Neuroethological studies of primate social perception

Abstract

The neuroethological basis of social signals was investigated using a multidisciplinary approach, involving connectional and comparative analysis of anatomical data, single cell recording and behavioural techniques. Previous literature implicates the amygdala, anterior temporal and prefrontal cortex in primate social functions. Non-metric multidimensional scaling (NMDS) and cluster analysis were used to analyse the connectional relatedness of macaque cortico-cortical and amygdalo-cortical connections. This objective analysis separated the amygdala nuclei into two groups, the basolateral (BL) and centromedial (CM) complexes. A comparative analysis was made of the possible functions of the amygdala nuclei by correlating amygdala nuclear volume with 5 socio-ecological indices, across 44 primate species. The lateral basal (LB) nucleus and BL size was found to correlate positively with social complexity. CM size correlated negatively. The LB nucleus receives information from the STS, which contains visual neurons responsive to eyes, heads and bodies. These cells were assessed for coding of socially relevant information. Single cell recording localised within the macaque superior temporal sulcus (STS) revealed neurons responsive to specific views, elevations and orientations of the head, eye position, and specific views of bodies walking in specific directions and reaching to objects. The tuning of these neurons could therefore support the function of recognition of another's purposive behaviour (e.g. direction of attention or intention). Visually responsive neurons in the STS also differentiated faces of different species (i.e. monkeys, humans and other animals). Behavioural studies suggest that monkeys do not follow the direction of attention of humans, yet monkeys appear to have the neural capacity. A behavioural study using video stimuli, revealed that monkeys spontaneously follow other monkeys' gaze onto an object or point in space. It is concluded that the amygdala and STS are part of a neural system which enable monkeys to interpret another's gaze and actions within a purposive behavioural framework.