Neuroanatomical causes and social consequences of variation in facial motor control: what comparative primatology can teach us about autism spectrum disorders

Abstract

A limited body of evidence suggests that people with severe Autism Spectrum Disorders (ASD) are generally less expressive than mental age‐matched controls. The objective of my talk is to demonstrate how a comparative primatological perspective can help to elucidate the neuroanatomical and social correlates of facial expressivity. I will review research on the evolution of facial nucleus volume in a wide range of nonhuman primates. Previous studies demonstrate that species that have relatively large facial motor nuclei tend to have larger brains, live in larger social groups, and spend more time social grooming than species with relatively small facial nuclei. These observations support the notion that the facial motor nucleus is an essential part of the network of brain regions that is impaired in ASD, i.e., the “social brain.” I will also present new results of a recent pilot study examining the correlation between emotional expressiveness and serotonin transporter genotype (5‐HTTLPR) in free‐ranging rhesus macaques (Macaca mulatta). Our preliminary findings suggest that female macaques with the SS genotype tend to be more expressive overall than LL individuals, even after controlling for dominance rank. This observation calls into question the hypothesis that the S allele is responsible for the relative lack of expressiveness observed in some people with ASD.