Lateralized use of the mouth in production of vocalizations by marmosets.

We have found that the common marmoset, Callithrix jacchus, displays a larger left hemimouth during production of fear expressions, with or without vocalization, and a larger right hemimouth when producing a social contact call. Thus, marmosets have right hemisphere specialization for the production of negative emotional expressions and vocalizations and left hemisphere specialization for the production of social contact communication. These hemispheric specializations for social communication in marmosets are the same as those found in humans for speech production and for the control of emotional expressions. We suggest that hemispheric specializations for communication in humans may well have precursors in primate evolution.

Eye preferences in common marmosets (Callithrix jacchus): influence of age, stimulus, and hand preference.

Eye preferences of the common marmoset ( Callithrix jacchus ) were examined, taking into account age, arousal, and hand preference. Monocular eye use for looking through a small hole at a stimulus was recorded. Of 21 marmosets, 20 displayed right-eye preferences for viewing a piece of familiar food. In subjects tested at 3-8, 12, 15-18, and 22 months, eye preferences were consistent across age. A group bias, indicative of right-eyedness, was also found for viewing other stimuli. The stimuli included a watch, mirror, model of a beetle, and the experimenter's hand. However, when the marmosets viewed a threatening stimulus, a model resembling two rearing snakes, they displayed increased arousal (indicated by longer duration between monocular viewing events and increased incidence of aroused vocalisations) and the eye preferences shifted away from a preference for the right eye to either no preference or a left-eye preference. No relationship between eye preference and hand preference for holding food was found. Therefore, we suggest that eye preferences may reflect hemispheric specialisations for perceptual processing, according to the emotional valence of the stimulus.

Hand, mouth and eye preferences in the common marmoset (Callithrix jacchus).

Motor and sensory preferences were measured for 8 common marmosets (Callithrix jacchus). Three types of motor function were examined: spontaneous feeding, mouth use when chewing and visuospatial reaching. Eye preference was used as an index of sensory lateralization. The marmosets displayed individual preferences for the performance of the motor tasks, but no group biases were revealed. Correlations between preferences displayed on the motor tests suggest that one hemisphere may control spontaneous feeding and lateralized mouth use while the other controls visuospatial reaching, but no consistent preference was present. In an eye preference test, 7 subjects displayed right-eye preferences, suggesting hemispheric specialisation for this function.