Abstract
As critical as waking brain function is to learning and memory, an established literature now describes an equally important yet complementary role for sleep in information processing. This overview examines the specific contribution of sleep to human hippocampal memory processing; both the detriments caused by a lack of sleep, and conversely, the proactive benefits that develop following the presence of sleep. First, a role for sleep before learning is discussed, preparing the hippocampus for initial memory encoding. Second, a role for sleep after learning is considered, modulating the post-encoding consolidation of hippocampal-dependent memory. Third, a model is outlined in which these encoding and consolidation operations are symbiotically accomplished, associated with specific NREM sleep physiological oscillations. As a result, the optimal network outcome is achieved: increasing hippocampal independence and hence overnight consolidation, while restoring next-day sparse hippocampal encoding capacity for renewed learning ability upon awakening. Finally, emerging evidence is considered suggesting that, unlike previous conceptions, sleep does not universally consolidate all information. Instead, and based on explicit as well as saliency cues during initial encoding, sleep executes the discriminatory offline consolidation only of select information. Consequently, sleep promotes the targeted strengthening of some memories while actively forgetting others; a proposal with significant theoretical and clinical ramifications.
Highlights
Beyond anecdotal, literary and even lyrical acknowledgments, a rapidly expanding corpus of scientific evidence supports a causal role for sleep in memory processing
SLEEP BEFORE LEARNING FOR MEMORY ENCODING Some of the earliest evidence describing the impact of prior sleep loss on subsequent learning of declarative memories was described by Morris et al (1960), and later by Harrison and Horne (2000), demonstrating impairment in encoding and retention of “temporal memory”
These findings have since been extended by Van Der Werf et al (2009), demonstrating that selective slow wave sleep deprivation alone is sufficient to impair hippocampal memory encoding ability
Summary
Literary and even lyrical acknowledgments, a rapidly expanding corpus of scientific evidence supports a causal role for sleep in memory processing. Pioneering work by Drummond et al (2000) has examined the neural basis of similar memory impairments using fMRI, investigating the effects of 35 h of total sleep deprivation on verbal learning. In those who were sleep-deprived, regions of the temporal lobe were significantly less active during learning, relative to a control group that had slept, while the prefrontal cortex expressed greater activation. The parietal lobes, which were not activated in the control group during learning, were significantly more active in the deprivation group Such findings suggest that sleep loss prior to learning (at least following one night) produces bi-directional changes in verbal encoding activity, involving the inability of the temporal lobe regions to engage normally during learning, combined with potential www.frontiersin.org
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