Memory Processing during Sleep Mechanisms and Evidence from Neuroimaging Studies

Abstract

————— A growing number of studies support the hypothesis that sleep participates in the off-line processing of recent memories. However, many determinants and outcomes of memory reprocessing during sleep remain to be identified. This review provides a summary description of the main behavioural, neurophysiological and hemodynamic features of sleep, with a special emphasis on sleep mechanisms deemed potentially important to support sleep-related brain plasticity and memory consolidation: PGO-waves, spindles and hippocampal rhythms. Next are presented brain imaging studies having demonstrated the reexpression and modulation of learning-related cerebral activity during posttraining sleep in humans. As a whole, functional neuroimaging results nowadays suggest that learning-dependent modulations in cerebral activity during human sleep reflect the offline processing of recent memory traces, which eventually leads to the plastic changes underlying the subsequent improvement in performance. Circulating blood in the cardiovascular system, respiring and sleeping have in common to be essential for the integrity and survival of most living organisms including humans. What makes unique the latter is that we have no certainty as to the function(s) supported by sleep, despite an increasing knowledge of its semiology, mechanisms and regulation. Nonetheless, the infinite repetition of sleep episodes, from the first to the last night of one’s life, suggests that sleep houses a series of processes necessary for normal brain function. Among several non-exclusive proposals, the hypothesis that sleep is a favourable period for brain plasticity has received increasing attention in the last 25 years (Maquet, Smith, & Stickgold, 2003). We will deal here with the corollary hypothesis that brain plasticity during sleep participates in the off-line processes of learning and memory consolidation.