Social Monitoring In Rhesus Monkeys with Lesions to Either the Amygdala, Hippocampus, or Orbitofrontal Cortex

Background: Investigation of the social skills of non-human primates in the wild and in the laboratory have revealed that monkeys, like humans, live in social groups characterized by complex and dynamic social organization through a variety of specific, long-term relationships between group members. This inherently social behavior has a neural basis, and indeed, the large cortex of primate brains is considered by some to be the direct result of an increase in social group size.  Although both ablation and neurophysiological recording studies in monkeys have implicated the amygdala and orbitofrontal cortex as two brain structures involved in social behavior, such as the visual monitoring of others, additional experiments are needed to elucidate the neural mechanisms guiding such actions.  Little is known about the role of these brain structures in the typical deployment of skills such as social monitoring.

Objectives: The current project aims to measure and analyze social monitoring mechanisms employed in both typical nonhuman primates and those with lesions to brain structures involved in social behavior. Specifically, we explore whether or not lesions to the amygdala, orbitofrontal cortex, or hippocampus alter social monitoring behavior while non-human primates view video scenes of conspecific interaction.

Methods: Four adult male rhesus monkeys (Macaca mulatta) received selective lesions to either the amygdala, hippocampus, orbitofrontal cortex, or a sham operation.  Eye-tracking data were collected while monkeys watched video scenes of primate social interaction taking place in naturalistic settings.  Eye movement data were analyzed in terms of frequency and duration of fixations as well as frequency, duration, amplitude, and velocity of saccades to control for basic oculomotor function.  Social monitoring was analyzed in terms of percentage of visual fixation time to discrete social behaviors (e.g., particular interactions between monkeys) and to conspecifics in general.

Results: Preliminary analyses suggest that monkeys with lesions to the amygdala, orbitofrontal cortex, and hippocampus each exhibit diminished social monitoring relative to control: lesioned monkeys make fewer saccadic shifts between monkeys and look less at monkeys in peripheral areas of the video scenes.  In contrast, the sham-lesioned monkey scans a larger portion of the video scenes and more frequently monitors the actions of monkeys in the backgrounds of scenes.  Frequency and duration of fixations appears similar across all monkeys.

Conclusions: The hippocampus, amygdala, and orbitofrontal cortex are thought to be part of a neural circuit crucial to associating social salience with perceptual information.  While preliminary, the results of the current study suggest that lesions to each of these regions may impact the ability to recruit the entirety of this circuit in order to successfully execute typical social monitoring.  These results provide avenues for further investigation into the building blocks of primate social behavior.