Learning-dependent neuronal responses recorded from limbic system brain structures during classical conditioning

Abstract

Multiple unit recordings were made from hippocampus and various other limbic system structures during classical conditioning of the rabbit nictitating membrane response. Hippocampal cells increase their firing rate substantially during the very early phases of conditioning—long before behavioral conditioning. A variety of evidence indicates that the hippocampal increase is learning-dependent, suggesting that hippocampal neurons are particularly sensitive to temporal parameters of stimuli used in conditioning. Single unit recordings of physiologically identified hippocampal cells indicate that pyramidal neurons are the cell type exhibiting the increase pattern of activation, while other putative cell types respond in different manners. Recordings from other limbic system brain areas during conditioning show a contrast between activity patterns of the major hippocampal afferents, important similarities between hippocampal and entorhinal cortical within-trial unit patterns of cell activation, and a selective projection of heightened hippocampal discharges along precommissural rather than post-commissurral fornix pathways.