The role of the Amygdala in the perception of reward

Abstract

This study set out to examine the role of the amygdala in a number of appetitively motivated tasks. Experiment one was a position discrimination task with reversals, which in later reversals involved manipulation of some secondary reinforcers associated with a correct response, and the introduction of a magnitude of reward component. Rats with NMDA-induced amygdala lesions performed at a similar level to shams at the initial discrimination and first three reversals, proceeding to reverse faster than controls in the subsequent three reversals. Manipulation of secondary reinforcers led to an equal and significant decline in performance for both groups, with the lesioned animals retaining their significant superiority in reversal performance. Alteration of the task from a 2 vs 0 pellet discrimination to a 2 vs 1 led to a drastic increase in task difficulty, but both groups completed three reversals and did not differ significantly in performance. Experience of handling the lesioned animals led to the confirmation, in experiment two, that they were significantly more hostile/reactive to handling than shams (using die "blind" ratings of experienced animal handlers). Experiment three attempted to refine die picture of this behavioural change by measuring gross activity levels - no differences between groups were found. The finding of enhanced reversal performance and the absence of a magnitude of reward deficit amongst lesioned animals in experiment one were unanticipated, problematic and demand replication. No strong support was provided for either of the principal contemporary theories of amygdala involvement in secondary reinforcement. Increased reactivity to handling was found to be consistent with a minority of the past literature, and activity levels were as anticipated. It is argued that the notion of "stimulus-reward associations" as an amygdala function is incoherent and unhelpful, and that references to the functions of the amygdale as a whole rather than of subnuclei can be equally misleading.