Asymmetries in the timing of facial and vocal expressions by rhesus monkeys: implications for hemispheric specialization

Research on fish, amphibians, birds and mammals now provides considerable evidence of both anatomical and motor asymmetries. Most of this work focuses on motor actions related to handling objects or moving towards or away from them. Considerably less work has been conducted on motor actions associated with social interactions such as facial and vocal expressions. In this paper we present analyses of timing asymmetries for rhesus monkey, Macaca mulatta, facial and vocal expressions. We collected video records of semifree-ranging adult and infant rhesus monkeys during a variety of social interactions, and subsequently subjected them to frame-by-frame analyses. Results of these analyses revealed that both facial and vocal expressions are consistently produced by adults and infants with a timing asymmetry, with one side opening earlier than the other. Specifically, for both adults and infants, the left side of the face initiates the expression before the right, thereby implicating right hemisphere dominance. Because some expressions are related to positive/approach emotions while others are associated with negative/withdrawal emotion, emotional valence does not appear to influence the direction of this motor asymmetry. Results are discussed in light of potential differences between the mechanisms underlying the production and perception of communicative expressions in human and nonhuman primates.