Acquisition of concurrent conditional discriminations in rats with ibotenate lesions of hippocampus and of subiculum

Abstract

Cells in the hippocampus and subiculum of rats were removed with injections of ibotenic acid, and the animals were subsequently trained on two concurrent Pavlovian conditional discriminations. In both discriminations, a stimulus (i.e., a facilitator) signaled the reinforcement of another stimulus (i.e., a target), but the discriminations differed in that one involved an increased memory load for its solution. Both the animals without a hippocampus and those with extensive damage to the subiculum learned the conditional discriminations at the same rate as did controls. Data obtained from transfer and probe trials indicated that discrimination performance was not due to either direct association of reinforcement with unique configural cues or the formation of direct associations between facilitators and reinforcement. The failure to find that removing the hippocampus affects either original learning (present results) or performance of complex Pavlovian conditional discriminations learned before the structure is removed (Davidson & Jarrard, 1989) seem to indicate that the hippocampus is not involved in acquisition, retention, or performance of complex conditional discriminations.