A Hippocampal Mechanism for Novelty Testing and Decision About Execution of a Response

Abstract

The activities of CA3, CA1 hippocampal pyramidal neurons and dentate fascia granule cells have been addressed during various experimental conditions to observe some bioelectric processes which a new or recurrent stimulus may evoke. In perforate path transsected cats stimulation of medial septal neurons resulted in increased burst activity of CA3 neurons. This outlasted the stimulus and was paralleled by activity of CA1(1) (stimulus unspecific) pyramidals. During stimulation of both the medial septal neurons and the entorhinal cortex (perforate path neurons) CA3 pyramidals responded with burst activity that usually decreased and was mirrored by CA1(1) activity. Activity of CA1(2) (stimulus modality specific neurons) increased during decrease of CA3 burst activity. During the earliest phases of attempted conditioning of the nictitating membrane of rabbits with an unusual light stimulus CA3 and CA1(1) burst activities increased. After completion of conditioning the CA3 and CA1(1) bursts significantly decreased after an initial increase and the CA1(2) activity increased during the decline of the CA3 activity. It was concluded that the entorhinal cortex (perforate path) carries codes of pre-existent comparable stimuli. These codes are compared in the dentate fascia and also in the CA3 pyramidals. The dentate fascia codes most of the match or mismatch between the MS and the ER-carried stimulus codes. These codes result in the CA3 pyramidals either in increased burst activity (mismatch, novelty value) that initiates in the CA1(1) (stimulus unspecific) pyramidals a code to be destined for incorporation into higher memory systems. In instances of match (not new stimulus) the interference pattern (or attenuation, etc.) generated through the interaction of the two codes is insufficient to maintain CA3 burst activity but results in increased activity of the CA1(2) (stimulus modality specific) pyramidals that program a decision for selection of the executory mechanisms to create a stimulus-adequate response. Encoding (graded EPSPs and dendritic spikes) depend on Ca gating.