L de Hoz | PubMed Health

PubMed1999

Reversible neural inactivation reveals hippocampal participation in several memory processes.

Studies of patients and animals with brain lesions have implicated the hippocampal formation in spatial, declarative/relational and episodic types of memory. These and other types of memory consist of a series of interdependent but potentially dissociable memory processes-encoding, storage, consolidation and retrieval. To identify whether hippocampal activity contributes to these processes independently, we used a novel method of inactivating synaptic transmission using a water-soluble antagonist of AMPA/kainate glutamate receptors. Once calibrated using electrophysiological and two-deoxyglucose techniques in vivo, drug or vehicle was infused chronically or acutely into the dorsal hippocampus of rats at appropriate times during or after training in a water maze. Our findings indicate that hippocampal neural activity is necessary for both encoding and retrieval of spatial memory and for either trace consolidation or long-term storage.

Animals↗

PubMed1998

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.

Acoustic Stimulation↗