Sleep and Memory

• Inadequate sleep prior to learning produces bi-directional changes in episodic encoding activity, involving the inability of the median temporal lobe to engage normally during learning, combined with potential compensation attempts by prefrontal regions, which in turn may facilitate recruitment of parietal lobe function

• The most profound impact of sleep deprivation is on the encoding of positive emotional stimuli, and to a lesser degree, emotionally neutral stimuli

• The encoding of negative memory appears to be more resistant to the effects of prior sleep loss

• The critical need for sleep before learning in preparing key neural structures for efficient next-day learning

Sleep and Memory Consolidation

• Memories tested after a night of sleep were significantly more resistant to interference

• Following learning of a spatial memory task that was paired with the smell of rose, the reperfusion of the rose scent at night resulted in significantly improved recall the following day and the representation of the odour resulted in greater re-activation of the hippocampus during SWS

Hippocampal-Neocortical Dialogue

• Neocortical structures become the eventual storage site for consolidated episodic memories

• The hippocampus plays a critical role in reactivating these networks, specifically during sleep

• Declarative memories from the day prior are more resistant to interference the next day, due to the increased cortico-cortical connections formed during overnight consolidation

• If the strengthening of cortico-cortical connections takes place during sleep then blocking sleep after hippocampal learning should negate this offline transfer, preventing the development of independence from the hippocampus, and, by doing so, decrease the capacity for new hippocampal learning the next day

• The duration of NREM SWS during the intervening nap correlated positively with later recognition memory performance, yet negatively with retrieval-related activity in the hippocampus

Synaptic Homeostasis Hypothesis

• Sleep regulates the synaptic connectivity of the brain, principally the neocortex

• The magnitude of slow-wave activity of SWS promotes the decrease of synaptic connections

• Learning and memory occurring during wakefulness result in a net increase in synaptic strength

• The role of subsequent SWS is to selectively downscale or depotentiate synaptic strength back to baseline levels, preventing synaptic over-potentiation, which would result in saturated brain plasticity

• The combination of both hypothesis would produce perhaps the most optimal and efficient memory trace

• Sleep deprivation, specifically the prevention of SWS, would also negate effective new learning the next day, due to over-potentiation of synaptic connections

• Any region that exhibits slow wave activity, and is involved in representing memory (hippocampus), would display a corresponding inability to code further information beyond a normal waking duration (16 h in humans)

• The mechanistic benefit of sleep spindles may be related to their faster stimulating frequency; a range suggested to facilitate long-term potentiation and not synaptic depression

Association and Integration

• Sleep can integrate related but novel phonemes into pre-existing long-term lexical memory stores overnight

• Sleep allowed the reinterpretation of prior experience, and supported the abstraction of commonalities, the ability to detect a general pattern in new information

• Human memory integration takes time to develop, requiring slow, offline associative processes

• A supplementary benefit of sleep-dependent memory association may be the improved efficiency of memory storage, in addition to a more generalized representation

• Forgetting individual items is the price we pay for remembering general rules

Creativity

• The highly associative properties of the REM sleep brain

• There is not a concrete episodic replay of daytime experiences, but instead, a much more associative process of semantic integration during sleep

• Sleep may offer the ability to test and build common informational schemas of knowledge, providing increasingly accurate statistic predictions about the world, and allowing for the discovery of novel, even creative next-day solution insights

Emotional Information Processing

• Sleep selectively favours the retention of previously learned emotional texts relative to neutral texts only present following late-night sleep

• The extent of emotional memory improvement is associated with specific REM sleep characteristics such as quantity and quality

Sleep to Forget and Sleep to Remember Hypothesis

• The strength of the memory (hippocampal-associated activity) remains at later recollection, yet the associated emotional reactivity to these items (amygdala activity) is reduced over time

• We sleep to forget the emotional tone, yet sleep to remember the tagged memory of that episode

• If the process of divorcing emotion from memory is not achieved across the first night following an emotional event, a repeat attempt would occur on the second night, since the strength of emotional ‘tag’ associated with the memory would remain high. Should this process fail a second time, the same events would continue to repeat across ensuing nights, potentially with an increasing progressive amount of REM in response

• PTSD patients continue to display hyperarousal reactions to associated trauma cues, indicating that the process of separating the affective tone from the emotional experience has not been accomplished