Lesions of the hippocampus or fornix do not interfere with the relative validity effect on a discrete stimulus or the context

Abstract

Two experiments with rats used an appetitively motivated instrumental discrimination procedure to examine whether damage to the hippocampal system would interfere with the relative validity effect in which a partially reinforced stimulus trained in compound acquires a weak conditioned response when concurrently trained CSs are perfect predictors of the presence or absence of the US [A.R. Wagner, F.A. Logan, K. Haberlandt, T. Price, Stimulus selection in animal discrimination learning, J. Exp. Psychol. 76 (1968) 171–180]. The true discrimination (TD) group received training with two compound cues containing a common element. One compound was reinforced and another never reinforced ( AX +, BX − ). Following TD training the common element ( X) failed to acquire a strong conditioned response in comparison to a control, pseudo discrimination (PD) group ( AX ±, BX ±), in which both compound cues were reinforced 50% of the time. Although ( X) was reinforced on a partial reinforcement schedule, (50%) in both groups, conditioning of X was affected by the reinforcement schedules of the accompanying elements ( A and B). Neither radio-frequency lesions of the fornix-fimbria (Experiment 1) nor neurotoxic lesions of the hippocampus (Experiment 2) interfered with this effect on X or on the same effect found during the inter trial interval (ITI). Rates of ITI responding were higher in the PD groups than in the TD groups suggesting stronger contextual conditioning following PD training. Furthermore, disruptions to the hippocampal system in Experiment 2 resulted in increased rates of lever pressing for food. These results do not support the notion that the hippocampus is critically involved in selective associations.