Orienting asymmetries in rhesus monkeys: the effect of time-domain changes on acoustic perception.

Humans exhibit left-hemisphere dominance for processing spoken language, a species-specific acoustic signal characterized by a suite of spectro-temporal parameters. Some nonhuman primates (genus Macaca) also exhibit left-hemisphere dominance for processing their species-specific vocalizations, as evidenced by right-ear biases in orienting and reaction-time studies, and more damaging effects from left- than right-hemisphere lesions. Little, however, is known about the acoustic features underlying such biases. We conducted field playback experiments on adult rhesus monkeys, Macaca mulatta, to determine whether asymmetries in perception (measured as an orienting bias) are sensitive to changes in the temporal characteristics of their calls. If the observed right-ear bias for perceiving conspecific calls (Hauser & Andersson 1994, Proceedings of the National Academy of Sciences, U.S.A., 91, 3946-3948) depends upon particular acoustic parameters, then experimental manipulations beyond the species-typical range of signal variation will cause a change in perceptual asymmetry, either reversing the pattern (i.e. right to left ear ) or wiping it out (i.e. no asymmetry). We presented manipulated and unmanipulated exemplars of three pulsatile call types within the rhesus repertoire: an affiliative signal 'grunt', an alarm signal 'shrill bark', and a mating signal 'copulation scream'. Signal manipulations involved either (1) a reduction of the interpulse interval to zero or the population minimum or (2) an expansion of the interpulse interval to the population maximum, or two times the maximum. For the grunt and shrill bark, manipulations of interpulse interval outside the range of natural variation either eliminated the orienting bias or caused a shift from right- to left-ear bias. For the copulation scream, however, a right-ear bias was observed in response to all stimuli, manipulated and unmanipulated. Results show that for some call types within the repertoire, temporal properties such as interpulse interval provide significant information to listeners about whether the signal is from a conspecific or not. We interpret the orienting bias as evidence that hemispheric asymmetries underly this perceptual effect.Copyright 1998 The Association for the Study of Animal Behaviour.