Sidney I Wiener, LPPA, Collège de France, CNRS, Paris, France
Hippocampal neurons present abstract, cue-invariant representations of behavioural episodes, for example, in the form of ‘place cell’ activity. But how is this activity engaged by other brain structures for instructing behavior and to create long term memory traces? Our recent studies have focussed on the prefrontal cortex (implicated in executive function and memory) and ventral striatum (implicated in goal-directed decision making and habit learning). Simultaneous recordings of ensembles of neurons as well as local field potentials (LFPs) in rats during learning and subsequent sleep reveal that communication between hippocampus and prefrontal cortex is privileged when their respective LFP slow waves oscillate coherently. This leads to the formation of co-active cell assemblies in prefrontal cortex upon new learning. During subsequent sleep the activity that occurred during learning is then replayed in the respective structures, and this occurs in a coordinated manner in synchrony with LFP ‘ripple’ events. This could underlie consolidation of labile experience-related hippocampal traces into long term memories stored in neocortex.