SYSTEMATIC COMPARISON OF THE EFFECTS OF HIPPOCAMPAL AND FORNIX-FIMBRIA LESIONS ON ACQUISITION OF 3 CONFIGURAL DISCRIMINATIONS

The effects of lesions to the hippocampal system on acquisition of thr ee different configural tasks by rats were tested. Lesions of either t he hippocampus (kainic acid/colchicine) or fornix-fimbria (radiofreque ncy current) were made before training. After recovery from surgery, r ats were trained to discriminate between simple and compound-configura l cues that signaled the availability or nonavailability of food when a bar was pressed. When positive cues were present, one food pellet co uld be earned by pressing a lever after a variable time had elapsed. T he trial terminated on food delivery (variable interval 15 s). This pr ocedure eliminates some possible alternative explanations of the resul ts of previous experiments on configural learning. Hippocampal lesions increased rates of responding and retarded acquisition of a negative patterning task (A(+), B+, AB(-)); using a ratio measure of discrimina tion performance these lesions had a milder retarding effect on a bico nditional discrimination (AX(+), AY(-), BY+, BX-), and they had no eff ect on a conditional context discrimination (X: A(+), B-; Y: A(-), B+) . Fornix-fimbria lesions did not affect acquisition of any of these ta sks but increased rates of responding. The results suggest that severa l task parameters determine the involvement of the hippocampus in conf igural learning; however, all tasks tested can also be learned to some extent in the absence of an intact hippocampal system, presumably by other learning/memory systems that remain intact following surgery. Th e lack of effect of fornix-fimbria lesions on any of these tasks sugge sts that retrohippocampal connections with other brain al eas may medi ate hippocampal contributions to the learning of some configural tasks . An analysis of these results and of: experiments on spatial learning situations suggests that involvement of the hippocampus is a function of the degree to which correct performance depends on a knowledge of relationships among cues in a situation. (C) 1997 Wiley-Liss, Inc.