Hippocampal lesions alter conditioning to conditional and contextual stimuli

The control of conditioned fear behaviour by a conditional stimulus (CS) and contextual stimuli (CXT) was compared in rats with lesions to the hippocampus (HPC) or neocortex (CO), and operated controls (OC). After classical fear conditioning in a distinctive context, rats were subsequently tested in the presence of the CS and CXT (CS + CXT), the CS alone (CS-only), or context alone (CXT-only). Two experiments were conducted in which conditioned fear was measured by an active avoidance response (experiment 1) or by response suppression (experiment 2). Groups did not differ in acquiring the conditioned fear response, as measured in the CS + CON test but, in both experiments, hippocampal (HPC) groups exhibited more conditioned fear behaviour than controls in the CXT-Only and CS-Only conditions. It was suggested that control rats conditioned the fear response to a stimulus complex that incorporated the CS and CTX. Rats with HPC lesions did not form this association between the stimulus elements; instead they segregated the CS and CXT and formed independent associations between the conditioned response (CR) and each component. In showing that HPC damage disrupts the process of forming associations between environmental stimuli and that the effect is not restricted to contextual cues, the results help to resolve apparently contradictory findings regarding the role of HPC in contextual information processing.