Hippocampal Lesions in Rhesus Monkeys Disrupt Emotional Responses but Not Reinforcer Devaluation Effects

Background: Although the role of the hippocampus in emotional behavior has long been recognized, the extent to which the hippocampus plays a role in the regulation and expression of emotion in rhesus monkeys has not been systematically explored. Methods: Rhesus monkeys (Macaca mulatta) with excitotoxic lesions of the hippocampal formation and unoperated control animals were assessed on two different types of emotional processing: defensive reactions to a potential predator (experiment 1) and ability to update the value of positive reinforcers, in this case food (experiment 2). Monkeys with aspiration lesions of the perirhinal cortex were also included in this study as an operated control group. Results: In experiment 1, whereas both operated groups showed reduced latencies to retrieve food located near an innately fear-provoking stimulus, a fake snake, only monkeys with hippocampal lesions displayed reduced defensive reactions to the snake. In experiment 2, both operated groups performed as well as control animals when choosing objects flexibly based on the current value of a food. Conclusions: These findings dissociate the hippocampus and perirhinal cortex in fear expression and specifically implicate the hippocampal formation in generating responses to stimuli that are potentially threatening.