Memory Reactivation in Rodents

• The firing rates of hippocampal neurons during behaviour tend to be correlated with firing rates of the same neurons during subsequent sleep

• Neurons that fired together during behaviour were more likely to fire together during non-REM sleep

• Replays occur 5–20 times faster than actual events

• Hippocampal replay can be accompanied by cortical replay of the same event

• Manipulating sharp wave ripples such that replay is suppressed (or enhanced) results in impairment (or improvement) in memory consolidation

How Can Sensory Stimulation in Sleep Influence Reactivation

• Targeted Memory Reactivation: manipulate reactivation on demand

• TMR influences what is replayed, but doesn’t increase the total number of replays

• TMR benefits to cued memories come at the cost of weaker non-cued memories

Why Does TMR Bias Replay?

• TMR may effectively trick the hippocampus by evoking what looks like a spontaneous reactivation of a memory in cortex and in turn influence what the hippocampus replays

• Because the hippocampus encodes spatiotemporal context, while other aspects of the memory such as emotional valence and sensory components are encoded in cortex, the reverberation of a replay event helps bind these different components together to form a coherent memory

• A cortical memory representation reactivates during the ‘up’ state of a slow oscillation and triggers a hippocampal replay of the same memory, allowing integration of this information across space and time

• This integrated representation is then sent back to the cortex for a second reactivation, with the repetition of this pattern potentially creating a series of temporally spaced hippocampal-cortical re-activations

• Memory triage is a sleep-dependent process by which memories with perceived future relevance are prioritized for consolidation

• Repeated replay events could gradually decrease the strength of the ‘tag’ associated with a cortical network, thus decreasing that network’s excitability

• Create a prioritization trade-off between how many times a memory has been replayed and its salience relative to other recent memories

• Form a critical feedback mechanism for the memory triage process