THEORETICAL MODEL OF THE HIPPOCAMPAL-CORTICAL MEMORY SYSTEM MOTIVATED BY PHYSIOLOGICAL FUNCTIONS IN THE HIPPOCAMPUS

Abstract

Electrical stimulation in the hippocampus leads to an increase in synaptic efficacy that lasts for many hours. This long-term potentiation (LTP) of synaptic transmission is presumed to play a crucial role in learning and memory in the brain. Our experimental data on the hippocampus show that the homosynaptic LTP and the associative LTP are highly sensitive to temporal pattern stimuli given by different correlations between successive interstimulus events, even when the mean rate of the stimuli is held constant; negatively correlated stimuli have relatively little LTP, whereas positively correlated stimuli have greater LTP. This suggests that the detailed temporal properties of the stimulus are an important factor in inducing LTP and supports the possibility that temporal codes are used as indexes in associating/dissociating memory events in the hippocampus. Based on the physiological evidence, we propose a hypothesis on how association and dissociation of event memories are done in the hippocampal-cortical memory system. It is postulated that the association/dissociation memory is carried out by indexing the representations of events (memory contents) with temporal codes. The memory contents are supplied from the sensory association cortices, while the temporal codes are supplied from the decision-making/motivation area. The two inputs are mixed (indexing) in the ento-perirhinal area. Indexed signals are fed to hippocampus, where connection or disconnection of memory contents occurs, depending on the kind of index. Finally, association/dissociation of event memories is done in the association cortex according to a covariance rule: two events memories are associated when direct cortico-cortical inputs and indirect inputs from the hippocampus are positively correlated through the consolidation in hippocampus, and they are dissociated when two inputs are negatively correlated as a consequence of the disconnection in the hippocampus.