FIMBRIA-FORNIX CUT AFFECTS SPONTANEOUS ACTIVITY, 2-WAY AVOIDANCE AND DELAYED NON MATCHING-TO-SAMPLE, BUT NOT LATENT INHIBITION

Latent inhibition (LI) consists of a decrement in conditioning to a st imulus as a result of its prior nonreinforced preexposure. Based on ev idence pointing to the involvement of the hippocampus and the nucleus accumbens (NAC) in LI disruption, it has been proposed that LI depends on the integrity of the subicular input to the NAG. Since fibers orig inating in the subiculum and destined for the NAC run through the fimb ria-fornix, we assessed the effects of fimbria-fornix lesion, made usi ng a knife cut, on LI. In addition, we assessed the effects of the fim bria-fornix cut in three tests known to be sensitive to lesions to the hippocampal region, namely, spontaneous activity, two-way active avoi dance and delayed-non-matching-to-sample. In accord with previously do cumented effects of lesions to the hippocampus and related structures, the fimbria-fornix cut increased spontaneous activity (Experiment 1), facilitated the acquisition of two-way active avoidance (Experiment 3 ), and produced a delay-dependent deficit in the delayed-non-match-to- sample task (Experiment 4), demonstrating that it disrupted hippocampa l functioning. In contrast, LI remained unaffected by the fimbria-forn ix cut (Experiment 2), indicating that disruption of subicular input t o the NAC is not responsible for the attenuation of LI following non-s elective hippocampal lesions. The implications of these results for th e neural circuitry of LI are discussed. (C) 1998 Elsevier Science B.V. All rights reserved.