Memory is a dynamical phenomenon, from the moment of encoding to retrieval. As shown in the mammalian brain, after encoding labile memories undergo consolidation, that is, they stabilize over time first in the hippocampus before being transferred to the neocortex for long-term storage (Battaglia et al., 2011, Frankland and Bontempi, 2005). In 1989, Gyorgy Buzsaki proposed the twostage model of memory consolidation, where each sequential phase of memory (encoding and consolidation) is related to different internal brain states (Buzsaki, 1989). Encoding would occur during awake hippocampal theta oscillations (5-10 Hz), and consolidation during slow wave sleep, involving reactivations that occur during hippocampal 200 Hz oscillations called the sharp-waves- ripples complexes (SPW-Rs). These reactivations are supposed to be the neuronal mechanism for the consolidation of the memory trace and its transfer to the cortex.
During a previous Post-Doc, Karim Benchenane has provided further results that validate and extend several of these hypotheses. First, he extended the two-stage memory consolidation theory (that was proposed in the sole hippocampus) to the hippocampal-prefrontal network and provided findings supporting the involvement of dopamine in these processes (Benchenane et al., 2010). Second, he identified the causal role of SPW-Rs in memory consolidation by showing that their selective suppression, during sleep following a spatial task, impaired learning (Girardeau et al., 2009). Altogether these results support the role of ripples and reactivation on memory consolidation and suggest a tight link between the reward dopaminergic system and these reactivation processes.
Recently, a more global picture shows that sleep reactivations occur in a coordinate fashion in a brain network including the hippocampus, the cortex and the thalamus. Moreover besides its role in memory consolidation, the coordination in brain rhythms associated with sleep reactivation has been shown to perturbed in model of neuropsychiatric diseases (Phillips et al., 2012). The aim of the team "Memory, Oscillations and Brain states" is to further develop innovative methodologies such as those used in Karim Benchenane's previous publications and to study the role of sleep in memory processes, the regulatory mechanisms of the coordination of oscillatory patterns among brain structures and test the hypothesis that a chronic deregulation of this coordinated network could lead to neuropsychiatric disorders.
First the team will develop a new brain-computer interface to test whether it is possible to create a new artificial spatial memory during sleep. Secondly, the MOBs team will study sleep reactivations in different auditory-related memory tasks that have the great advantage to span all the different forms of memory (sensory memory, short- and long-term memory, declarative/explicit and non-declarative/implicit memory). Finally, the MOBs team will investigate the mechanisms of the coordination between hippocampal and thalamo-cortical oscillations during sleep normal sleep and in pathological situations mimicking neuropsychiatric diseases in rodents.
Altogether, this project will provide new information about the role of sleep oscillatory patterns on memory processes and how their modification impacts cognitive capabilities in pathological situations.