MEMORY REPROCESSING IN CORTICOCORTICAL AND HIPPOCAMPOCORTICAL NEURONAL ENSEMBLES

Hippocampal cells that fire together during behaviour exhibit enhanced activity correlations during subsequent sleep, with some preservation of temporal order information. Thus, information reflecting experienc es during behaviour is re-expressed in hippocampal circuits during sub sequent 'offline' periods, as postulated by some theories of memory co nsolidation If the hippocampus orchestrates the reinstatement of exper ience-specific activity patterns in the neocortex, as also postulated by such theories, then correlation patterns both within the neocortex and between hippocampus and neocortex should also re-emerge during sle ep. Ensemble recordings were made in the posterior parietal neocortex, in CA1d and simultaneously in both areas, in seven rats. Each session involved an initial sleep episode (S1), behaviour on a simple maze (M ), and subsequent sleep (S2). The ensemble activity-correlation struct ure within and between areas during S2 resembled that of M more closel y than did the correlation pattern of S1. Temporal order (i.e. the asy mmetry of the cross-correlogram) was also preserved within, but not be tween, structures. Thus, traces of recent experience are re-expressed in both hippocampal and neocortical circuits during sleep, and the rep resentations in the two areas tend to correspond to the same experienc e. The poorer preservation of temporal firing biases between neurons i n the different regions may reflect the less direct synaptic coupling between regions than within them. Alternatively, it could result from a shift, between behavioural states, in the relative dominance relatio ns in the corticohippocampal dialogue. Between structure order will be disrupted, for example, if during behaviour, neocortical patterns ten d to drive corresponding hippocampal patterns, whereas during sleep th e reverse occurs. This possibility remains to be investigated.