The Associative Representation of Fear Memories Mediated by the Amygdala.

Abstract

This dissertation investigates how memories for emotional events are represented within the brain. The amygdala is a region of the brain that has been identified as playing a major role in emotional memory. The amygdala contains two subregions, the basolateral complex and the central nucleus, which have been found to be important in the acquisition and expression of emotional memories, respectively. For example, damage to either brain area in rats or humans prevents the acquisition of Pavlovian fear conditioning, a form of emotional learning in which neutral stimuli that do not provoke fear come to do some after being paired with an aversive event.

However, rats with damage to the basolateral amygdala can acquire fear if given sufficient experience (e.g., many conditioning trials) and it appears that the central nucleus acquires fear memories under these conditions. These findings suggest that either the nature of the memories acquired during limited or extensive training are different, which therefore recruits different brain areas, or the amygdala subregions serve similar roles in fear learning. To address this, the author first examined whether the nature of fear memories formed after limited and extensive training are in fact different. Previous work has suggested that early in conditioning the ability of a conditioned stimulus to produce learned responses is based on retrieving a memory of the aversive outcome, but that after extensive training the conditioned stimulus directly retrieves the learned response. The author therefore manipulated the animals’ representation of the aversive outcome after fear conditioning to assess whether fear memories depend on an expectation of an aversive event. Even after extensive training fear memories were dependent on the expectation of an aversive event, suggesting that the nature of learned fear associations does not change as a function of training.

Next, the author investigated whether the nature of learned fear associations mediated by the basolateral complex and central nucleus are similar. Using either permanent or temporary lesions of the amygdala, it was found that both basolateral and central nucleus-dependent memories are affected by post-conditioning changes in the value of the aversive outcome, suggesting that they depend on the