Contingent versus incidental context processing during conditioning: dissociation after excitotoxic hippocampal plus dentate gyrus lesions.

This experiment explored whether excitotoxic hippocampus plus dentate gyrus (HPC/DG) lesions in rats would dissociate the differential processing of contextual cues during the performance of learned associations when (1) their processing during training is incidental to successful learning or (2) the solution of a discrimination problem is contingent on their processing. A series of training stages were conducted, beginning with appetitive conditioning to two stimuli (X and Y), each of which was trained in one of two different contexts (operant chambers A and B) (i.e., AX+, BY+). Conditioning was indexed as appetitive responding. The animals were then trained on a biconditional contextual discrimination with these same stimuli (AX+, AY-; BY+, BX-). The next stage involved conditioning trials to two new stimuli (W and Z), one in each context, while the animals were actively discriminating contexts A and B by continuing to perform the original biconditional discrimination (AX+, AY-, AW+; BY+, BX-, BZ+). Finally, they were trained on a second biconditional discrimination involving these new stimuli (AX+, AY-, AW+, AZ-; BY+, BX-, BZ+, BW-). The incidental use of context cues was examined by looking at the rate of conditioned responding to cues X, Y, W, and Z in their original training contexts or a different context; HPC/DG lesioned rats differed from controls in being unaffected by a change of context. The contingent use of context cues was examined by looking at performance of each of the two biconditional tasks; HPC/DG lesioned rats reached levels of conditional performance indistinguishable from those of controls. These findings point to two distinct ways in which contextual information is processed in the brain, revealing a dissociation between incidental and contingent processing of contextual cues after HPC/DG lesions.