to vocalize for food reinforce- ment. Our results showed that the monkeys increased their voice level under a masking noise band of 200–500 Hz (funda- mental frequencies for these monkeys ranged between 250 and 400 Hz. The slope of the function relating vocal intensity to external noise level was approximately 0.2, comparable to our own data from humans. The monkeys' voice amplitude was not noticeably affected by a high-frequency nonmasking noise band of 8–16 kHz. Thus both humans and monkeys ap- pear to have a similar self-regulatory system of voice control in masking noise, despite alleged differences in the neural basis of human speech and monkey calls. [Research supported by grants from NIH and Sigma Xi.]