Historically, the term 'memory consolidation' refers to a process whereby a memory becomes increasingly resistant to interference from competing or disrupting factors with the continued passage of time. Recent findings regarding the learning of skilled sensory and motor tasks ('procedural learning') have refined this definition, suggesting that consolidation can be more strictly determined by time spent in specific brain states such as wake, sleep or certain stages of sleep. There is also renewed interest in the possibility that recalling or 'reactivating' a previously consolidated memory renders it once again fragile and susceptible to interference, therefore requiring periods of reconsolidation. Using a motor skill finger-tapping task, here we provide evidence for at least three different stages of human motor memory processing after initial acquisition. We describe the unique contributions of wake and sleep in the development of different forms of consolidation, and show that waking reactivation can turn a previously consolidated memory back into a labile state requiring subsequent reconsolidation.

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During the sleep cycle in cats, neurons localized to the posterolateral pole of the nucleus locus coeruleus and the nucleus subcoeruleus undergo discharge rate changes that are the opposite of those of the pontine reticular giant cells. The inverse rate ratios and activity curves of these two interconnected populations are compatible with reciprocal interaction as a physiological basis of sleep cycle oscillation.

Performance on a visual discrimination task showed maximal improvement 48-96 hours after initial training, even without intervening practice. When subjects were deprived of sleep for 30 hours after training and then tested after two full nights of recovery sleep, they showed no significant improvement, despite normal levels of alertness. Together with previous findings 11 that subjects show no improvement when retested the same day as training, this demonstrates that sleep within 30 hours of training is absolutely required for improved performance.Skill learning represents one of several classes of procedural memory, and is defined as experience-dependent improvement in performance on perceptual, perceptuomotor or motor tasks 1 . Such learning is one example of memory consolidation 2 . Once a behavioral training session ends, consolidation of learning continues for some time, and manipulations such as drug treatments can either enhance or reverse this consolidation if administered shortly after training 3 .Sleep deprivation hours after training can interfere with consolidation, which suggests involvement of sleep in consolidation 4 , with rapid-eye-movement (REM) sleep 5 and deeper, slow-wave sleep (SWS) 6 subserving distinct functions 7 . In one such procedure 8 , improvement on a visual discrimination task was only observed after several hours 9 ; overnight improvement was blocked by REM deprivation, although REM sleep was concluded to be a permissive rather than obligatory condition for consolidation of this learning 10 . We subsequently showed that improvement on this task only occurs when subjects are tested following a night of sleep, and that this overnight improvement is proportional to the amount of SWS in the first quarter of the night and of REM sleep in the last quarter, with these two sleep parameters explaining 80% of the intersubject variance in improvement 11 . These results suggest that it is the occurrence of sleep, rather than the simple passage of time, that leads to consolidation and improvement on this task. However, these studies were correlative in nature, and did not demonstrate a clear causal link between sleep and improvement. We now report that performance following a single training session improves beyond the first 24 hours, and improves more after a second night of sleep. We show that this improvement is absolutely dependent on the first night of sleep, and that subsequent sleep cannot replace the first night requirement. These findings add to related data from the last three decades, which suggest that sleep after training can be important in consolidation, integration and maintenance of memories 5-7,12 .Subjects (n = 133) were 18 to 25 years old and gave informed consent before participating in the study, which was approved by the Human Studies Committee of the Massachusetts Mental Health Center. Each subject was trained in a single session lasting 60-90 minutes, and was subsequently tested in a second, identical session, 3 hours to 7 days after training. One group (n = 11) was de...

Dreaming has fascinated and mystified humankind for ages: the bizarre and evanescent qualities of dreams have invited boundless speculation about their origin, meaning and purpose. For most of the twentieth century, scientific dream theories were mainly psychological. Since the discovery of rapid eye movement (REM) sleep, the neural underpinnings of dreaming have become increasingly well understood, and it is now possible to complement the details of these brain mechanisms with a theory of consciousness that is derived from the study of dreaming. The theory advanced here emphasizes data that suggest that REM sleep may constitute a protoconscious state, providing a virtual reality model of the world that is of functional use to the development and maintenance of waking consciousness.

Improvement in motor skill performance is known to continue for at least 24 hr following training, yet the relative contributions of time spent awake and asleep are unknown. Here we provide evidence that a night of sleep results in a 20% increase in motor speed without loss of accuracy, while an equivalent period of time during wake provides no significant benefit. Furthermore, a significant correlation exists between the improved performance overnight and the amount of stage 2 NREM sleep, particularly late in the night. This finding of sleep-dependent motor skill improvement may have important implications for the efficient learning of all skilled actions in humans.

Sleep can be addressed across the entire hierarchy of biological organization. We discuss neuronal-network and regional forebrain activity during sleep, and its consequences for consciousness and cognition. Complex interactions in thalamocortical circuits maintain the electroencephalographic oscillations of non-rapid eye movement (NREM) sleep. Functional neuroimaging affords views of the human brain in both NREM and REM sleep, and has informed new concepts of the neural basis of dreaming during REM sleep -- a state that is characterized by illogic, hallucinosis and emotionality compared with waking. Replay of waking neuronal activity during sleep in the rodent hippocampus and in functional images of human brains indicates possible roles for sleep in neuroplasticity. Different forms and stages of learning and memory might benefit from different stages of sleep and be subserved by different forebrain regions.

Converging evidence and new research methodologies from across the neurosciences permit the neuroscientific study of the role of sleep in off-line memory reprocessing, as well as the nature and function of dreaming. Evidence supports a role for sleep in the consolidation of an array of learning and memory tasks. In addition, new methodologies allow the experimental manipulation of dream content at sleep onset, permitting an objective and scientific study of this dream formation and a renewed search for the possible functions of dreaming and the biological processes subserving it.