Functional asymmetry of emotions in primates: new findings in chimpanzees

In the past 15 years, there have been a number of studies conducted on asymmetries in the perception and production of facial expressions in human and non-human primates as a means of inferring hemispheric specialization for emotions. We review these studies to assess continuity and discontinuity between species in these emotional processes. We further present new data on asymmetries in the production of facial expressions in a sample of captive chimpanzees. Objective measures (hemimouth length and area) and subjective measures (human judgement's of chimeric stimuli) indicate that chimpanzees' facial expressions are asymmetric, with a greater involvement of the left side of the face (right hemisphere) in the production of emotional responses. Left hemimouth was bigger than the right in the facial expressions of pant-hooting, play, and silent bared-teeth (p < 0.05) and it extended laterally more than the right in the categories of pant-hooting, silent bared-teeth, and scream face (p < 0.05). Human judges also reported that the left side of the faces was emotionally more intense in the case of the play and silent bared-teeth categories (p < 0.01). Thus, chimpanzees, like humans and some other non-human primates, show a right hemisphere specialization for facial expression of emotions, which suggests that this functional asymmetry is homologous in all these species.