Trains of sleep sequences are indices of learning capacity in rats

Sleep memory processing: the sequential hypothesis

According to the sequential hypothesis (SH) memories acquired during wakefulness are processed during sleep in two serial steps respectively occurring during slow wave sleep (SWS) and rapid eye movement (REM) sleep. During SWS memories to be retained are distinguished from irrelevant or competing traces that undergo downgrading or elimination. Processed memories are stored again during REM sleep which integrates them with preexisting memories. The hypothesis received support from a wealth of EEG, behavioral, and biochemical analyses of trained rats. Further evidence was provided by independent studies of human subjects. SH basic premises, data, and interpretations have been compared with corresponding viewpoints of the synaptic homeostatic hypothesis (SHY). Their similarities and differences are presented and discussed within the framework of sleep processing operations. SHY's emphasis on synaptic renormalization during SWS is acknowledged to underline a key sleep effect, but this cannot marginalize sleep's main role in selecting memories to be retained from downgrading traces, and in their integration with preexisting memories. In addition, SHY's synaptic renormalization raises an unsolved dilemma that clashes with the accepted memory storage mechanism exclusively based on modifications of synaptic strength. This difficulty may be bypassed by the assumption that SWS-processed memories are stored again by REM sleep in brain subnuclear quantum particles. Storing of memories in quantum particles may also occur in other vigilance states. Hints are provided on ways to subject the quantum hypothesis to experimental tests.

Different types of avoidance behavior in rats produce dissociable post-training changes in sleep

Avoidance learning affects post-training sleep, and post-training sleep deprivation impairs performance. However, not all rats learn to make avoidance responses, and some rats fail to escape; a definitive behavior of learned helplessness, a model of depression. This study investigated the changes in sleep associated with different behaviors adopted following avoidance training. Rats (n=53) were trained for 100 trials over 2 days (50 trials/day), followed by 23-24 h of post-training polysomnography, then re-tested (25 trials). At re-test, rats were categorized into: 1) Active Avoiders (AA; n=22), 2), Non-learning (NL; n=21), or 3) Escape Failures (EF; n=10). AA rats increased avoidances over days, whereas the NL and EF groups did not. EF rats increased escape failures over days, whereas the NL and AA rats did not. EF rats had increased rapid eye movement (REM) sleep in the first 4h on training day 1. They also had increased non-REM sleep in the first 4h and last 4h on both training days. AA rats had increased REM sleep 13-20 h post-training. The type of behavioral strategy adopted throughout training is associated with a unique pattern of changes in post-training sleep. Training-dependent changes in post-acquisition sleep may reflect distinct processes involved in the consolidation of these different memory traces.

A role for sleep in brain plasticity

The idea that sleep might be involved in brain plasticity has been investigated for many years through a large number of animal and human studies, but evidence remains fragmentary. Large amounts of sleep in early life suggest that sleep may play a role in brain maturation. In particular, the influence of sleep in developing the visual system has been highlighted. The current data suggest that both Rapid Eye Movement (REM) and non-REM sleep states would be important for brain development. Such findings stress the need for optimal paediatric sleep management. In the adult brain, the role of sleep in learning and memory is emphasized by studies at behavioural, systems, cellular and molecular levels. First, sleep amounts are reported to increase following a learning task and sleep deprivation impairs task acquisition and consolidation. At the systems level, neurophysiological studies suggest possible mechanisms for the consolidation of memory traces. These imply both thalamocortical and hippocampo-neocortical networks. Similarly, neuroimaging techniques demonstrated the experience-dependent changes in cerebral activity during sleep. Finally, recent works show the modulation during sleep of cerebral protein synthesis and expression of genes involved in neuronal plasticity.

Temporal pattern of hippocampal high-frequency oscillations during sleep after stimulant-evoked waking

Hippocampal ripple oscillations (140-200 Hz) are believed to be critically involved in the consolidation of memory traces during slow-wave sleep (SWS). We investigated the temporal pattern of ripple occurrence in relation to sleep phases following different types of waking. Amphetamine, the atypical wakening drug modafinil or non-pharmacological sleep deprivation lead to an increased ripple occurrence ("rebound") during the subsequent SWS episode. Waking of the same duration evoked by amphetamine or sleep deprivation led to a ripple rebound of similar extent (approximately 200%). The mean intraripple frequency was also elevated by up to 20 Hz during SWS following all treatments. Ripple amplitude was significantly increased only in experiments with amphetamine. Ripple occurrence but not intraripple frequency clearly correlated with the antecedent waking duration independent of treatment. Recovery of ripple occurrence and frequency to the pretreatment level during SWS depended on SWS duration. At the end of the recovery period paradoxical sleep (PS) acted like waking, elevating ripple occurrence during subsequent SWS episodes. On the other hand, PS decreased ripple occurrence if recovery from the rebound was not yet complete. Thus occurrence and structure of ripple oscillations are regulated by the timing and duration of previous SWS, PS and waking episodes.

Waking EEG power spectra in the rat: correlations with training performance

Adult rats chronically implanted with supradural electrodes were telemetrically EEG recorded during a baseline session, a training session for a two-way active avoidance task, and a retention session. Rats were assigned to a fast learning (FL), slow learning (SL) and non learning (NL) group if they achieved criterion during the training session, the retention session, or in neither session. High-resolution EEG analyses indicated that intergroup differences were present in the low frequency range of waking baseline power spectra. Moreover, baseline delta emissions directly correlated with freezings, and inversely correlated with avoidances, while emissions at 7-10 Hz directly correlated with avoidances and inversely correlated with freezings. Interestingly, during the first training period, waking delta emission selectively increased in FL rats in concomitance with a marked performance improvement; instead, SL and NL rats displayed increments at 7-9 Hz. In addition, freezings scored during the first two training periods directly correlated with post-training waking emission at 2 Hz, and inversely correlated with emission at 7-10 Hz. Conversely, escapes and avoidances directly correlated with waking emission at 7-10 Hz. The data indicate that (i) waking baseline power spectra differ among behavioral groups, and correlate with behavioral performance the following day; (ii) selective modifications of waking power spectra occur in each behavioral group during training; and (iii) behavioral responses during training correlate with post-training waking power spectra. Notably, the delta increment selectively occurring in training FL rats is assumed to reflect online memory processing leading to better performance. The latter observation supports the primary involvement of delta waves in learning.

Cellular and molecular connections between sleep and synaptic plasticity

The hypothesis that sleep promotes learning and memory has long been a subject of active investigation. This hypothesis implies that sleep must facilitate synaptic plasticity in some way, and recent studies have provided evidence for such a function. Our knowledge of both the cellular neurophysiology of sleep states and of the cellular and molecular mechanisms underlying synaptic plasticity has expanded considerably in recent years. In this article, we review findings in these areas and discuss possible mechanisms whereby the neurophysiological processes characteristic of sleep states may serve to facilitate synaptic plasticity. We address this issue first on the cellular level, considering how activation of T-type Ca(2+) channels in nonREM sleep may promote either long-term depression or long-term potentiation, as well as how cellular events of REM sleep may influence these processes. We then consider how synchronization of neuronal activity in thalamocortical and hippocampal-neocortical networks in nonREM sleep and REM sleep could promote differential strengthening of synapses according to the degree to which activity in one neuron is synchronized with activity in other neurons in the network. Rather than advocating one specific cellular hypothesis, we have intentionally taken a broad approach, describing a range of possible mechanisms whereby sleep may facilitate synaptic plasticity on the cellular and/or network levels. We have also provided a general review of evidence for and against the hypothesis that sleep does indeed facilitate learning, memory, and synaptic plasticity.

Sleeping brain, learning brain. The role of sleep for memory systems

The hypothesis that sleep participates in the consolidation of recent memory traces has been investigated using four main paradigms: (1) effects of post-training sleep deprivation on memory consolidation, (2) effects of learning on post-training sleep, (3) effects of within sleep stimulation on the sleep pattern and on overnight memories, and (4) re-expression of behavior-specific neural patterns during post-training sleep. These studies convincingly support the idea that sleep is deeply involved in memory functions in humans and animals. However, the available data still remain too scarce to confirm or reject unequivocally the recently upheld hypothesis that consolidations of non-declarative and declarative memories are respectively dependent upon REM and NREM sleep processes.

Learning and sleep: the sequential hypothesis

During the last 30 years, paradoxical sleep (PS) has been generally considered as the only type of sleep involved in memory processing, mainly for the consistent increase of PS episodes in laboratory animals learning a relatively complex task, and for the retention deficits induced by post-training PS deprivation. The vicissitudes of this idea, examined in detail by several laboratories, have been critically presented in a number of review articles However, according to a more comprehensive unitary proposal (the sequential hypothesis), memory processing during sleep does require the initial participation of slow-wave sleep (SS) in addition to the subsequent involvement of PS. The evidence supporting this hypothesis, largely derived from experiments concerning rats trained for a two-way active avoidance task, is reviewed here in some detail. Recent studies of human sleep are in full agreement with this view. In the rat, the main effect of learning on post-training SS consists in the selective increment in the average duration of SS episodes initiating different types of sleep sequences. Notably, following training for a two-way active avoidance task, the occurrence of this effect in sleep sequences including transition sleep (TS), such as SS-->TS-->W and SS-->TS-->PS, appears related to the processing of memories of the novel avoidance response. Conversely, the occurrence of the same effect in sleep sequences lacking TS may reflect the processing of memories of innate responses (escapes and freezings). Memories of innate and novel responses are assumed to engage in a dynamic competitive interaction to attain control of waking behaviour. Interestingly, in baseline sleep, variables of SS-->TS-->W and SS-->TS-->PS sequences, such as the average duration of SS, TS, and PS episodes, have proved to be good indices of the capacity to learn, as shown by their strong correlations with the number of avoidances scored by rats the following day. Comparable correlations have not been displayed by variables of baseline SS-->W and SS-->PS sequences.