Lack of delta waves and sleep disturbances during non-rapid eye movement sleep in mice lacking alpha1G-subunit of T-type calcium channels

T-type calcium channels have been implicated as a pacemaker for brain rhythms during sleep but their contribution to behavioral states of sleep has been relatively uncertain. Here, we found that mice lacking alpha1(G) T-type Ca(2+) channels showed a loss of the thalamic delta (1-4 Hz) waves and a reduction of sleep spindles (7-14 Hz), whereas slow (<1 Hz) rhythms were relatively intact, when compared with the wild-type during urethane anesthesia and non-rapid eye movement (NREM) sleep. Analysis of sleep disturbances, as defined by the occurrence of brief awakening (BA) episodes during NREM sleep, revealed that mutant mice exhibited a higher incidence of BAs of >16 sec compared with the wild-type, whereas no difference was seen in BAs of <16 sec between the two genotypes. These results are consistent with the previous idea of the distinct nature of delta oscillations and sleep spindles from cortically generated slow waves. These results also suggest that the alpha1(G)-subunit of T-type calcium channels plays a critical role in the genesis of thalamocortical oscillations and contributes to the modulation of sleep states and the transition between NREM sleep and wake states.     

Ultradian oscillations in plasma renin activity: their relationships to meals and sleep stages

The 24-h pattern of PRA was studied in 6 supine normal subjects, and the relationship between sleep stages and PRA oscillations was analyzed using 18 nighttime profiles and the concomitant polygraphic recordings of sleep. Blood was collected at 10-min intervals. The slow trends obtained by adjusting a third degree polynomial to the 24-h data were not reproducible among individuals, and no circadian pattern was detected. Sustained oscillations in PRA occurred throughout the day. Spectral analysis revealed that PRA oscillated at a regular periodicity of about 100 min during the night. This periodicity was modified during the daytime by meal intake, which induced PRA peaks with large interindividual variations in size. A close relationship was found between the nocturnal PRA oscillations and the alternance of rapid eye movement (REM) sleep and non-REM sleep. Non-REM sleep invariably coincided with increasing or peaking PRA levels. REM sleep occurred as PRA was declining or at nadirs. More precisely, increases in PRA marked the transition from REM sleep to stage II, whereas stages III and IV usually occurred when PRA was highest. This relationship between the periodic nocturnal oscillations in PRA and the alternance of the REM-non-REM cycles may translate a similar oscillatory process in the central nervous system or may be linked to hemodynamic changes during sleep that might be partly controlled by the renin-angiotensin system.     

Differential responses of hippocampal subfields to cortical up-down states

The connectivity of the hippocampal trisynaptic circuit, formed by the dentate gyrus, the CA3 and the CA1 region, is well characterized anatomically and functionally in vitro. The functional connectivity of this circuit in vivo remains to be understood. Toward this goal, we investigated the influence of the spontaneous, synchronized oscillations in the neocortical local field potential, reflecting up-down states (UDS) of cortical neurons, on the hippocampus. We simultaneously measured the extracellular local field potential in association cortex and the membrane potential of identified hippocampal excitatory neurons in anesthetized mice. Dentate gyrus granule cells showed clear UDS modulation that was phase locked to cortical UDS with a short delay. In contrast, CA3 pyramidal neurons showed mixed UDS modulation, such that some cells were depolarized during the cortical up state and others were hyperpolarized. CA1 pyramidal neurons, located farther downstream, showed consistent UDS modulation, such that when the cortical and dentate gyrus neurons were depolarized, the CA1 pyramidal cells were hyperpolarized. These results demonstrate the differential functional connectivity between neocortex and hippocampal subfields during UDS oscillations.     

Inhibition recruitment in prefrontal cortex during sleep spindles and gating of hippocampal inputs

During light slow-wave sleep, the thalamo-cortical network oscillates in waxing-and-waning patterns at about 7 to 14 Hz and lasting for 500 ms to 3 s, called spindles, with the thalamus rhythmically sending strong excitatory volleys to the cortex. Concurrently, the hippocampal activity is characterized by transient and strong excitatory events, Sharp-Waves-Ripples (SPWRs), directly affecting neocortical activity--in particular the medial prefrontal cortex (mPFC)--which receives monosynaptic fibers from the ventral hippocampus and subiculum. Both spindles and SPWRs have been shown to be strongly involved in memory consolidation. However, the dynamics of the cortical network during natural sleep spindles and how prefrontal circuits simultaneously process hippocampal and thalamo-cortical activity remain largely undetermined. Using multisite neuronal recordings in rat mPFC, we show that during sleep spindles, oscillatory responses of cortical cells are different for different cell types and cortical layers. Superficial neurons are more phase-locked and tonically recruited during spindle episodes. Moreover, in a given layer, interneurons were always more modulated than pyramidal cells, both in firing rate and phase, suggesting that the dynamics are dominated by inhibition. In the deep layers, where most of the hippocampal fibers make contacts, pyramidal cells respond phasically to SPWRs, but not during spindles. Similar observations were obtained when analyzing γ-oscillation modulation in the mPFC. These results demonstrate that during sleep spindles, the cortex is functionnaly "deafferented" from its hippocampal inputs, based on processes of cortical origin, and presumably mediated by the strong recruitment of inhibitory interneurons. The interplay between hippocampal and thalamic inputs may underlie a global mechanism involved in the consolidation of recently formed memory traces.     

Interplay between spontaneous and induced brain activity during human non-rapid eye movement sleep

Humans are less responsive to the surrounding environment during sleep. However, the extent to which the human brain responds to external stimuli during sleep is uncertain. We used simultaneous EEG and functional MRI to characterize brain responses to tones during wakefulness and non-rapid eye movement (NREM) sleep. Sounds during wakefulness elicited responses in the thalamus and primary auditory cortex. These responses persisted in NREM sleep, except throughout spindles, during which they became less consistent. When sounds induced a K complex, activity in the auditory cortex was enhanced and responses in distant frontal areas were elicited, similar to the stereotypical pattern associated with slow oscillations. These data show that sound processing during NREM sleep is constrained by fundamental brain oscillatory modes (slow oscillations and spindles), which result in a complex interplay between spontaneous and induced brain activity. The distortion of sensory information at the thalamic level, especially during spindles, functionally isolates the cortex from the environment and might provide unique conditions favorable for off-line memory processing.     

Learning increases human electroencephalographic coherence during subsequent slow sleep oscillations

Learning is assumed to induce specific changes in neuronal activity during sleep that serve the consolidation of newly acquired memories. To specify such changes, we measured electroencephalographic (EEG) coherence during performance on a declarative learning task (word pair associations) and subsequent sleep. Compared with a nonlearning control condition, learning performance was accompanied with a strong increase in coherence in several EEG frequency bands. During subsequent non-rapid eye movement sleep, coherence only marginally increased in a global analysis of EEG recordings. However, a striking and robust increase in learning-dependent coherence was found when analyses were performed time-locked to the occurrence of slow oscillations (<1 Hz). Specifically, the surface-positive half-waves of the slow oscillation resulting from widespread cortical depolarization were associated with distinctly enhanced coherence after learning in the slow-oscillatory, delta, slow-spindle, and gamma bands. The findings identify the depolarizing phase of the slow oscillations in humans as a time period particularly relevant for a reprocessing of memories in sleep.     

A nonsynaptic mechanism underlying interictal discharges in human epileptic neocortex

Very fast oscillations (VFOs, >80 Hz) are important for physiological brain processes and, in excess, with certain epilepsies. Putative mechanisms for VFO include interneuron spiking and network activity in coupled pyramidal cell axons. It is not known whether either, or both, of these apply in pathophysiological conditions. Spontaneously occurring interictal discharges occur in human tissue in vitro, resected from neocortical epileptic foci. VFO associated with these discharges was manifest in both field potential and, with phase delay, in excitatory synaptic inputs to fast spiking interneurons. Recruitment of somatic pyramidal cell and interneuron spiking was low, with no correlation between VFO power and synaptic inputs to principal cells. Reducing synaptic inhibition failed to affect VFO occurrence, but they were abolished by reduced gap junction conductance. These data suggest a lack of a causal role for interneurons, and favor a nonsynaptic pyramidal cell network origin for VFO in epileptic human neocortex.     

Two distinct activity patterns of fast-spiking interneurons during neocortical UP states

During sleep, neocortical neuronal networks oscillate slowly (<1 Hz) between periods of activity (UP states) and silence (DOWN states). UP states favor the interaction between thalamic-generated spindles (7-14 Hz) and cortically generated gamma (30-80 Hz) waves. We studied how these three nested oscillations modulate fast-spiking interneuron (FSi) activity in vivo in VGAT-Venus transgenic rats. Our data describe a population of FSi that discharge "early" within UP states and another population that discharge "late." Early FSi tended to be silent during epochs of desynchronization, whereas late FSi were active. We hypothesize that late FSi may be responsible for generating the gamma oscillations associated with cognitive processing during wakefulness. Remarkably, FSi populations were differently modulated by spindle and gamma rhythms. Early FSi were robustly coupled to spindles and always discharged earlier than late FSi within spindle and gamma cycles. The preferred firing phase during spindle and gamma waves was strongly correlated in each cell, suggesting a cross-frequency coupling between oscillations. Our results suggest a precise spatiotemporal pattern of FSi activity during UP states, whereby information rapidly flows between early and late cells, initially promoted by spindles and efficiently extended by local gamma oscillations.     

Modulation of episodic renin release during sleep in humans

We previously described a strong concordance between nocturnal oscillations in plasma renin activity and sleep cycles. To examine whether modifying renal renin content or release influences the response to central stimuli linked to sleep stage alternation, plasma renin activity was measured every 10 minutes from 11:00 PM to 8:00 AM in three groups of six subjects. The first group received one 40 mg dose of the diuretic furosemide; the second group underwent the night experiment after 3 days on a low sodium diet; the third group received one 100 mg dose of the beta-blocker atenolol. Each subject underwent a control night when a placebo was given. The nocturnal curves were analyzed with a pulse detection program. For the control nights, 74 of the 83 sleep cycles were associated with significant plasma renin activity oscillations; non-rapid eye movement sleep occurred in the ascending portions and rapid eye movement sleep in the declining portions of the oscillations. These oscillations persisted in the three groups of subjects during the experimental nights and the relation with the sleep stages was not disturbed. Acute stimulation by furosemide amplified the oscillations and led to a general upward trend of the nocturnal profiles. Similarly, a low sodium diet, which led to a slow increase in renal renin content, provoked large oscillations with high initial levels. However, in both cases the mean relative amplitude of the oscillations, expressed as a percentage of the nocturnal means, was similar to that of the control nights and approximated 60%.(ABSTRACT TRUNCATED AT 250 WORDS)     

Sleep,Brain, and Cognition Special Feature: Hippocampal gamma and sharp wave/ripples mediate bidirectional interactions with cortical networks during sleep

Hippocampus–neocortex interactions during sleep are critical for memory processes: Hippocampally initiated replay contributes to memory consolidation in the neocortex and hippocampal sharp wave/ripples modulate cortical activity. Yet, the spatial and temporal patterns of this interaction are unknown. With voltage imaging, electrocorticography, and laminarly resolved hippocampal potentials, we characterized cortico-hippocampal signaling during anesthesia and nonrapid eye movement sleep. We observed neocortical activation transients, with statistics suggesting a quasi-critical regime, may be helpful for communication across remote brain areas. From activity transients, we identified, in a data-driven fashion, three functional networks. A network overlapping with the default mode network and centered on retrosplenial cortex was the most associated with hippocampal activity. Hippocampal slow gamma rhythms were strongly associated to neocortical transients, even more than ripples. In fact, neocortical activity predicted hippocampal slow gamma and followed ripples, suggesting that consolidation processes rely on bidirectional signaling between hippocampus and neocortex.

Spontaneous activity during quiescent periods and sleep is likely to be crucial for a multitude of cognitive functions. Functional connectivity studies from human neuroimaging and other brain monitoring modalities have identified several “resting-state networks” whose activity fluctuations are coordinated. One of them, the default mode network (DMN), increases its activity when the brain does not actively drive overt behavior and is involved in functions such as imagery, planning, self-reflection, and memory mechanism (1, 2). The dynamical interplay between hippocampus and the neocortex is another hallmark of the spontaneous activity during behavioral idleness. Hippocampal sharp waves/ripples (SWRs) (3), bursts of hippocampal activity, have attracted most attention as a conduit for this interplay, as they modulate neocortical activity (4–6). Interestingly, the DMN areas are among those most strongly activated at SWR times (7). SWRs are also linked to the bulk of memory replay in the hippocampus, which is the spontaneous repetition of neural activity patterns that were initially elicited during experience (8, 9). Replay has been observed also in many cortical areas (see, e.g., refs. 10–13), most strongly in correspondence with hippocampal SWRs.The interaction between the hippocampus and the neocortex figures prominently in most theories of explicit memory: The hippocampus is seen as rapidly forming rapid memories and “index codes” that summarize and point to activity patterns (14) that simultaneously (albeit more slowly) form in the neocortex and elsewhere in the brain. At memory retrieval, and during offline periods, those codes would propagate to the neocortex. Here, they would help seamlessly updating a large memory repository, as postulated by the complementary learning systems theory (15), or would create new, multiple memory traces [MMT theory (16)] with either similar content or semantic, gist-like representations (17).According to all of these theoretical accounts, however, cortico-hippocampal interplay likely involves the entirety of the neocortex. Yet, previous work has concentrated mostly on single neocortical areas, chosen among those with strongest anatomical links to the hippocampus. Functional magnetic resonance imaging (fMRI) (5) and voltage-sensitive imaging data (18) highlight the global character of neocortical activity modulations related to SWR, with a prominent role of the DMN (7), but how information propagates in neocortical networks is not known yet.During sleep cortical networks are capable of self-sustained high activity transients (UP states, periods of neuronal activation) delimited by silent periods (DOWN states, periods of neuronal silence) (19), generated by recurrent excitation across cortical neurons (20, 21). UP/DOWN state fluctuations have often been described as traveling waves (22–25), but little is known about the structure of each transient activation. Here, with voltage imaging in neocortex of mice combined with layer-resolved hippocampal local field potentials (LFPs), we characterize the probability distribution of transient sizes, showing that it approximates a power-law shape, signature of a near-critical state (in which long-range spatial and temporal correlation are maximized, facilitating global brain coordination), thought to maximize information transmission between far apart cortical sites (26, 27). Thus, these dynamics may be an effective support for the formation of neocortical memory traces.We further delineate the structure of activity transients in a data-driven manner and we find three functional networks centered respectively on the retrosplenial cortex and medial cortical bank of the cortex, on somatosensory cortex, and on lateral cortex. These networks closely match the results of a large anatomical projections dataset (28). Crucially, we show that they are differentially involved in hippocampal communication, with a “retrosplenial” network, overlapping with the standard DMN playing a prominent role.All the theoretical accounts mentioned above emphasize the hippocampal influence on the neocortex and a unidirectional flow from the hippocampus to neocortex, embodied in SWRs. Our data from voltage imaging and LFPs from hippocampal subfield CA1 suggest two refinements of this picture. First, the strongest correlate of cortical activity transients is the slow gamma rhythm (20 to 50 Hz), even outpacing SWRs. Slow gamma has been linked with the routing of information from hippocampal subfields CA3 to CA1. The CA3 subfield is rich in recurrent connections, features of an auto-associative memory (29–31). During sleep, increased slow gamma during SWR events correlates with greater replay (32). Furthermore, using pseudocausality analysis we show here that cortical transients in the DMN/retrosplenial network precede bouts of hippocampal slow gamma.Together, these data point toward a bidirectional interaction as a constituent of the overall architecture of cortico-hippocampal interactions, which provides a potential dynamical scenario. This picture opens up a theoretical view explaining the involvement of the DMN in memory and the two-way exchanges between hippocampus and neocortex, regions crucial for memory and cognition.     

Triggering sleep slow waves by transcranial magnetic stimulation

During much of sleep, cortical neurons undergo near-synchronous slow oscillation cycles in membrane potential, which give rise to the largest spontaneous waves observed in the normal electroencephalogram (EEG). Slow oscillations underlie characteristic features of the sleep EEG, such as slow waves and spindles. Here we show that, in sleeping subjects, slow waves and spindles can be triggered noninvasively and reliably by transcranial magnetic stimulation (TMS). With appropriate stimulation parameters, each TMS pulse at <1 Hz evokes an individual, high-amplitude slow wave that originates under the coil and spreads over the cortex. TMS triggering of slow waves reveals intrinsic bistability in thalamocortical networks during non-rapid eye movement sleep. Moreover, evoked slow waves lead to a deepening of sleep and to an increase in EEG slow-wave activity (0.5-4.5 Hz), which is thought to play a role in brain restoration and memory consolidation.     

Declarative memory consolidation in humans: a prospective functional magnetic resonance imaging study

Retrieval of recently acquired declarative memories depends on the hippocampus, but with time, retrieval is increasingly sustainable by neocortical representations alone. This process has been conceptualized as system-level consolidation. Using functional magnetic resonance imaging, we assessed over the course of three months how consolidation affects the neural correlates of memory retrieval. The duration of slow-wave sleep during a nap/rest period after the initial study session and before the first scan session on day 1 correlated positively with recognition memory performance for items studied before the nap and negatively with hippocampal activity associated with correct confident recognition. Over the course of the entire study, hippocampal activity for correct confident recognition continued to decrease, whereas activity in a ventral medial prefrontal region increased. These findings, together with data obtained in rodents, may prompt a revision of classical consolidation theory, incorporating a transfer of putative linking nodes from hippocampal to prelimbic prefrontal areas.     

Neuronal migration disorders: Heterotopic neocortical neurons in CA1 provide a bridge between the hippocampus and the neocortex

Neuronal migration disorders have been involved in various pathologies, including epilepsy, but the properties of the neural networks underlying disorders have not been determined. In the present study, patch clamp recordings were made from intrahippocampal heterotopic as well as from neocortical and hippocampal neurons from brain slices of rats with prenatally methylazoxymethanol-induced cortical malformation. We report that heterotopic neurons have morphometrical parameters and cellular properties of neocortical supragranular neurons and are integrated in both neocortical and hippocampal networks. Thus, stimulation of the white matter induces both antidromic and orthodromic response in heterotopic and neocortical neurons. Stimulation of hippocampal afferents evokes a monosynaptic response in the majority of heterotopic neurons and a polysynaptic all-or-none epileptiform burst in the presence of bicuculline to block γ-aminobutyric acid type A inhibition. Furthermore, hippocampal paroxysmal activity generated by bath application of bicuculline can spread directly to the neocortex via the heterotopia in methylazoxymethanol-treated but not in naive rats. We conclude that heterotopias form a functional bridge between the limbic system and the neocortex, providing a substrate for pathological conditions.     

Neuronal metabolism governs cortical network response state

The level of arousal in mammals is correlated with metabolic state and specific patterns of cortical neuronal responsivity. In particular, rhythmic transitions between periods of high activity (up phases) and low activity (down phases) vary between wakefulness and deep sleep/anesthesia. Current opinion about changes in cortical response state between sleep and wakefulness is split between neuronal network-mediated mechanisms and neuronal metabolism-related mechanisms. Here, we demonstrate that slow oscillations in network state are a consequence of interactions between both mechanisms. Specifically, recurrent networks of excitatory neurons, whose membrane potential is partly governed by ATP-modulated potassium (K(ATP)) channels, mediate response-state oscillations via the interaction between excitatory network activity involving slow, kainate receptor-mediated events and the resulting activation of ATP-dependent homeostatic mechanisms. These findings suggest that K(ATP) channels function as an interface between neuronal metabolic state and network responsivity in mammalian cortex.     

Slow oscillation electrical brain stimulation during waking promotes EEG theta activity and memory encoding

The application of transcranial slow oscillation stimulation (tSOS; 0.75 Hz) was previously shown to enhance widespread endogenous EEG slow oscillatory activity when applied during a sleep period characterized by emerging endogenous slow oscillatory activity. Processes of memory consolidation typically occurring during this state of sleep were also enhanced. Here, we show that the same tSOS applied in the waking brain also induced an increase in endogenous EEG slow oscillations (0.4–1.2 Hz), although in a topographically restricted fashion. Applied during wakefulness tSOS, additionally, resulted in a marked and widespread increase in EEG theta (4–8 Hz) activity. During wake, tSOS did not enhance consolidation of memories when applied after learning, but improved encoding of hippocampus-dependent memories when applied during learning. We conclude that the EEG frequency and related memory processes induced by tSOS critically depend on brain state. In response to tSOS during wakefulness the brain transposes stimulation by responding preferentially with theta oscillations and facilitated encoding.     

Infraslow oscillations modulate excitability and interictal epileptic activity in the human cortex during sleep

Human cortical activity has been intensively examined at frequencies ranging from 0.5 Hz to several hundred Hz. Recent studies have, however, reported also infraslow fluctuations in neuronal population activity, magnitude of electroencephalographic oscillations, discrete sleep events, as well as in the occurrence of interictal events. Here we use direct current electroencephalography to demonstrate large-scale infraslow oscillations in the human cortex at frequencies ranging from 0.02 to 0.2 Hz. These oscillations, which are not detectable in conventional electroencephalography because of its limited recording bandwidth (typical lower limit 0.5 Hz), were observed in widespread cortical regions. Notably, the infraslow oscillations were strongly synchronized with faster activities, as well as with the interictal epileptic events and K complexes. Our findings suggest that the infraslow oscillations represent a slow, cyclic modulation of cortical gross excitability, providing also a putative mechanism for the as yet enigmatic aggravation of epileptic activity during sleep.     

Intracortical and corticothalamic coherency of fast spontaneous oscillations.

We report that fast (mainly 30- to 40-Hz) coherent electric field oscillations appear spontaneously during brain activation, as expressed by electroencephalogram (EEG) rhythms, and they outlast the stimulation of mesopontine cholinergic nuclei in acutely prepared cats. The fast oscillations also appear during the sleep-like EEG patterns of ketamine/xylazine anesthesia, but they are selectively suppressed during the prolonged phase of the slow (<1-Hz) sleep oscillation that is associated with hyperpolarization of cortical neurons. The fast (30- to 40-Hz) rhythms are synchronized intracortically within vertical columns, among closely located cortical foci, and through reciprocal corticothalamic networks. The fast oscillations do not reverse throughout the depth of the cortex. This aspect stands in contrast with the conventional depth profile of evoked potentials and slow sleep oscillations that display opposite polarity at the surface and midlayers. Current-source-density analyses reveal that the fast oscillations are associated with alternating microsinks and microsources across the cortex, while the evoked potentials and the slow oscillation display a massive current sink in midlayers, confined by two sources in superficial and deep layers. The synchronization of fast rhythms and their high amplitudes indicate that the term "EEG desynchronization," used to designate brain-aroused states, is incorrect and should be replaced with the original term, "EEG activation" [Moruzzi, G. & Magoun, H.W. (1949) Electroencephalogr. Clin. Neurophysiol. 1, 455-473].     

fMRI spectral signatures of sleep

Sleep can be distinguished from wake by changes in brain electrical activity, typically assessed using electroencephalography (EEG). The hallmark of nonrapid-eye-movement (NREM) sleep is the shift from high-frequency, low-amplitude wake EEG to low-frequency, high-amplitude sleep EEG dominated by spindles and slow waves. Here we identified signatures of sleep in brain hemodynamic activity, using simultaneous functional MRI (fMRI) and EEG. We found that, at the transition from wake to sleep, fMRI blood oxygen level–dependent (BOLD) activity evolved from a mixed-frequency pattern to one dominated by two distinct oscillations: a low-frequency (<0.1 Hz) oscillation prominent in light sleep and correlated with the occurrence of spindles, and a high-frequency oscillation (>0.1 Hz) prominent in deep sleep and correlated with the occurrence of slow waves. The two oscillations were both detectable across the brain but exhibited distinct spatiotemporal patterns. During the falling-asleep process, the low-frequency oscillation first appeared in the thalamus, then the posterior cortex, and lastly the frontal cortex, while the high-frequency oscillation first appeared in the midbrain, then the frontal cortex, and lastly the posterior cortex. During the waking-up process, both oscillations disappeared first from the thalamus, then the frontal cortex, and lastly the posterior cortex. The BOLD oscillations provide local signatures of spindle and slow wave activity. They may be employed to monitor the regional occurrence of sleep or wakefulness, track which regions are the first to fall asleep or wake up at the wake–sleep transitions, and investigate local homeostatic sleep processes.

Traditionally, sleep is considered to be a global state that affects the whole brain uniformly and simultaneously. Correspondingly, brain activity during human sleep is typically measured using scalp electroencephalography (EEG). The hallmark of nonrapid-eye-movement (NREM) sleep is the shift from high-frequency, low-amplitude wake EEG to low-frequency, high-amplitude sleep EEG dominated by slow waves and spindles. Slow waves are associated with the near-synchronous transitions in large populations of neurons between depolarized up states of intense firing and hyperpolarized down states of silence (1). They are generated primarily in the cerebral cortex and affect virtually all cortical neurons, as well as neurons in several subcortical structures (2). By contrast, spindles are associated with cycles of depolarization and hyperpolarization triggered by the interactions between reticular thalamic nucleus and specific thalamic nuclei and amplified by the thalamo-cortico-thalamic circuits. Based on the prominence of slow waves and spindles, NREM sleep can be subdivided into transitional (N1), intermediate (N2), and deep (N3) sleep stages.Recently, the view of sleep as a global state has been overturned by the intracranial findings of local sleep and local wakefulness (3). During wakefulness, individual neurons were found to display brief periods of slow wave activity, accompanied by transient behavioral impairments (4). Conversely, during deep NREM sleep, subsets of brain regions were found to display wake-like activity (5), which was associated with dreaming (6). These findings establish that sleep-like and wakefulness-like states are not mutually exclusive, but can occur simultaneously in the same brain, with some neuronal populations showing one state and the rest the other. They highlight the importance to monitor the local state of individual neuronal populations, as opposed to the global state of the brain as a whole. However, EEG lacks both the spatial resolution and the brain coverage required for monitoring local neuronal state. It is difficult to identify the brain regions that generate the scalp EEG signal, where different source configurations can give rise to the same EEG topography. Moreover, the scalp and the intracranial EEG signals are both insensitive to neuronal activities in deep brain structures, making it difficult to monitor the neuronal state in these brain regions.Here we employed functional MRI (fMRI) to explore, with a full brain coverage and higher spatial resolution, local signatures of sleep in brain hemodynamic activity. We reasoned that the frequency content of fMRI blood oxygen level–dependent (BOLD) activity would show systematic changes from wake to sleep, reflecting the local groupings of spindles or slow waves by infra-slow fluctuations within the frequency range of brain hemodynamic activity. Our hypothesis builds upon previous reports of BOLD spectral changes from wake to sleep. Previous studies reported increases in low-frequency BOLD activity (<0.1 Hz) from wake to light sleep (7–9), as well as increases in higher-frequency BOLD activity (>0.1 Hz) from wake to propofol anesthesia (10). Although the relationships between these BOLD spectral changes and spindle or slow wave activity were not examined, it is interesting to note that propofol anesthesia can induce slow waves similar to those of NREM sleep (11), which might underlie the observed increase in high-frequency BOLD activity; moreover, the emergence of sleep spindles during child development (12) coincides with an increase in low-frequency BOLD activity (13). These studies hinted at a possible link between BOLD frequency content and spindle or slow wave activity. However, the exact link has remained unclear.Using simultaneous fMRI and EEG, we found that, during the transition from wake to sleep, fMRI BOLD activity evolved from a mixed-frequency pattern to one dominated by two distinct oscillations: a low-frequency oscillation (<0.1 Hz) prominent in light sleep and a higher-frequency oscillation (>0.1 Hz) in deep sleep. The time courses of low-frequency and high-frequency BOLD oscillation power correlated, respectively, with the time courses of spindle and slow wave activities. Moreover, the regional distributions and the onset, offset patterns of low-frequency and high-frequency BOLD oscillation were similar to those of spindle and slow wave activity. By providing local signatures of spindle and slow wave activity, these two BOLD oscillations may be employed to monitor the local neuronal state and detect local sleep or local wakefulness.     

Hierarchical clustering of brain activity during human nonrapid eye movement sleep

Consciousness is reduced during nonrapid eye movement (NREM) sleep due to changes in brain function that are still poorly understood. Here, we tested the hypothesis that impaired consciousness during NREM sleep is associated with an increased modularity of brain activity. Cerebral connectivity was quantified in resting-state functional magnetic resonance imaging times series acquired in 13 healthy volunteers during wakefulness and NREM sleep. The analysis revealed a modification of the hierarchical organization of large-scale networks into smaller independent modules during NREM sleep, independently from EEG markers of the slow oscillation. Such modifications in brain connectivity, possibly driven by sleep ultraslow oscillations, could hinder the brain's ability to integrate information and account for decreased consciousness during NREM sleep.