Learning and retention of a visual discrimination task in rats with various combinations of lesions in the temporal-hippocampal region

Abstract

The temporal-hippocampal region appears to be critically involved in cognitive functions. Hippocampal or para-hippocampal lesions have been reported to impair learning and memory. Radical hippocampal lesions may, however, encroach upon the neighboring parahippocampal cortex, and the effects obtained are often attributed to hippocampal dysfunction alone. The present study was undertaken to examine whether damage to neighboring structures along with the hippocampus might have additive disruptive effects on learning and memory. Rats received either selective hippocampal (hippocampus proper, fascia dentata, subiculum) lesions alone or hippocampal lesions (Hipp) combined with damage to the temporal cortex (TC), the lateral entorhinal cortex (LEC), or the fiber connections between TC and LEC. Hipp lesions alone resulted in only impairment of the acquisition of a visual discrimination task, whereas Hipp + LEC lesions and Hipp + TC/LEC lesions produced marked deficits in both acquiring and retaining the same task. Hipp + TC lesions caused a milder impairment of both acquisition and retention. These results suggest that profound effects on learning and memory can be obtained when hippocampal lesions are combined with parahippocampal lesions.