Slow waves, synaptic plasticity and information processing: insights from transcranial magnetic stimulation and high-density EEG experiments

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.

Technical advances in the characterization of the complexity of sleep and sleep disorders

The current clinical standard for quantifying sleep physiology is the laboratory polysomnogram, from which basic sleep-wake stages are determined. However, the complexity of sleep physiology has inspired alternative metrics that are providing additional insights into the rich dynamics of sleep. Electro-encephalography, magneto-encephalography, and functional magnetic resonance imaging represent advanced imaging modalities for understanding brain dynamics. These methods are complemented by autonomic measurements that provide additional important insights. We review here the spectrum of approaches that have been leveraged towards improved understanding of the complexity of sleep.

A dynamical role for acetylcholine in synaptic renormalization

Although sleep is a fundamental behavior observed in virtually all animal species, its functions remain unclear. One leading proposal, known as the synaptic renormalization hypothesis, suggests that sleep is necessary to counteract a global strengthening of synapses that occurs during wakefulness. Evidence for sleep-dependent synaptic downscaling (or synaptic renormalization) has been observed experimentally, but the physiological mechanisms which generate this phenomenon are unknown. In this study, we propose that changes in neuronal membrane excitability induced by acetylcholine may provide a dynamical mechanism for both wake-dependent synaptic upscaling and sleep-dependent downscaling. We show in silico that cholinergically-induced changes in network firing patterns alter overall network synaptic potentiation when synaptic strengths evolve through spike-timing dependent plasticity mechanisms. Specifically, network synaptic potentiation increases dramatically with high cholinergic concentration and decreases dramatically with low levels of acetylcholine. We demonstrate that this phenomenon is robust across variation of many different network parameters.

Napping to renew learning capacity: enhanced encoding after stimulation of sleep slow oscillations

As well as consolidating memory, sleep has been proposed to serve a second important function for memory, i.e. to free capacities for the learning of new information during succeeding wakefulness. The slow wave activity (SWA) that is a hallmark of slow wave sleep could be involved in both functions. Here, we aimed to demonstrate a causative role for SWA in enhancing the capacity for encoding of information during subsequent wakefulness, using transcranial slow oscillation stimulation (tSOS) oscillating at 0.75 Hz to induce SWA in healthy humans during an afternoon nap. Encoding following the nap was tested for hippocampus-dependent declarative materials (pictures, word pairs, and word lists) and procedural skills (finger sequence tapping). As compared with a sham stimulation control condition, tSOS during the nap enhanced SWA and significantly improved subsequent encoding on all three declarative tasks (picture recognition, cued recall of word pairs, and free recall of word lists), whereas procedural finger sequence tapping skill was not affected. Our results indicate that sleep SWA enhances the capacity for encoding of declarative materials, possibly by down-scaling hippocampal synaptic networks that were potentiated towards saturation during the preceding period of wakefulness.

NREM Sleep Oscillations and Brain Plasticity in Aging

The human electroencephalogram (EEG) during non-rapid eye movement sleep (NREM) is characterized mainly by high-amplitude (>75 μV), slow-frequency (<4 Hz) waves (slow waves), and sleep spindles (∼11-15 Hz; >0.25 s). These NREM oscillations play a crucial role in brain plasticity, and importantly, NREM sleep oscillations change considerably with aging. This review discusses the association between NREM sleep oscillations and cerebral plasticity as well as the functional impact of age-related changes on NREM sleep oscillations. We propose that age-related reduction in sleep-dependent memory consolidation may be due in part to changes in NREM sleep oscillations.

Arousal state feedback as a potential physiological generator of the ultradian REM/NREM sleep cycle

Human sleep episodes are characterized by an approximately 90-min ultradian oscillation between rapid eye movement (REM) and non-REM (NREM) sleep stages. The source of this oscillation is not known. Pacemaker mechanisms for this rhythm have been proposed, such as a reciprocal interaction network, but these fail to account for documented homeostatic regulation of both sleep stages. Here, two candidate mechanisms are investigated using a simple model that has stable states corresponding to Wake, REM sleep, and NREM sleep. Unlike other models of the ultradian rhythm, this model of sleep dynamics does not include an ultradian pacemaker, nor does it invoke a hypothetical homeostatic process that exists purely to drive ultradian rhythms. Instead, only two inputs are included: the homeostatic drive for Sleep and the circadian drive for Wake. These two inputs have been the basis for the most influential Sleep/Wake models, but have not previously been identified as possible ultradian rhythm generators. Using the model, realistic ultradian rhythms are generated by arousal state feedback to either the homeostatic or circadian drive. For the proposed 'homeostatic mechanism', homeostatic pressure increases in Wake and REM sleep, and decreases in NREM sleep. For the proposed 'circadian mechanism', the circadian drive is up-regulated in Wake and REM sleep, and is down-regulated in NREM sleep. The two mechanisms are complementary in the features they capture. The homeostatic mechanism reproduces experimentally observed rebounds in NREM sleep duration and intensity following total sleep deprivation, and rebounds in both NREM sleep intensity and REM sleep duration following selective REM sleep deprivation. The circadian mechanism does not reproduce sleep state rebounds, but more accurately reproduces the temporal patterns observed in a normal night of sleep. These findings have important implications in terms of sleep physiology and they provide a parsimonious explanation for the observed ultradian rhythm of REM/NREM sleep.

Morning-evening variation in human brain metabolism and memory circuits

It has been posited that a critical function of sleep is synaptic renormalization following a net increase in synaptic strength during wake. We hypothesized that wake would alter the resting-state functional organization of the brain and increase its metabolic cost. To test these hypotheses, two experiments were performed. In one, we obtained morning and evening resting-state functional MRI scans to assess changes in functional brain organization. In the second experiment, we obtained quantitative positron emission tomography measures of glucose and oxygen consumption to assess the cost of wake. We found selective changes in brain organization. Most prominently, bilateral medial temporal regions were locally connected in the morning but in the evening exhibited strong correlations with frontal and parietal brain regions involved in memory retrieval. We speculate that these changes may reflect aspects of memory consolidation recurring on a daily basis. Surprisingly, these changes in brain organization occurred without increases in brain metabolism.

Effect of low intensity low-frequency stimuli on long-term depression in the rat hippocampus area CA1 in vivo

Normally long-term depression (LTD) is difficult to be induced in naïve adult rats in vivo, but it can be induced in the juvenile females and acute-stressed adult males. Using these rats as LTD models, we find in our previous study that LTD induction by the classical low-frequency stimuli (LFS) may be associated with sleep. During sleep, endogenous field potential oscillations presented in the neocortical and hippocampal circuits play important roles in synaptic downscaling as well as memory consolidations. Generally, LTD can be considered as a special synaptic downscaling and the classical LFS is very similar to such endogenous oscillations. Thus, we speculate whether we can design a new LFS which is more similar to such oscillations and whether LTD can be induced by it in naïve adult rats? In this study, we found that in the naïve adult rats anesthetized in sleep stage, the classical LFS could not induce LTD, however, a low-intensity LFS, an endogenous oscillation-like one, could induce LTD. Furthermore, in the rats anesthetized in wakefulness stage, neither the classical nor the low-intensity LFS could induce LTD. Our study showed that in the naïve adult rats, LTD could be induced by the oscillation-like LFS in the sleep stage anesthesia, suggesting that LTD may physiologically occur during sleep and be inhibited in wakefulness stage. Our study suggested that in the hippocampus LTD may be a potential long-term synaptic plasticity underlying sleep-dependent memory consolidations.

Overnight changes in waking auditory evoked potential amplitude reflect altered sleep homeostasis in major depression

OBJECTIVE

Sleep homeostasis is altered in major depressive disorder (MDD). Pre- to postsleep decline in waking auditory evoked potential (AEP) amplitude has been correlated with sleep slow wave activity (SWA), suggesting that overnight changes in waking AEP amplitude are homeostatically regulated in healthy individuals. This study investigated whether the overnight change in waking AEP amplitude and its relation to SWA is altered in MDD.

METHOD

Using 256-channel high-density electroencephalography, all-night sleep polysomnography and single-tone waking AEPs pre- and postsleep were collected in 15 healthy controls (HC) and 15 non-medicated individuals with MDD.

RESULTS

N1 and P2 amplitudes of the waking AEP declined after sleep in the HC group, but not in MDD. The reduction in N1 amplitude also correlated with fronto-central SWA in the HC group, but a comparable relationship was not found in MDD, despite equivalent SWA between groups. No pre- to postsleep differences were found for N1 or P2 latencies in either group. These findings were not confounded by varying levels of alertness or differences in sleep variables between groups.

CONCLUSION

MDD involves altered sleep homeostasis as measured by the overnight change in waking AEP amplitude. Future research is required to determine the clinical implications of these findings.

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.

Good morning creativity: task reactivation during sleep enhances beneficial effect of sleep on creative performance

Both scientists and artists have suggested that sleep facilitates creativity, and this idea has received substantial empirical support. In the current study, we investigate whether one can actively enhance the beneficial effect of sleep on creativity by covertly reactivating the creativity task during sleep. Individuals' creative performance was compared after three different conditions: sleep-with-conditioned-odor; sleep-with-control-odor; or sleep-with-no-odor. In the evening prior to sleep, all participants were presented with a problem that required a creative solution. In the two odor conditions, a hidden scent-diffuser spread an odor while the problem was presented. In the sleep-with-conditioned-odor condition, task reactivation during sleep was induced by means of the odor that was also presented while participants were informed about the problem. In the sleep-with-control-odor condition, participants were exposed to a different odor during sleep than the one diffused during problem presentation. In the no odor condition, no odor was presented. After a night of sleep with the conditioned odor, participants were found to be: (i) more creative; and (ii) better able to select their most creative idea than participants who had been exposed to a control odor or no odor while sleeping. These findings suggest that we do not have to passively wait until we are hit by our creative muse while sleeping. Task reactivation during sleep can actively trigger creativity-related processes during sleep and thereby boost the beneficial effect of sleep on creativity.

Recovery of cortical effective connectivity and recovery of consciousness in vegetative patients

Patients surviving severe brain injury may regain consciousness without recovering their ability to understand, move and communicate. Recently, electrophysiological and neuroimaging approaches, employing simple sensory stimulations or verbal commands, have proven useful in detecting higher order processing and, in some cases, in establishing some degree of communication in brain-injured subjects with severe impairment of motor function. To complement these approaches, it would be useful to develop methods to detect recovery of consciousness in ways that do not depend on the integrity of sensory pathways or on the subject's ability to comprehend or carry out instructions. As suggested by theoretical and experimental work, a key requirement for consciousness is that multiple, specialized cortical areas can engage in rapid causal interactions (effective connectivity). Here, we employ transcranial magnetic stimulation together with high-density electroencephalography to evaluate effective connectivity at the bedside of severely brain injured, non-communicating subjects. In patients in a vegetative state, who were open-eyed, behaviourally awake but unresponsive, transcranial magnetic stimulation triggered a simple, local response indicating a breakdown of effective connectivity, similar to the one previously observed in unconscious sleeping or anaesthetized subjects. In contrast, in minimally conscious patients, who showed fluctuating signs of non-reflexive behaviour, transcranial magnetic stimulation invariably triggered complex activations that sequentially involved distant cortical areas ipsi- and contralateral to the site of stimulation, similar to activations we recorded in locked-in, conscious patients. Longitudinal measurements performed in patients who gradually recovered consciousness revealed that this clear-cut change in effective connectivity could occur at an early stage, before reliable communication was established with the subject and before the spontaneous electroencephalogram showed significant modifications. Measurements of effective connectivity by means of transcranial magnetic stimulation combined with electroencephalography can be performed at the bedside while by-passing subcortical afferent and efferent pathways, and without requiring active participation of subjects or language comprehension; hence, they offer an effective way to detect and track recovery of consciousness in brain-injured patients who are unable to exchange information with the external environment.

Induction of visual dream reports after transcranial direct current stimulation (tDCs) during Stage 2 sleep

REM sleep is a unique brain state characterized by frontal deactivation alongside activation of the posterior association and limbic cortices. Human brain lesion studies have found that the loss of dreaming is characterized by damage to the frontal and posterior parieto-temporo-occipital association cortex. Therefore, it is reasonable to assume that the function of these brain regions might encapsulate the neural processes of dreaming. The aim of the following two experiments was to investigate the effect of transcranial direct current stimulation (tDCs), applied simultaneously to the frontal and right posterior parietal cortex during Stage 2 sleep, on dreaming. In Experiment 1, 17 healthy participants received tDCs (cathodal-frontal, anodal-parietal) and low-intensity tDCs as well as no tDCs (blank control) during Stage 2 sleep in a counterbalanced order across the night. Dream reports were collected upon awakening after each of the three conditions. In Experiment 2, 10 participants received tDCs (cathodal-frontal, anodal-parietal), no tDCs (blank control) and two additional control conditions (reversed polarity and other-cephalic tDCs). In both experiments a significantly greater number of imagery reports were found on awakening after tDCs (cathodal-frontal, anodal-parietal), compared to the blank control conditions. However, in Experiment 2 the frequency of imagery reports from the tDCs (cathodal-frontal, anodal-parietal) was not significantly different from the other two tDC conditions, suggesting a non-specific effect of tDCs. Overall, it was concluded that tDCs (cathodal-frontal, anodal-parietal) increased the frequency of dream reports with visual imagery, possibly via a general arousing effect and/or recreating specific cortical neural activity involved in dreaming.

Spontaneous hemodynamic oscillations during human sleep and sleep stage transitions characterized with near-infrared spectroscopy

Understanding the interaction between the nervous system and cerebral vasculature is fundamental to forming a complete picture of the neurophysiology of sleep and its role in maintaining physiological homeostasis. However, the intrinsic hemodynamics of slow-wave sleep (SWS) are still poorly known. We carried out 30 all-night sleep measurements with combined near-infrared spectroscopy (NIRS) and polysomnography to investigate spontaneous hemodynamic behavior in SWS compared to light (LS) and rapid-eye-movement sleep (REM). In particular, we concentrated on slow oscillations (3-150 mHz) in oxy- and deoxyhemoglobin concentrations, heart rate, arterial oxygen saturation, and the pulsation amplitude of the photoplethysmographic signal. We also analyzed the behavior of these variables during sleep stage transitions. The results indicate that slow spontaneous cortical and systemic hemodynamic activity is reduced in SWS compared to LS, REM, and wakefulness. This behavior may be explained by neuronal synchronization observed in electrophysiological studies of SWS and a reduction in autonomic nervous system activity. Also, sleep stage transitions are asymmetric, so that the SWS-to-LS and LS-to-REM transitions, which are associated with an increase in the complexity of cortical electrophysiological activity, are characterized by more dramatic hemodynamic changes than the opposite transitions. Thus, it appears that while the onset of SWS and termination of REM occur only as gradual processes over time, the termination of SWS and onset of REM may be triggered more abruptly by a particular physiological event or condition. The results suggest that scalp hemodynamic changes should be considered alongside cortical hemodynamic changes in NIRS sleep studies to assess the interaction between the autonomic and central nervous systems.

Large-scale functional brain networks in human non-rapid eye movement sleep: insights from combined electroencephalographic/functional magnetic resonance imaging studies

This paper reviews the existing body of knowledge on the neural correlates of spontaneous oscillations, functional connectivity and brain plasticity in human non-rapid eye movement (NREM) sleep. The first section reviews the evidence that specific sleep events as slow waves and spindles are associated with transient increases in regional brain activity. The second section describes the changes in functional connectivity during NREM sleep, with a particular focus on changes within a low-frequency, large-scale functional brain network. The third section will discuss the possibility that spontaneous oscillations and differential functional connectivity are related to brain plasticity and systems consolidation, with a particular focus on motor skill acquisition. Implications for the mode of information processing per sleep stage and future experimental studies are discussed.

Both sleep and wakefulness support consolidation of continuous, goal-directed, visuomotor skill

Sleep has been shown to benefit memory consolidation for certain motor skills, but it remains unclear if this relationship exists for motor skills with direct rehabilitation applications. We aimed to determine the neurobehavioral relationship between finger-tracking skill development and sleep following skill training in young, healthy subjects. Forty subjects received tracking training in the morning (n = 20) or the evening (n = 20). Measures of tracking skill and cortical excitability were collected before and after training. Following training, tracking skill and measures of cortical excitability were assessed at two additional follow-up visits (12 and 24 h post-training) for each subject following an episode of sleep or waking activity. Two-way repeated-measures ANOVAs with Bonferroni-adjusted post hoc tests were conducted for tracking accuracy and measures of cortical excitability. Skill performance improved after training and continued to develop offline during the first post-training interval (12 h). This development was not further enhanced by sleep during this interval. Level of skill improvement was maintained for at least one day in both training groups. Cortical excitability was reduced following training and was related to level of skill performance at follow-up assessment. These data suggest offline memory consolidation of a continuous, visuospatial, finger-tracking skill is not dependent on sleep. These findings are in agreement with recent literature, indicating characteristics of a motor skill may be sensitive to the beneficial effect of sleep on post-training information processing.

Complex propagation patterns characterize human cortical activity during slow-wave sleep

Cortical electrical activity during nonrapid eye movement (non-REM) sleep is dominated by slow-wave activity (SWA). At larger spatial scales (∼2-30 cm), investigated by scalp EEG recordings, SWA has been shown to propagate globally over wide cortical regions as traveling waves, which has been proposed to serve as a temporal framework for neural plasticity. However, whether SWA dynamics at finer spatial scales also reflects the orderly propagation has not previously been investigated in humans. To reveal the local, finer spatial scale (∼1-6 cm) patterns of SWA propagation during non-REM sleep, electrocorticographic (ECoG) recordings were conducted from subdurally implanted electrode grids and a nonlinear correlation technique [mutual information (MI)] was implemented. MI analysis revealed spatial maps of correlations between cortical areas demonstrating SWA propagation directions, speed, and association strength. Highest correlations, indicating significant coupling, were detected during the initial positive-going deflection of slow waves. SWA propagated predominantly between adjacent cortical areas, albeit spatial noncontinuities were also frequently observed. MI analysis further uncovered significant convergence and divergence patterns. Areas receiving the most convergent activity were similar to those with high divergence rate, while reciprocal and circular propagation of SWA was also frequent. We hypothesize that SWA is characterized by distinct attributes depending on the spatial scale observed. At larger spatial scales, the orderly SWA propagation dominates; at the finer scale of the ECoG recordings, non-REM sleep is characterized by complex SWA propagation patterns.

Functional ADA polymorphism increases sleep depth and reduces vigilant attention in humans

Homeostatically regulated slow-wave oscillations in non-rapid eye movement (REM) sleep may reflect synaptic changes across the sleep-wake continuum and the restorative function of sleep. The nonsynonymous c.22G>A polymorphism (rs73598374) of adenosine deaminase (ADA) reduces the conversion of adenosine to inosine and predicts baseline differences in sleep slow-wave oscillations. We hypothesized that this polymorphism affects cognitive functions, and investigated whether it modulates electroencephalogram (EEG), behavioral, subjective, and biochemical responses to sleep deprivation. Attention, learning, memory, and executive functioning were quantified in healthy adults. Right-handed carriers of the variant allele (G/A genotype, n = 29) performed worse on the d2 attention task than G/G homozygotes (n = 191). To test whether this difference reflects elevated homeostatic sleep pressure, sleep and sleep EEG before and after sleep deprivation were studied in 2 prospectively matched groups of G/A and G/G genotype subjects. Deep sleep and EEG 0.75- to 1.5-Hz oscillations in non-REM sleep were significantly higher in G/A than in G/G genotype. Moreover, attention and vigor were reduced, whereas waking EEG alpha activity (8.5-12 Hz), sleepiness, fatigue, and α-amylase in saliva were enhanced. These convergent data demonstrate that genetic reduction of ADA activity elevates sleep pressure and plays a key role in sleep and waking quality in humans.

Flow detection of propagating waves with temporospatial correlation of activity

Voltage-sensitive dye imaging (VSDI) allows population patterns of cortical activity to be recorded with high temporal resolution, and recent findings ascribe potential significance to these spatial propagation patterns--both for normal cortical processing and in pathologies such as epilepsy. However, analysis of these spatiotemporal patterns has been mostly qualitative to date. In this report, we describe an algorithm to quantify fast local flow patterns of cortical population activation, as measured with VSDI. The algorithm uses correlation of temporal features across space, and therefore differs from conventional optical flow algorithms which use correlation of spatial features over time. This alternative approach allows us to take advantage of the characteristics of fast optical imaging data, which have very high temporal resolution but less spatial resolution. We verify the method both on artificial and biological data, and demonstrate its use.

Melatonin has membrane receptor-independent hypnotic action on neurons: an hypothesis

Melatonin, which is known to have sleep-promoting properties, has no morpho-physiological barriers and readily enters neurons and their subcellular compartments from both the blood and cerebrospinal fluid. It has multiple receptor-dependent and receptor-independent functions. Sleep is a neuronal function, and it can no longer be postulated that one or more anatomical structures fully control sleep. Neurons require sleep for metabolically driven restorative purposes, and as a result, the process of sleep is modulated by peripheral and central mechanisms. This is an important finding because it suggests that melatonin should have intracellular sleep-inducing properties. Based on recent evidence, it is proposed that melatonin induces sleep at the neuronal level independently of its membrane receptors. Thus, the hypnotic action of melatonin and the mechanisms involving the circadian rhythms are separate neurological functions. This is contrary to the presently accepted view.

A postsleep decline in auditory evoked potential amplitude reflects sleep homeostasis

OBJECTIVE

It has been hypothesized that slow wave activity, a well established measure of sleep homeostasis that increases after waking and decreases after sleep, may reflect changes in cortical synaptic strength. If so, the amplitude of sensory evoked responses should also vary as a function of time awake and asleep in a way that reflects sleep homeostasis.

METHODS

Using 256-channel, high-density electroencephalography (EEG) in 12 subjects, auditory evoked potentials (AEP) and spontaneous waking data were collected during wakefulness before and after sleep.

RESULTS

The amplitudes of the N1 and P2 waves of the AEP were reduced after a night of sleep. In addition, the decline in N1 amplitude correlated with low-frequency EEG power during non-rapid eye movement sleep and spontaneous wakefulness, both homeostatically regulated measures of sleep need.

CONCLUSIONS

The decline in AEP amplitude after a night of sleep may reflect a homeostatic reduction in synaptic strength.

SIGNIFICANCE

These findings provide further evidence for a connection between synaptic plasticity and sleep homeostasis.

The dreaming brain/mind, consciousness and psychosis

Several independent lines of research in neurobiology seem to support the phenomenologically-grounded view of the dreaming brain/mind as a useful model for psychosis. Hallucinatory phenomena and thought disorders found in psychosis share several peculiarities with dreaming, where internally generated, vivid sensorimotor imagery along with often heightened and incongruous emotion are paired with a decrease in ego functions which ultimately leads to a severe impairment in reality testing. Contemporary conceptualizations of severe mental disorders view psychosis as one psychopathological dimension that may be found across several diagnostic categories. Some experimental data have shown cognitive bizarreness to be equally elevated in dreams and in the waking cognition of acutely psychotic subjects and in patients treated with pro-dopaminergic drugs, independent of the underlying disorder. Further studies into the neurofunctional underpinnings of both conditions will help to clarify the use and validity of this model.

Phase advancement and nucleus-specific timing of thalamocortical activity during slow cortical oscillation

The exact timing of cortical afferent activity is instrumental for the correct coding and retrieval of internal and external stimuli. Thalamocortical inputs represent the most significant subcortical pathway to the cortex, but the precise timing and temporal variability of thalamocortical activity is not known. To examine this question, we studied the phase of thalamic action potentials relative to cortical oscillations and established correlations among phase, the nuclear location of the thalamocortical neurons, and the frequency of cortical activity. The phase of thalamic action potentials depended on the exact frequency of the slow cortical oscillation both on long (minutes) and short (single wave) time scales. Faster waves were accompanied by phase advancement in both cases. Thalamocortical neurons located in different nuclei fired at significantly different phases of the slow waves but were active at a similar phase of spindle oscillations. Different thalamic nuclei displayed distinct burst patterns. Bursts with a higher number of action potentials displayed progressive phase advancement in a nucleus-specific manner. Thalamic neurons located along nuclear borders were characterized by mixed burst and phase properties. Our data demonstrate that the temporal relationship between cortical and thalamic activity is not fixed but displays dynamic changes during oscillatory activity. The timing depends on the precise location and exact activity of thalamocortical cells and the ongoing cortical network pattern. This variability of thalamic output and its coupling to cortical activity can enable thalamocortical neurons to actively participate in the coding and retrieval of cortical signals.

Hippocampal memory consolidation during sleep: a comparison of mammals and birds

The transition from wakefulness to sleep is marked by pronounced changes in brain activity. The brain rhythms that characterize the two main types of mammalian sleep, slow-wave sleep (SWS) and rapid eye movement (REM) sleep, are thought to be involved in the functions of sleep. In particular, recent theories suggest that the synchronous slow-oscillation of neocortical neuronal membrane potentials, the defining feature of SWS, is involved in processing information acquired during wakefulness. According to the Standard Model of memory consolidation, during wakefulness the hippocampus receives input from neocortical regions involved in the initial encoding of an experience and binds this information into a coherent memory trace that is then transferred to the neocortex during SWS where it is stored and integrated within preexisting memory traces. Evidence suggests that this process selectively involves direct connections from the hippocampus to the prefrontal cortex (PFC), a multimodal, high-order association region implicated in coordinating the storage and recall of remote memories in the neocortex. The slow-oscillation is thought to orchestrate the transfer of information from the hippocampus by temporally coupling hippocampal sharp-wave/ripples (SWRs) and thalamocortical spindles. SWRs are synchronous bursts of hippocampal activity, during which waking neuronal firing patterns are reactivated in the hippocampus and neocortex in a coordinated manner. Thalamocortical spindles are brief 7-14 Hz oscillations that may facilitate the encoding of information reactivated during SWRs. By temporally coupling the readout of information from the hippocampus with conditions conducive to encoding in the neocortex, the slow-oscillation is thought to mediate the transfer of information from the hippocampus to the neocortex. Although several lines of evidence are consistent with this function for mammalian SWS, it is unclear whether SWS serves a similar function in birds, the only taxonomic group other than mammals to exhibit SWS and REM sleep. Based on our review of research on avian sleep, neuroanatomy, and memory, although involved in some forms of memory consolidation, avian sleep does not appear to be involved in transferring hippocampal memories to other brain regions. Despite exhibiting the slow-oscillation, SWRs and spindles have not been found in birds. Moreover, although birds independently evolved a brain region--the caudolateral nidopallium (NCL)--involved in performing high-order cognitive functions similar to those performed by the PFC, direct connections between the NCL and hippocampus have not been found in birds, and evidence for the transfer of information from the hippocampus to the NCL or other extra-hippocampal regions is lacking. Although based on the absence of evidence for various traits, collectively, these findings suggest that unlike mammalian SWS, avian SWS may not be involved in transferring memories from the hippocampus. Furthermore, it suggests that the slow-oscillation, the defining feature of mammalian and avian SWS, may serve a more general function independent of that related to coordinating the transfer of information from the hippocampus to the PFC in mammals. Given that SWS is homeostatically regulated (a process intimately related to the slow-oscillation) in mammals and birds, functional hypotheses linked to this process may apply to both taxonomic groups.

Thalamic dysfunction in schizophrenia suggested by whole-night deficits in slow and fast spindles

OBJECTIVE

Slow waves and sleep spindles are the two main oscillations occurring during non-REM sleep. While slow oscillations are primarily generated and modulated by the cortex, sleep spindles are initiated by the thalamic reticular nucleus and regulated by thalamo-reticular and thalamo-cortical circuits. In a recent high-density EEG study, the authors found that 18 medicated schizophrenia patients had reduced sleep spindles, compared with healthy and depressed subjects, during the first non-REM episode. In the present study, the authors investigated whether spindle deficits were present in a larger sample of schizophrenia patients, were consistent across the night, were related to antipsychotic medications, and were suggestive of impairments in specific neuronal circuits.

METHOD

Whole-night high-density EEG recordings were performed in 49 schizophrenia patients, 20 nonschizophrenia patients receiving antipsychotic medication, and 44 healthy subjects. In addition to sleep spindles, several parameters of slow waves were assessed.

RESULTS

Schizophrenia patients had whole-night deficits in spindle power (12-16 Hz) and in slow (12-14 Hz) and fast (14-16 Hz) spindle amplitude, duration, number, and integrated activity in the prefrontal, centroparietal, and temporal regions. Integrated spindle activity and spindle number had the largest effect sizes (effect size: ≥ 2.21). In contrast, no slow wave deficits were found in schizophrenia patients.

CONCLUSIONS

These results indicate that spindle deficits can be reliably established in schizophrenia, are stable across the night, are unlikely to be due to antipsychotic medications, and point to deficits in the thalamic reticular nucleus and thalamo-reticular circuits.

Sleep-like states modulate functional connectivity in the rat olfactory system

The present study was an examination of state-dependent functional connectivity during spontaneous activity between the piriform cortex and its upstream and downstream connections. Rats were anesthetized with urethan and allowed to spontaneously cycle between fast- and slow-wave states similar to fast- and slow-wave sleep states. Local field potential recordings were made from the olfactory bulb, piriform cortex, dorsal hippocampus, amygdala, and primary visual cortex. The results demonstrate that during slow-wave sleep-like states, when the piriform cortex shows reduced sensitivity to odor input via the olfactory bulb, there is enhanced coherence with other forebrain structures. Granger causality analyses suggest that the link between piriform cortical and hippocampal activity during slow-wave state is in the direction of the hippocampus to the piriform cortex rather than the reverse. The results suggest that slow-wave sleep-like states may provide an opportunity for the transfer and/or consolidation of information related to odor memories, specifically at a time when the piriform cortex is less sensitive to sensory input.

Neurophysiological and computational principles of cortical rhythms in cognition

Synchronous rhythms represent a core mechanism for sculpting temporal coordination of neural activity in the brain-wide network. This review focuses on oscillations in the cerebral cortex that occur during cognition, in alert behaving conditions. Over the last two decades, experimental and modeling work has made great strides in elucidating the detailed cellular and circuit basis of these rhythms, particularly gamma and theta rhythms. The underlying physiological mechanisms are diverse (ranging from resonance and pacemaker properties of single cells to multiple scenarios for population synchronization and wave propagation), but also exhibit unifying principles. A major conceptual advance was the realization that synaptic inhibition plays a fundamental role in rhythmogenesis, either in an interneuronal network or in a reciprocal excitatory-inhibitory loop. Computational functions of synchronous oscillations in cognition are still a matter of debate among systems neuroscientists, in part because the notion of regular oscillation seems to contradict the common observation that spiking discharges of individual neurons in the cortex are highly stochastic and far from being clocklike. However, recent findings have led to a framework that goes beyond the conventional theory of coupled oscillators and reconciles the apparent dichotomy between irregular single neuron activity and field potential oscillations. From this perspective, a plethora of studies will be reviewed on the involvement of long-distance neuronal coherence in cognitive functions such as multisensory integration, working memory, and selective attention. Finally, implications of abnormal neural synchronization are discussed as they relate to mental disorders like schizophrenia and autism.

Cortical reactivity and effective connectivity during REM sleep in humans

We recorded the electroencephalographic (EEG) responses evoked by transcranial magnetic stimulation (TMS) during the first rapid eye movement (REM) sleep episode of the night and we compared them with the responses obtained during previous wakefulness and NREM sleep. Confirming previous findings, upon falling into NREM sleep, cortical activations became more local and stereotypical, indicating a significant impairment of the intracortical dialogue. During REM sleep, a state in which subjects regain consciousness but are almost paralyzed, TMS triggered more widespread and differentiated patterns of cortical activation, that were similar to the ones observed in wakefulness. Similarly, TMS/hd-EEG may be used to probe the internal dialogue of the thalamocortical system in brain injured patients that are unable to move and communicate.

Resting our cortices by going DOWN to sleep

In this issue of Neuron, Vyazovskiy et al. reports on progressive changes in cortical unit activity within extended wakefulness and within extended sleep paralleling changes in EEG slow-wave sleep activity. Sleep debt may be integrated at the level of individual cortical neurons, providing support for the synaptic homeostasis theory.

The functions of sleep

Here we report on The Satellite Symposium on Sleep Function that was held in Lausanne during 6(th) FENS forum and brought together neuroscientists from basic and clinical sleep research. We illustrate the principal questions that arose during this interdisciplinary gathering and introduce the contents of nine review articles on aspects of sleep that are contained in this Special Issue of the European Journal of Neuroscience.

A perturbational approach for evaluating the brain's capacity for consciousness

How do we evaluate a brain's capacity to sustain conscious experience if the subject does not manifest purposeful behaviour and does not respond to questions and commands? What should we measure in this case? An emerging idea in theoretical neuroscience is that what really matters for consciousness in the brain is not activity levels, access to sensory inputs or neural synchronization per se, but rather the ability of different areas of the thalamocortical system to interact causally with each other to form an integrated whole. In particular, the information integration theory of consciousness (IITC) argues that consciousness is integrated information and that the brain should be able to generate consciousness to the extent that it has a large repertoire of available states (information), yet it cannot be decomposed into a collection of causally independent subsystems (integration). To evaluate the ability to integrate information among distributed cortical regions, it may not be sufficient to observe the brain in action. Instead, it is useful to employ a perturbational approach and examine to what extent different regions of the thalamocortical system can interact causally (integration) and produce specific responses (information). Thanks to a recently developed technique, transcranial magnetic stimulation and high-density electroencephalography (TMS/hd-EEG), one can record the immediate reaction of the entire thalamocortical system to controlled perturbations of different cortical areas. In this chapter, using sleep as a model of unconsciousness, we show that TMS/hd-EEG can detect clear-cut changes in the ability of the thalamocortical system to integrate information when the level of consciousness fluctuates across the sleep-wake cycle. Based on these results, we discuss the potential applications of this novel technique to evaluate objectively the brain's capacity for consciousness at the bedside of brain-injured patients.