The Role of Sleep in Declarative Memory Consolidation—Direct Evidence by Intracranial EEG

Effects of a brief afternoon nap on declarative and procedural memory

The relationship between sleep and memory consolidation has not been fully revealed. The current study aimed to investigate how a brief afternoon nap contributed to the consolidation of declarative and procedural memory by exploring the relationship between sleep characteristics (i.e., the durations of sleep stages and slow oscillation, slow-wave activity, and spindle activity extracted from sleep) and task performance and the relationship between delta, theta, alpha, and beta bands extracted from wake during task performance and task performance. Twenty-three healthy young adults underwent a paired associates learning task and a sequential finger-tapping task with easy and difficult levels and were tested for memory performance before and after the intervention (i.e., an about 30-min nap or stay awake). Electroencephalogram (EEG) signals were continously recorded during the whole experiment. Results revealed that a short afternoon nap improved movement speed for the procedural memory task, regardless of the task difficulty, but unaffected the performance on the declarative memory task. Besides, the improvement in movement speed for the easy procedural memory task was positively correlated with slow-wave activity (SWA) during non-rapid-eye-movement (NREM) sleep but negatively correlated with slow oscillation and spindle activity during sleep stage 2 and NREM sleep, and the improvement in the difficult procedural memory task correlated positively with SWA during NREM sleep. Moreover, performance on the easy declarative and procedural memory tasks was negatively correlated with the relative power of alpha or theta; whereas the alpha band was positively correlated with the difficult declarative memory performance. These findings suggested that a brief afternoon nap with NREM sleep would benefit procedural memory consolidation but not declarative memory; such contribution of napping to memory consolidation would be either explained by the sleep characteristics or physiological arousal during performing tasks; task difficulty would moderate the relationship between the declarative memory performance and EEGs during task performance.

Electrophysiological mechanisms of human memory consolidation

Consolidation stabilizes memory traces after initial encoding. Rodent studies suggest that memory consolidation depends on replay of stimulus-specific activity patterns during fast hippocampal “ripple” oscillations. Here, we measured replay in intracranial electroencephalography recordings in human epilepsy patients, and related replay to ripples. Stimulus-specific activity was identified using representational similarity analysis and then tracked during waking rest and sleep after encoding. Stimulus-specific gamma (30–90 Hz) activity during early (100–500 ms) and late (500–1200 ms) encoding is spontaneously reactivated during waking state and sleep, independent of later memory. Ripples during nREM sleep, but not during waking state, trigger replay of activity from the late time window specifically for remembered items. Ripple-triggered replay of activity from the early time window during nREM sleep is enhanced for forgotten items. These results provide the first electrophysiological evidence for replay related to memory consolidation in humans, and point to a prominent role of nREM ripple-triggered replay in consolidation processes.

It is believed that fast “ripple” oscillations in the hippocampus play a role in consolidation, a process by which memory traces are stabilized. Here, the authors show that ripples occuring during non-REM sleep trigger “replay” of brain activity associated with previously experienced stimuli.

Nap-mediated benefit to implicit information processing across age using an affective priming paradigm

Understanding how sleep‐related information processing affects behaviour may allow targeted cognitive enhancement to improve quality of life. Previous evidence demonstrates that implicitly‐presented cues are processed during subsequent sleep, resulting in enhanced cognition upon waking. We used a masked priming task to investigate this further. To assess sleep‐mediated effects on reactions to implicitly presented primes, participants performed an Affective Priming Task pre‐and‐post 90 min of sleep, compared with an equal period of wakefulness. The Choice Reaction Time Task—a similar binary choice task but without the implicit aspect—was used as a control. Sixteen healthy participants across a range of ages were tested and sleep monitored using electroencephalogram. In stark contrast to the control task, in the Affective Priming Task reaction times significantly improved across all prime types after sleep, but not an equal period of wake. There was no significant change in reaction times on Choice Reaction Time Task after wakefulness or sleep. Rather than a general suppression of all primes, the data are more in keeping with specific strategic optimisation of prime processing during sleep. We plan future work to probe the mechanisms and neuroanatomical substrate of sleep‐mediated prime processing.

Parsing the neural signatures of reduced error detection in older age

Recent work has demonstrated that explicit error detection relies on a neural evidence accumulation process that can be traced in the human electroencephalogram (EEG). Here, we sought to establish the impact of natural aging on this process by recording EEG from young (18-35 years) and older adults (65-88 years) during the performance of a Go/No-Go paradigm in which participants were required to overtly signal their errors. Despite performing the task with equivalent accuracy, older adults reported substantially fewer errors, and the timing of their reports were both slower and more variable. These behavioral differences were linked to three key neurophysiological changes reflecting distinct parameters of the error detection decision process: a reduction in medial frontal delta/theta (2-7 Hz) activity, indicating diminished top-down input to the decision process; a slower rate of evidence accumulation as indexed by the rate of rise of a centro-parietal signal, known as the error positivity; and a higher motor execution threshold as indexed by lateralized beta-band (16-30 Hz) activity. Our data provide novel insight into how the natural aging process affects the neural underpinnings of error detection.

Automated approach to detecting behavioral states using EEG-DABS

Electrocorticographic (ECoG) signals represent cortical electrical dipoles generated by synchronous local field potentials that result from simultaneous firing of neurons at distinct frequencies (brain waves). Since different brain waves correlate to different behavioral states, ECoG signals presents a novel strategy to detect complex behaviors. We developed a program, EEG Detection Analysis for Behavioral States (EEG-DABS) that advances Fast Fourier Transforms through ECoG signals time series, separating it into (user defined) frequency bands and normalizes them to reduce variability. EEG-DABS determines events if segments of an experimental ECoG record have significantly different power bands than a selected control pattern of EEG. Events are identified at every epoch and frequency band and then are displayed as output graphs by the program. Certain patterns of events correspond to specific behaviors. Once a predetermined pattern was selected for a behavioral state, EEG-DABS correctly identified the desired behavioral event. The selection of frequency band combinations for detection of the behavior affects accuracy of the method. All instances of certain behaviors, such as freezing, were correctly identified from the event patterns generated with EEG-DABS. Detecting behaviors is typically achieved by visually discerning unique animal phenotypes, a process that is time consuming, unreliable, and subjective. EEG-DABS removes variability by using defined parameters of EEG/ECoG for a desired behavior over chronic recordings. EEG-DABS presents a simple and automated approach to quantify different behavioral states from ECoG signals.

Obstructive sleep apnea exaggerates cognitive dysfunction in stroke patients

BACKGROUND

Obstructive sleep apnea (OSA) is very common in stroke survivors. It potentially worsens the cognitive dysfunction and inhibits their functional recovery. However, whether OSA independently damages the cognitive function in stroke patients is unclear. A simple method for evaluating OSA-induced cognitive impairment is also missing.

METHODS

Forty-four stroke patients six weeks after onset and 24 non-stroke patients with snoring were recruited for the polysomnographic study of OSA and sleep architecture. Their cognitive status was evaluated with a validated Chinese version of Cambridge Prospective Memory Test. The relationship between memory deficits and respiratory, sleeping, and dementia-related clinical variables were analyzed with correlation and multiple linear regression tests.

RESULTS

OSA significantly and independently damaged time- and event-based prospective memory in stroke patients, although it had less power than the stroke itself. The impairment of prospective memory was correlated with increased apnea-hypopnea index, decreased minimal and mean levels of peripheral oxygen saturation, and disrupted sleeping continuity (reduced sleep efficiency and increased microarousal index). The further regression analysis identified minimal levels of peripheral oxygen saturation and sleep efficiency to be the two most important predictors for the decreased time-based prospective memory in stroke patients.

CONCLUSIONS

OSA independently contributes to the cognitive dysfunction in stroke patients, potentially through OSA-caused hypoxemia and sleeping discontinuity. The prospective memory test is a simple but sensitive method to detect OSA-induced cognitive impairment in stroke patients. Proper therapies of OSA might improve the cognitive function and increase the life quality of stroke patients.

Is There a Relation between EEG-Slow Waves and Memory Dysfunction in Epilepsy? A Critical Appraisal

Is there a relationship between peri-ictal slow waves, loss of consciousness, memory, and slow-wave sleep, in patients with different forms of epilepsy? We hypothesize that mechanisms, which result in peri-ictal slow-wave activity as detected by the electroencephalogram, could negatively affect memory processes. Slow waves (≤4 Hz) can be found in seizures with impairment of consciousness and also occur in focal seizures without impairment of consciousness but with inhibited access to memory functions. Peri-ictal slow waves are regarded as dysfunctional and are probably caused by mechanisms, which are essential to disturb the consolidation of memory entries in these patients. This is in strong contrast to physiological slow-wave activity during deep sleep, which is thought to group memory-consolidating fast oscillatory activity. In patients with epilepsy, slow waves may not only correlate with the peri-ictal clouding of consciousness, but could be the epiphenomenon of mechanisms, which interfere with normal brain function in a wider range. These mechanisms may have transient impacts on memory, such as temporary inhibition of memory systems, altered patterns of hippocampal-neocortical interactions during slow-wave sleep, or disturbed cross-frequency coupling of slow and fast oscillations. In addition, repeated tonic-clonic seizures over the years in uncontrolled chronic epilepsy may cause a progressive cognitive decline. This hypothesis can only be assessed in long-term prospective studies. These studies could disentangle the reversible short-term impacts of seizures, and the impacts of chronic uncontrolled seizures. Chronic uncontrolled seizures lead to irreversible memory impairment. By contrast, short-term impacts do not necessarily lead to a progressive cognitive decline but result in significantly impaired peri-ictal memory performance.

Semantic congruence reverses effects of sleep restriction on associative encoding

Encoding and memory consolidation are influenced by factors such as sleep and congruency of newly learned information with prior knowledge (i.e., schema). However, only a few studies have examined the contribution of sleep to enhancement of schema-dependent memory. Based on previous studies showing that total sleep deprivation specifically impairs hippocampal encoding, and that coherent schemas reduce the hippocampal consolidation period after learning, we predict that sleep loss in the pre-training night will mainly affect schema-unrelated information whereas sleep restriction in the post-training night will have similar effects on schema-related and unrelated information. Here, we tested this hypothesis by presenting participants with face-face associations that could be semantically related or unrelated under different sleep conditions: normal sleep before and after training, and acute sleep restriction either before or after training. Memory was tested one day after training, just after introducing an interference task, and two days later, without any interference. Significant results were evident on the second retesting session. In particular, sleep restriction before training enhanced memory for semantically congruent events in detriment of memory for unrelated events, supporting the specific role of sleep in hippocampal memory encoding. Unexpectedly, sleep restriction after training enhanced memory for both related and unrelated events. Although this finding may suggest a poorer encoding during the interference task, this hypothesis should be specifically tested in future experiments. All together, the present results support a framework in which encoding processes seem to be more vulnerable to sleep loss than consolidation processes.

How sleep activates epileptic networks?

Background. The relationship between sleep and epilepsy has been long ago studied, and several excellent reviews are available. However, recent development in sleep research, the network concept in epilepsy, and the recognition of high frequency oscillations in epilepsy and more new results may put this matter in a new light. Aim. The review address the multifold interrelationships between sleep and epilepsy networks and with networks of cognitive functions. Material and Methods. The work is a conceptual update of the available clinical data and relevant studies. Results and Conclusions. Studies exploring dynamic microstructure of sleep have found important gating mechanisms for epileptic activation. As a general rule interictal epileptic manifestations seem to be linked to the slow oscillations of sleep and especially to the reactive delta bouts characterized by A1 subtype in the CAP system. Important link between epilepsy and sleep is the interference of epileptiform discharges with the plastic functions in NREM sleep. This is the main reason of cognitive impairment in different forms of early epileptic encephalopathies affecting the brain in a special developmental window. The impairment of cognitive functions via sleep is present especially in epileptic networks involving the thalamocortical system and the hippocampocortical memory encoding system.

Impact of nocturnal sleep deprivation on declarative memory retrieval in students at an orphanage: a psychoneuroradiological study

BACKGROUND AND METHODS

This study investigated the effects of sleep deprivation on total and partial (early and late) declarative memory and activation in the areas of the brain involved in these activities. The study included two experiments. Experiment 1 included 40 male residents of an orphanage aged 16-19 years, who were divided into four groups (n = 10 each) and subjected to total sleep deprivation, normal sleep, early-night sleep deprivation, or late-night sleep deprivation. Experiment 2 included eight students from the same institution who were divided into the same four groups (n = 2) as in experiment 1. Declarative memory was tested using lists of associated word pairs in both experiments, and activation of the relevant brain regions was measured before and after retrieval by single-photon emission computed tomography for subjects in experiment 2 only.

RESULTS

Students subjected to normal sleep had significantly higher scores for declarative memory retrieval than those subjected to total sleep deprivation (P = 0.002), early-night sleep deprivation (P = 0.005), or late-night sleep deprivation (P = 0.02). The left temporal lobe showed the highest rate of activity during memory retrieval after normal sleep, whereas the frontal, parietal, and right temporal lobes were more active after sleep deprivation.

CONCLUSION

Both slow wave sleep and rapid eye movement sleep play an active role in consolidation of declarative memory, which in turn allows memory traces to be actively reprocessed and strengthened during sleep, leading to improved performance in memory recall.

Functional dissociation between anterior and posterior temporal cortical regions during retrieval of remote memory

Retrieval of remote memory is considered to differentially involve the anterior and posterior temporal neocortices. Previous neuropsychological studies suggest that the different posterior temporal cortical regions are involved in the retrieval of remote memory of different categories of stimuli, whereas the anterior region is involved more generally in remote memory retrieval. In the present study, using functional magnetic resonance imaging of human brains, we tested this dissociation by examining the more precise characteristics of the anterior and posterior temporal cortical regions. Two categories of stimuli, faces and scenes, were used for paired stimuli to be retrieved, and the brain activity during retrieval of paired stimuli that were learned immediately before the scanning was compared with that during retrieval of paired stimuli that were learned ∼8 weeks earlier. We found that the different posterior temporal cortical regions were activated during retrieval of different categories of remote memory in a category-specific manner, whereas the anterior temporal cortical region was activated during retrieval of remote memory in a category-general manner. Furthermore, by applying a multivariate pattern analysis to psychophysiological interactions during retrieval of remote memory relative to recent memory, we revealed the significant interaction from the category-specific posterior temporal cortical regions to the category-general anterior temporal region. These results suggest that the posterior temporal cortical regions are involved in representation and retrieval of category-specific remote memory, whereas the anterior cortical temporal region is involved in category-general retrieval process of remote memory.

Hippocampal sleep features: relations to human memory function

The recent spread of intracranial electroencephalographic (EEG) recording techniques for presurgical evaluation of drug-resistant epileptic patients is providing new information on the activity of different brain structures during both wakefulness and sleep. The interest has been mainly focused on the medial temporal lobe, and in particular the hippocampal formation, whose peculiar local sleep features have been recently described, providing support to the idea that sleep is not a spatially global phenomenon. The study of the hippocampal sleep electrophysiology is particularly interesting because of its central role in the declarative memory formation. Recent data indicate that sleep contributes to memory formation. Therefore, it is relevant to understand whether specific patterns of activity taking place during sleep are related to memory consolidation processes. Fascinating similarities between different states of consciousness (wakefulness, REM sleep, non-REM sleep) in some electrophysiological mechanisms underlying cognitive processes have been reported. For instance, large-scale synchrony in gamma activity is important for waking memory and perception processes, and its changes during sleep may be the neurophysiological substrate of sleep-related deficits of declarative memory. Hippocampal activity seems to specifically support memory consolidation during sleep, through specific coordinated neurophysiological events (slow waves, spindles, ripples) that would facilitate the integration of new information into the pre-existing cortical networks. A few studies indeed provided direct evidence that rhinal ripples as well as slow hippocampal oscillations are correlated with memory consolidation in humans. More detailed electrophysiological investigations assessing the specific relations between different types of memory consolidation and hippocampal EEG features are in order. These studies will add an important piece of knowledge to the elucidation of the ultimate sleep function.

Procedural and declarative memory task performance, and the memory consolidation function of sleep, in recent and abstinent ecstasy/MDMA users

Ecstasy/MDMA use has been associated with various memory deficits. This study assessed declarative and procedural memory in ecstasy/MDMA users. Participants were tested in two sessions, 24 h apart, so that the memory consolidation function of sleep on both types of memory could also be assessed. Groups were: drug-naive controls (n = 24); recent ecstasy/MDMA users, who had taken ecstasy/MDMA 2-3 days before the first testing session (n = 25), and abstinent users, who had not taken ecstasy/MDMA for at least 8 days before testing (n = 17). Procedural memory did not differ between groups, but greater lifetime consumption of ecstasy was associated with poorer procedural memory. Recent ecstasy/MDMA users who had taken other drugs (mainly cannabis) 48-24 h before testing exhibited poorer declarative memory than controls, but recent users who had not taken other drugs in this 48-24-h period did not differ from controls. Greater lifetime consumption of ecstasy, and of cocaine, were associated with greater deficits in declarative memory. These results suggest that procedural, as well as declarative, memory deficits are associated with the extent of past ecstasy use. However, ecstasy/MDMA did not affect the memory consolidation function of sleep for either the declarative or the procedural memory task.

Sleep and rest facilitate implicit memory in a visual search task

Several forms of learning have been demonstrated to show improvements with sleep. Based on rodent models, it has been suggested that replay of waking events in the hippocampus during sleep may underlie memory consolidation in humans. However, behavioral data for the role of sleep in human hippocampal-related memory have been inconsistent. To further investigate the role of sleep in hippocampal-mediated learning, we tested subjects in two sessions of a contextual cueing paradigm, a form of hippocampus-dependent implicit learning, separated by intervals of sleep, active wake, or carefully controlled quiet rest. Participants completed a visual search task, and unbeknownst to them, some search displays were occasionally repeated in the experiment. Contextual cueing was revealed by faster search speed on repeated trials (Old) than unrepeated ones (New), even though subjects were unaware of the trial repetition. Notably, performance in a second testing session was equivalent for participants who underwent quiet resting, daytime sleep, or nocturnal sleep between the two sessions. These four groups showed equivalent transfer of learning from Session 1. Notably, learning of New configurations in Session 2 was absent in the active wake group, but was equally strong among the other three groups. These results indicate that this form of hippocampal learning is independent of sleep, and vulnerable to proactive interference during active wake. They prompt a reevaluation of the hippocampal replay hypothesis as a general model of sleep-dependent learning.

Memory processes during sleep: beyond the standard consolidation theory

Two-step theories of memory formation suggest that an initial encoding stage, during which transient neural assemblies are formed in the hippocampus, is followed by a second step called consolidation, which involves re-processing of activity patterns and is associated with an increasing involvement of the neocortex. Several studies in human subjects as well as in animals suggest that memory consolidation occurs predominantly during sleep (standard consolidation model). Alternatively, it has been suggested that consolidation may occur during waking state as well and that the role of sleep is rather to restore encoding capabilities of synaptic connections (synaptic downscaling theory). Here, we review the experimental evidence favoring and challenging these two views and suggest an integrative model of memory consolidation.

Memory loss in Alzheimer's disease: implications for development of therapeutics

Alzheimer's disease (AD) is a progressive neurodegenerative disease marked by a constellation of cognitive disturbances, the earliest and most prominent being impaired episodic memory. Episodic memory refers to the memory system that allows an individual to consciously retrieve a previously experienced item or episode of life. Many recent studies have focused on characterizing how AD pathology impacts particular aspects of episodic memory and underlying mental and neural processes. This review summarizes the findings of those studies and discusses the effects of current and promising treatments for AD on episodic memory. The goal of this review is to raise awareness of the strides that cognitive neuroscientists have made in understanding intact and dysfunctional memory. Knowledge of the specific memorial processes that are impaired in AD may be of great value to basic scientists developing novel therapies and to clinical researchers assessing the efficacy of those therapies.

Procedural learning and sleep hippocampal low frequencies in humans

Recent evidence suggests that slow EEG rhythms are involved in post-learning plasticity. However, the relationships between memory consolidation and hippocampal EEG features remain unclear. Here, we assessed the effects of both procedural and declarative learning on qualitative and quantitative measures of sleep by recording stereo-EEG (SEEG) directly from the hippocampus and the neocortex in a group of epileptic patients undergoing pre-surgical evaluations. Following a baseline night, sleep was recorded after administration of a declarative (paired-associate word list learning task) and a procedural (sequential finger tapping) task. Patients were tested before going to bed (test) and after sleep in the following morning (retest). At retest, we found that patients recalled correctly more word pairs compared to the pre-sleep test (declarative task), and they were slightly faster in performing the motor task (procedural task). Standard polysomnography showed an increase in the amount of slow-wave sleep (SWS) only after procedural learning, paralleled by an increase of hippocampal SEEG power in the very low frequency range (VLF, 0.5-1 Hz) during the first NREM sleep cycle. Moreover, procedural performance enhancement and SEEG power increase in the hippocampal VLF were significantly correlated, indicating a link between procedural memory consolidation and slow hippocampal SEEG rhythms. These findings are consistent with the hypothesis of synaptic homeostasis occurring during sleep, suggesting that hippocampal slow oscillations are associated with local processes of post-learning synaptic downscaling.

Ripples in the medial temporal lobe are relevant for human memory consolidation

High-frequency oscillations (ripples) have been described in the hippocampus and rhinal cortex of both animals and human subjects and have been linked to replay and consolidation of previously acquired information. More specifically, studies in rodents suggested that ripples are generated in the hippocampus and are then transferred into the rhinal cortex, and that they occur predominantly during negative half waves of neocortical slow oscillations. Recordings in human epilepsy patients used either microelectrodes or foramen ovale electrodes; it is thus unclear whether macroelectrodes, which are routinely used for pre-surgical investigations, allow the recording of ripples as well. Furthermore, no direct link between ripples and behavioural performance has yet been established. Here, we recorded intracranial electroencephalogram with macroelectrodes from the hippocampus and rhinal cortex contralateral to the seizure onset zone in 11 epilepsy patients during a memory consolidation task while they were having an afternoon 'nap', i.e. a sleep period of approximately 1 h duration. We found that ripples could reliably be detected both in the hippocampus and in the rhinal cortex and had a similar frequency composition to events recorded previously with microelectrodes in humans. Results from cross-correlation analysis revealed that hippocampal events were closely locked to rhinal events and were consistent with findings on transmission of ripples from the hippocampus into the rhinal cortex. Furthermore, hippocampal ripples were significantly locked to the phase of hippocampal delta band activity, which might provide a mechanism for the reported phase-locking to neocortical slow oscillations. Ripples occurred with the highest incidence during periods when subjects lay awake during the nap time. Finally, we found that the number of rhinal, but not hippocampal, ripples was correlated with the number of successfully recalled items (post-nap) learned prior to sleep. These data confirm previous recordings in animals and humans, but move beyond them in several respects: they are the first recordings of ripples in humans during a cognitive task and suggest that ripples are indeed related to behavioural performance; furthermore, they propose a mechanism for phase-locking of ripples to neocortical slow waves via phase coupling to hippocampal delta activity; finally, they show that ripples can be recorded reliably with standard macroelectrodes in the human brain.

Stress-induced changes in sleep and associated neuronal activity in rat hippocampus and amygdala

Stress increases vulnerability to anxiety and depression. We have investigated the effect of acute immobilization stress in amygdalohippocampal circuits by measuring the electroencephalogram (EEG) in male Wistar rats during rapid eye movement (REM) sleep. Electrodes were implanted stereotaxically in the hippocampus (CA1 and CA3 subregions of the hippocampus) and the amygdala (lateral nucleus). Prior to the stress, two baseline recordings were taken. Twenty-four hours later rats were exposed once to acute immobilization stress (AIS) session for 2 h. After the release and on subsequent days, electrophysiological changes that occurred due to stress during REM sleep were analyzed by comparing them with baseline measurements. Our results suggest that acute immobilization stress induced significant increase in REM sleep in the first 24 h after the exposure. In addition to changes in the sleep patterns, we have observed increased theta oscillations in CA1 area of the hippocampus with decreased coherence at theta range (4-8 Hz) between hippocampus and amygdala. These results suggest that single exposure to aversive experience such as immobilization stress can lead to dynamic changes in neuronal activities with altered sleep morphology. The results obtained in the present study are comparable to those seen in human patients suffering from panic, and anxiety due to posttraumatic stress disorder (PTSD).