No cognitive processing in the unconscious, anesthetic-like, state of sleep.

Summary

The brain during non-REM sleep is in an unconscious state akin to general anesthesia, making meaningful cognitive processing impossible. Waking experiences are never faithfully reproduced in sleep but appear in distorted forms. Both non-REM sleep and general anesthesia share features such as sensory blockade, immobility, amnesia, and loss of awareness, characterized by delta oscillations across the cortex, reduced neural activity especially in frontal and parietal regions, and disrupted functional connectivity in thalamocortical and corticocortical networks. Disrupting the cortex, particularly the orbitofrontal cortex, impairs higher-order cognitive functions. The profound cortical deactivation in non-REM sleep would negate any possibility of memory processing or consolidation.

Study at a glance

Abstract

We review evidence challenging the hypothesis that memories are processed or consolidated in sleep. We argue that the brain is in an unconscious state in sleep, akin to general anesthesia (GA), and hence is incapable of meaningful cognitive processing-the sole purview of waking consciousness. At minimum, the encoding of memories in sleep would require that waking events are faithfully transferred to and reproduced in sleep. Remarkably, however, this has never been demonstrated, as waking experiences are never truly replicated in sleep but rather appear in very altered or distorted forms. General anesthetics (GAs) exert their effects through endogenous sleep-wake control systems and accordingly GA and sleep share several common features: sensory blockade, immobility, amnesia and lack of awareness (unconsciousness). The loss of consciousness in non-REM (NREM) sleep or to GAs is characterized by: (a) delta oscillations throughout the cortex; (b) marked reductions in neural activity (from waking) over widespread regions of the cortex, most pronounced in frontal and parietal cortices; and (c) a significant disruption of the functional connectivity of thalamocortical and corticocortical networks, particularly those involved in "higher order" cognitive functions. Several (experimental) reports in animals and humans have shown that disrupting the activity of the cortex, particularly the orbitofrontal cortex, severely impairs higher order cognitive and executive functions. The profound and widespread deactivation of the cortex in the unconscious states of NREM sleep or GA would be expected to produce an equivalent, or undoubtedly a much greater, disruptive effect on mnemonic and cognitive functions. In conclusion, we contend that the unconscious, severely altered state of the brain in NREM sleep would negate any possibility of cognitive processing in NREM sleep.