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How do brain dynamics support the emergence of new memories, and what network mechanisms underlie the strengthening of those that finally persist? These central questions of neuroscience can be addressed in the case of spatial memories by observing and manipulating the activity of the hippocampus, a circuit that provides the rodent brain with a map-like representation of space. In this talk I will present a series of experiments where we monitored hippocampal network activity in rodents during the acquisition, consolidation and recall stages of spatial memory tasks. First, I will show that during spatial learning of new goal locations, at times when the network is dominated by theta-band (5-12Hz) oscillations, emerging cell assemblies compete for expression against previously established ones. This 'flickering' of assemblies is associated with changes in the firing associations of interneurons to the new pyramidal-cell assemblies. Newly-formed hippocampal representations, however, may degrade with time unless stabilized by additional processes. In another set of experiments we found compelling evidence that hippocampal sleep reactivation during sharp wave/ripple (150-250Hz) events can be primed by midbrain dopaminergic afferents active during learning to further consolidate newly-encoded spatial representations. Our findings highlight the importance of elaborate neuronal coordination between the hippocampus and related circuits during spatial learning to support both the assignment of behavioural experiences to separate neuronal assemblies and their selective persistence.

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