Right hemisphere dominance for emotion processing in baboons.

Asymmetries of emotional facial expressions in humans offer reliable indexes to infer brain lateralization and mostly revealed right hemisphere dominance. Studies concerned with oro-facial asymmetries in nonhuman primates largely showed a left-sided asymmetry in chimpanzees, marmosets and macaques. The presence of asymmetrical oro-facial productions was assessed in Olive baboons in order to determine the functional cerebral asymmetries. Two affiliative behaviors (lipsmack, copulation call) and two agonistic ones (screeching, eyebrow-raising) were recorded. For screeching, a strong and significant left hemimouth bias was found, but no significant bias was observed for the other behaviors. These results are discussed in the light of the available literature concerning asymmetrical oro-facial productions in nonhuman primates. In addition, these findings suggest that human hemispheric specialization for emotions has precursors in primate evolution.