Amygdaloid and hippocampal function in short-term retention of a classically conditioned fear response

Abstract

An experiment was performed to investigate the effects of bilateral lesions in the amygdala and dorsal hippocampus on the retention and reconditioning of a classically conditioned fear response. The results of the experiment showed that both the amygdaloid and dorsal hip~ campallesions produced temporary retention deficits that were followed by permanent recovery of function after reconditioning. It was concluded that these areas are not the loci of the memory trace for the classically conditioned fear response. Perhaps more enduring memory deficits would be produced by larger lesions placed in the ventral hippocampus. Horvath (1963) reported retention deficits after amygdalectomy in cats trained on a complex shuttle avoidance task that did not contain a discriminable cue as a background CS. Thatcher and Kimble (1966) found that overtrained rats showed little or no retention deficits of an avoidance response after bilateral amygda­ lectomy, whereas animals given no overtraining showed a marked retention deficit. Goldstein (1975) subsequently demonstrated that rats with bilateral lesions of the basolateral amygdaloid complex could learn the classically conditioned fear response if the shock and safe compartments were made discriminably different. When the amount of fear reduction in the test situation was low, there was a greater performance deficit. The performance of amygda­ lectomized rats could be improved by introducing a discriminative cue into the test situation. The amount of fear reduction in the test situation was, therefore, probably a more important variable than the amygda­ lectomy in contributing to performance of the classi­ cally conditioned fear response. The integrity of the amygdaloid complex is probably not directly involved in the retention of classical fear conditiOning and avoidance conditioning. The amygda­ loid complex may be essential for the temporary motiva­ tional processes involved in fear conditioning and avoid­ ance conditiOning, whereas the hippocampus may be important in processes of longer duration, such as short­ term memory or retention (Uretsky & McCleary, 1969). This hypothesis may be tested by determining whether permanent retention deficits can be demonstrated in hippocampal-Iesioned rats under the conditions most conducive to maximal performance of the classically conditioned fear response (Le., with the addition of the discriminative cue). Demonstration of permanent reten­ tion deficits in such preparations would suggest that the hippocampus rather than the amygdaloid complex is the significant structure in the retention of a classically conditioned fear response. The main purpose of the present experiment was to determine the relative roles of the amygdaloid complex and the hippocampus in the short-term retention of a classically conditioned fear response. The test response used is simple, in the sense that the shock and safe compartments are discriminably different (i.e., the animals are run from a white shock compartment to a black safe compartment). An additional purpose of the experiment was to determine whether, if there is a reten· tion deficit after placement of amygdaloid or hippo­ campal lesions, retraining can restore ~e original func­ tion. If a permanent retention deficit is found for the amygdaloid- or the hippocampal-Iesioned rats, the specific area involved in the deficit may be a locus of the memory trace for the classically conditioned fear response.