Neurobiology of Learning and Memory

Considerable evidence indicates an important role for amygdaloid nuclei in both the acquisition and expression of Pavlovian fear conditioning. Recent reports from my laboratory have focused on the impact of neurotoxic lesions of the basolateral complex of the amygdala (BLA) on conditional freezing behavior in rats. In these studies, I have observed severe effects of posttraining BLA lesions on the expression of conditional freezing even after extensive presurgical overtraining (25–75 trials). Moreover, I have found no evidence for sparing of fear memory (i.e., savings) in these rats when I assess their rate of reacquisition relative to BLA rats receiving minimal training (1 trial). In these experiments, freezing behavior was assessed using a conventional time-sampling procedure and expressed as a response probability. Although this measure is well established in the literature, it is conceivable that it is not sensitive to spared memory in rats with BLA lesions. To address this issue, I present a more detailed analysis of freezing behavior that quantifies latency to freeze, the number of freezing bouts, the duration of freezing bouts, and the probability distribution of bout lengths. I also include control data from untrained (noshock) rats. Consistent with my earlier reports, I find no evidence of savings of fear memory in rats with neurotoxic BLA lesions using several measures of freezing behavior. These results reiterate the conclusion that fear memory, as it is expressed in freezing behavior, requires neurons in the BLA. q 2001 Academic Press