ORIENTING ASYMMETRIES IN RHESUS-MONKEYS - THE EFFECT OF TIME-DOMAIN CHANGES ON ACOUSTIC PERCEPTION

Humans exhibit left-hemisphere dominance for processing spoken languag e, a species-specific acoustic signal characterized by a suite of spec tre-temporal parameters. Some nonhuman primates (genus Macaca) also ex hibit left-hemisphere dominance for processing their species-specific vocalizations, as evidenced by right-ear biases in orienting and react ion-time studies, and more damaging effects from left- than right-hemi sphere lesions. Little, however, is known about the acoustic features underlying such biases, We conducted field playback experiments on adu lt rhesus monkeys, Macaca mulatta, to determine whether asymmetries in perception (measured as an orienting bias) are sensitive to changes i n the temporal characteristics of their calls. If the observed right-e ar bias for perceiving conspecific calls (Hauser & Andersson 1994, Pro ceedings of the National Academy of Sciences, U.S.A., 91, 3946-3948) d epends upon particular acoustic parameters, then experimental manipula tions beyond the species-typical range of signal variation will cause a change in perceptual asymmetry, either reversing the pattern (i.e. r ight to left ear) or wiping it out (i.e. no asymmetry). We presented m anipulated and unmanipulated exemplars of three pulsatile call types w ithin the rhesus repertoire: an affiliative signal 'grunt', an alarm s ignal 'shrill bark', and a mating signal 'copulation scream'. Signal m anipulations involved either (1) a reduction of the interpulse interva l to zero or the population minimum or (2) an expansion of the interpu lse interval to the population maximum, or two times the maximum. For the grunt and shrill bark, manipulations of interpulse interval outsid e the range of natural variation either eliminated the orienting bias or caused a shift from right- to left-ear bias. For the copulation scr eam, 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 interva l provide significant information to listeners about whether the signa l is from a conspecific or not. We interpret the orienting bias as evi dence that hemispheric asymmetries underly this perceptual effect. (C) 1998 the Association for lire Study of Animal Behaviour.