The Memory Function of Sleep By: Susanne Diekelmann and Jan Born

1. The Memory Function of SleepBy: Susanne Diekelmann and Jan Born Youngjin Kang Alyssa Nolde Toni Sellers

2. 6. Studies on Re-activation • Neuronal activity in specific circuits promotes enduring synaptic changes • It is thought that consolidation after encoding relies on reactivation of neuronal circuits that were active during the initial encoding process

3. Major breakthrough: • Study on rats. Spatiotemporal patterns of neuronal firing that occur in the hippocampus during exploration or simple spatial tasks are reactivated (fire) in the same order • Later shown in humans as well • This occurs mostly during SWS and during the first few hours after the initial learning has occurred • It is always in the order it was experienced • Noisier, less accurate, fast firing rate

4. How reactivation differs between SWS and REM sleep • First discovered: • Study of human learning spatial locations in the presence of a smell • Re-exposure to the odor during SWS, but not REM, enhanced the spatial memories

5. The odors induced stronger hippocampal activation than what was present during wakefulness • This is because during SWS, hippocampal networks are sensitive to inputs that can reactivate that information • It is thought that reactivations during consolidation stimulate redistribution of hippocampal memories to neocortical storage sites • Short term to long term

6. 7.“Synaptic Consolidation” & “System Consolidation” • Synaptic Consolidation. • the enduring synaptic changes that are necessary to stabilize memories. • long-term potentiation (LTP) is considered a key mechanism of synaptic consolidation. • System Consolidation. • Consolidation involves the redistribution of representations between different neuronal systems that is, the fast- and slow-learning stores • For declarative memories, the fast- and slow-learning stores are represented by the hippocampus and neocortex, respectively. “Synaptic consolidation and systems consolidation support each other.”

7. 8. How synaptic consolidation and systems consolidation support each other Dilemma of “Plasticity vs. Stability” “How the brain’s neuronal networks can acquire new information (plasticity) without overriding older knowledge (stability)?” 2. New events are encoded in parallel in both Stores. The two-stage model of memory consolidation 1. (1) Temporary store: learning at a fast rate, serving as an intermediate buffer, holding the information only temporarily; (2) Long-term store: learning at a slower rate and serving as the long-term store 3. The newly encoded memory are repeatedly re-activated in the temporal store with concurrent re-activation in the long term store 4. New memories become gradually redistributed such that representations in the slow-learning, long-term store are strengthened.

8. 9. What is important about LTP and LTD and what is the connection between them and various parts of sleep? • Long-term potentiation (LTP) is considered a key mechanism of synaptic consolidation • It is less clear whether re-activation during sleep promotes the redistribution of memories by inducing new LTP or whether it is because re-activation enhances the maintenance of LTP that occurs during encoding. • LTP can be induced in the hippocampus more reliably than during SWS • Sleep strengthens LTP-like plasticity induced in the neocortex with the use of TMS prior to sleeping

9. 9. What is important about LTP and LTD and what is the connection between them and various parts of sleep? • Stimulation during SWS could induce either LTP or LTD (long-term depression), depending on the pattern of stimulation • These finding suggest that local, off-line (sleep) re-activation of specific glutamatergic circuits supports both LTP induction and maintenance, and the molecular processes underlying synaptic consolidation (strengthening of memory reps. at the synaptic level)

10. 10. What is “global sleep?” What findings about global sleep have taught us about memory consolidation? What have they taught us? • Global sleep: measures in whole-brain or large cortical samples • Global sleep suppresses the molecular signals that mediate LTP-related synaptic remodeling but enhances LTD-related signaling, which seems to be mediated by SWS