On the hippocampus and learned conditional responding: effects of aspiration versus ibotenate lesions

To study the possible involvement of the hippocampus in learned conditional responding, rats with aspiration lesions of the hippocampus and others that had the hippocampus removed with ibotenic acid were trained on concurrent Pavlovian conditional and nonconditional discriminations. In agreement with the results reported by Ross et al. (1984), animals that had the hippocampus removed with aspiration were unable to learn the conditional discrimination, but learned the simple nonconditional discrimination without difficulty. In contrast, rats that had the hippocampus removed with ibotenic acid did not differ from controls in learning either discrimination. Furthermore, transfer test results were consistent with the hypothesis that ibotenate-lesioned rats performed conditional operations to solve the conditional discrimination problem. Histological analysis indicated that the ibotenate lesions resulted in the removal of the hippocampus and dentate gyrus with minimal involvement of other structures. In aspiration hippocampal animals there was less overall damage to the hippocampus, but there was also extensive damage to the axons passing in alveus and fimbria, together with bilateral loss of cells in the subicular complex, the cingulate gyrus, and the cortex. Since rats with ibotenate lesions had more complete removal of the hippocampus, yet learned both discriminations, the impaired conditional responding found in aspiration hippocampals must be due to extrahippocampal damage.