LEARNING AND RETENTION OF A VISUAL-DISCRIMINATION TASK IN RATS WITH VARIOUS COMBINATIONS OF LESIONS IN THE TEMPORAL HIPPOCAMPAL REGION

The temporal-hippocampal region appears to be critically involved in c ognitive functions. Hippocampal or parahippocampal lesions have been r eported to impair learning and memory. Radical hippocampal lesions may , however, encroach upon the neighboring parahippocampal cortex, and t he effects obtained are often attributed to hippocampal dysfunction al one. The present study was undertaken to examine whether damage to nei ghboring structures along with the hippocampus might have additive dis ruptive effects on learning and memory. Rats received either selective hippocampal (hippocampus proper, fascia dentata, subiculum) lesions a lone or hippocampal lesions (Hipp) combined with damage to the tempora l cortex (TC), the lateral entorhinal cortex (LEG), or the fiber conne ctions between TC and LEG. Hipp lesions alone resulted in only impairm ent of the acquisition of a visual discrimination task, whereas Hipp LEC lesions and Hipp + TC/LEC lesions produced marked deficits in bot h acquiring and retaining the same task. Hipp + TC lesions caused a mi lder impairment of both acquisition and retention. These results sugge st that profound effects on learning and memory can be obtained when h ippocampal lesions are combined with parahippocampal lesions.