Limbic networks and associative learning: I. Entorhinal contributions to instrumental conditioning

The hippocampal formation is a highly delineated brain structure that is believed to play a prominent role in learning and memory. The present experiment evaluated the contributions of medial and lateral perforant path input to bar press-conditioning under (a) continuous and (b) differential reinforcement of low rates of responding (DRL) schedules, and (c) shuttlebox avoidance conditioning. Bilateral deafferentation of either pathway had no effect on the acquisition of bar press responses or on performance under the DRL schedule. Deafferentation of the medial pathway facilitated acquisition of avoidance responses in a manner much like the effects seen in hippocampectomized animals. It is suggested that the medial perforant path participates in the expression of correlated patterns of neuronal discharge known to develop within the hippocampus and that this “model” serves to modulate the temporal characteristics of simple conditioned reflexes. Loss of the modulatory influence of the model may affect acquisition and extinction rates. Contributions of other hippocampal circuits are discussed in relation to established deficits. Preliminary results of this experiment were presented at the second annual convention of the American Psychological Society, Dallas, Texas, June 1990.