Hippocampal-cortical interactions and the dynamics of memory trace reactivation

The formation of memory and extraction of knowledge from it is the basis of intelligence. It is believed that, during slow-wave sleep, the brain reorganizes its connectivity matrix so as to store new information optimally. As the probability of direct synaptic connection between arbitrarily chosen neurons in the cortex is extremely low (on the order of 10(- 6)), a combination of modular and hierarchical organization appears to be necessary to enable rapid association of arbitrary items. During waking, an "index" of the neural pattern in lower order cortical modules may be created and stored in the highest order association cortex, the hippocampus, and broadcast back to the relevant cortical modules, where it is stored with the local data. In this manner, the pattern can be spontaneously reactivated and reinstated in all modules to enable the establishment of crossmodular connections, and replay of such patterns of neural activity or "phase sequences" has been observed in hippocampus and neocortex. In prefrontal cortex, the playback of "phase sequences" is associated with periods of intense upstate/downstate transitions and can be accelerated five- to eightfold relative to the waking state. The playback speed declines over time as does the strength of the replay, which is consistent with a simple decay of an asymmetric component of the synaptic weight matrix induced during the experience itself. Since the hippocampal events associated with memory reactivation (sharp-wave ripple events) tend to be correlated with up transitions in the neocortex, hippocampus may coordinate reactivation in neocortex, at least under some conditions.