LEFT-HEMISPHERE DOMINANCE FOR PROCESSING VOCALIZATIONS IN ADULT, BUT NOT INFANT, RHESUS-MONKEYS - FIELD EXPERIMENTS

In humans, the left hemisphere of the brain is dominant for processing language. To assess the evolutionary origins of this neuropsychologic al mechanism, playback experiments were conducted on a large populatio n of free-ranging rhesus monkeys (Macaca mulatta). Playbacks provided an equal opportunity to orient the right or left ear toward the speake r. Results revealed that 61 of 80 adult rhesus favored the right ear ( left hemisphere) when vocalizations from their own repertoire were hea rd but favored the left ear when listening to heterospecific vocalizat ions. In contrast, infants less than a year old showed no perceptual a symmetry for conspecific or heterospecific calls. Thus, like humans, a dult rhesus monkeys also evidence left hemisphere dominance for proces sing species-specific vocalizations. The emergence of such asymmetry, however, may depend on both differential maturation of the two hemisph eres and experience with the species-typical vocal repertoire.