We reconsider sleep/wake brain states as being highly dynamic and regionally complex, rather than stationary and global. We review recent evidence for spatiotemporal complexity and advances in understanding the systems that regulate vigilance state, such as neuromodulatory systems – which may be more heterogenous than originally assumed. A modular and dynamic perspective highlights novel avenues for spatiotemporal optimization of sleep/wake states and has implications for how we view brain states, their functional roles, and how to design future experiments.
We recorded single-unit spiking (713 clusters), microwire LFPs and iEEG from 13 patients implanted with depth electrodes while playing sounds (click-trains, words, music) during wakefulness and sleep, studying auditory responses (observed mainly in lateral temporal lobe). Amplitude of spike and gamma responses was similar in wake and NREM sleep, with modest or no attenuation in A1 and moderate attenuation beyond A1. Even responses outside A1 were highly robust and informative about the stimulus during sleep, but alpha-beta (10-30Hz) power decrease induced by auditory stimulation, often termed ABD (Alpha Beta Desynchronization), was reduced during sleep, even during REM sleep when we often dream but remain largely disconnected from the environment. Our results suggest that the “feedforward sweep” is intact during sleep, but feedback signaling is impaired.
Alzheimer’s disease (AD) begins with a decades-long presymptomatic phase, well before the onset of memory decline and global disturbances in sleep architecture. Does hippocampal circuit activity and its homeostasis change at these early stages, and does that occur in particular behavioral states? A collaborative study with the lab of Inna Slutsky combines electrophysiology and calcium imaging to establish that familial AD (fAD) model mice do not show abnormalities in CA1 firing rates during wakefulness, but CA1 hyperexcitability is clearly evident in low-arousal states of NREM sleep and anesthesia, well before memory impairments or global sleep EEG impairments can be observed. This hyperexcitability is associated with disrupted homeostatic down-regulation of CA1 mean firing rates, and spikes in field potentials that resemble epileptiform interictal discharges.
Loss of consciousness happens abruptly, but does the neuronal activity in our brains also change suddenly? Here we gradually deepen anaesthesia in rats whilst playing aounds and recording how their neurons fire in the auditory cortex. We found a range of different changes with anaesthesia, but all of them changed gradually with increased anaesthesia, and none changed suddenly with loss of consciousness.
Sleep involves infra-slow ∼50-second fluctuations between disengagement and sensory reactivity.
New findings reveal that the brain’s noradrenaline system controls these dynamics by acting in the thalamus to affect sleep spindles,
and by modulating coordinated heart rate variations.
The timing, duration, and quality of sleep are regulated by an interaction between the circadian clock and homeostatic sleep pressure, which builds up during extended wakefulness. Homeostatic factors are thought to accumulate with increasing duration and intensity of wakefulness prior to sleep. In this study, a combination of behavioral monitoring and EMG/EEG recording under a pharmacological manipulation of DNA damage response markers revealed that DNA damage in neurons is a homeostatic driver for sleep. In turn, sleep increases the movements of DNA to enable efficient recruitment and activity of the DNA repair system.
Engagement is a major determinant of performance. Hyper-engagement risks impulsivity and is fatiguing over time, while hypo-engagement could lead to missed opportunities. Even in sleep, when engagement levels are minimal, sensory responsiveness varies. Thus, maintaining an optimal engagement level with the environment is a fundamental cognitive ability. In this study we investigated the activity of claustro-frontal circuits using calcium fiber photometry. We find that a moderate level of activity in claustro-cingulate projections defines optimal engagement. Low activity of this pathway is associated with impulsive actions, while high activity is associated with behavioral lapses. In addition, cingulate projecting claustrum neurons are most active during deep unresponsive slow-wave sleep, when mice are less prone to awakening by sensory stimuli.
We studied the immediate effects of transient non-invasive transcutaneous vagal nerve stimulation (tVNS) on heathy naïve human participants using high density EEG and pupilometery. We found that tVNS induce transient pupil dilation as in in animal models, and attenuates EEG alpha oscillations anti correlated with arousal. Our results support the use of tVNS as non-invasive neuormodulation tool for neuroscientific research and for developing novel clinical applications.
One of the most obvious affects of anaesthesia is that our patients are disconnected from their sensory environment: they don’t respond to questions, they don’t see the operation and they don’t feel the pain of surgery. However it is far from obvious where in the sensory hierarchy this disconnection occures. In this project we used intracranial depth-electrode recordings from single neurons in neurosurgical patients to map auditory responses as they went from wakefulness to anaesthesia. We showed that signals continued to arrive relatively unchanged to primary auditory cortex, but even only a little higher in the hierarchy, only millimeters away, the signals were disrupted. Furthermore we found evidence that feed-back signals in the brain were more affected by loss of consciousness, and a perhaps surprising difference between changes in high gamma responses (increased with anaesthesia) and neuronal firing (decreased with anaesthesia).
In this research, we performed targeted memory reactivation (TMR) locally in one side of the brain to improved memories stored in a single hemisphere: Participants learned to associate words with left or right visual field locations while contextual odor was present. Presenting this odor again to a single nostril during post-learning naps selectively promoted memory for specific words stored in the related hemisphere (ipsilateral to the cued nostril) and modulates local sleep oscillations and coupling.
A reduced responsiveness to external stimuli is the main characteristic of sleep. In this study we show that the LC, a small nucleus in the brainstem and the main source of noradrenaline, plays a central role in our ability to disconnect from the environment during sleep. We recorded rats cerebral cortical activity (Electroencephalogram; EEG) in order to identify epochs of sleep while playing sounds indifferent intensities. We showed that the activity of the LC just before the sound was played was correlated to the probability to wake up. Next, increase of LC activity by optogenetics cause rats awakenings more frequently in a response to a sound compared to the exact same sound, when the LC is not activated. Also, an activity decrease of the LC, makes it harder to wake up the rats, meaning that they are more disconnected from the environment.
We have recorded simultaneously the auditory responses of neurons in the auditory and perirhinal cortices. While the former, a low order sensory region, showed similar response magnitudes across vigilance states, the higher order area showed robust attenuations in both NREM and REM sleep. Furthermore, the latency of neurons has been found to predict the response attenuation.
Memory consolidation during sleep is thought to depend on coordinated interplay of cortical slow waves, sleep spindles, and hippocampal ripples, but direct evidence is lacking. I developed a real-time, closed-loop system triggering intra-cortical electrical stimulation based on brain activity, that will be used to study the neurophysiology of memory consolidation processes. Synchronizing stimulation, locked to slow waves in medial temporal lobe (MTL), but not identical stimulation without precise time-locking, enhanced slow waves and sleep spindles, increased locking of brain-wide neural activity to MTL slow waves, and improved coupling between hippocampal-ripples and neocortical oscillations. Additionally, synchronizing stimulation improved memory performance in a manner highly correlated with electrophysiological effects. Our results establish that hippocampo-cortical synchronization during sleep causally supports human memory consolidation via coordination between ripples and neocortical oscillations, and suggest potential avenues for treatment of memory disorders during sleep.
REM sleep is considered vital for supporting well-being and normal cognition. In 1984, Lavie and colleagues described the case of Y.C. – a man with a pontine lesion and near-total absence of REM sleep who led a normal life. Here, we set out to re-evaluate this individual’s REMS status 30 years after the original report, and formally assess his cognitive abilities. We find a near-total absence of REM sleep with no signs of significant compensation throughout adult life, along with normal cognitive status. The results provide a unique perspective on the ongoing debate regarding the functional role of REMS in supporting cognition.
An identical sensory stimulus may or may not be incorporated into perceptual experience, depending on the behavioral and cognitive state of the organism. What determines whether a sensory stimulus will be perceived? Here we tested whether noradrenaline signaling may play a key role. We pharmacologically down- and upregulated noradrenaline signaling in healthy volunteers using clonidine and reboxetine in double-blind placebo-controlled experiments, testing the effects on perceptual abilities and visually evoked electroencephalography (EEG) and fMRI responses. We found that detection sensitivity, discrimination accuracy, and subjective visibility change in accordance with noradrenaline (NE) levels, whereas decision bias (criterion) is not affected. Similarly, noradrenaline increases the consistency of EEG visually evoked potentials, while lower noradrenaline levels delay response components around 200ms. Furthermore, BOLD fMRI activations in high-order visual cortex selectively vary along with noradrenaline signaling. The results point to noradrenaline as a key factor causally linking visual awareness to external world events.
Despite the persistence of some sensory processing during sleep, it is unclear whether high-level cognitive processes such as speech parsing are also preserved. We used a novel Concurrent Hierarchical Tracking (CHT) approach for studying the depth of speech processing across wakefulness and sleep while tracking neuronal activity with EEG. We found that responses to the auditory sound stream remained intact; however, the sleeping brain did not show signs of hierarchical parsing of the continuous stream of syllables into words, phrases, and sentences. The results suggest that sleep imposes a functional barrier between basic sensory processing and high-level cognitive processing. This paradigm also holds promise for studying residual cognitive abilities in a wide array of unresponsive states.
We studied how natural loss of consciousness during sleep affects visual processing using high density scalp EEG in healthy human participants. We found attenuated fast frequency following responses in the visual cortex in sleep (N/REM) compared to wakefulness while slow and onset responses were stronger in sleep. These results supports the view that during sleep the cortex ability to process basic information is very much limited but factors shared in NREM and REM sleep, for example low noradrenergic tone.
Sleep deprivation has widespread health effects, including increased risk of hypertension, diabetes, obesity, heart attack, and stroke. In addition, it leads to car accidents and medical errors. During sleep deprivation, homeostatic and circadian processes interact to build up sleep pressure, which results in slow behavioral performance (cognitive lapses). Here we used intracranial electrodes to record single-neuron activities and LFPs in human neurosurgical patients performing a face/nonface categorization psychomotor vigilance task (PVT) over multiple experimental sessions, including after full-night sleep deprivation. We find that, just before cognitive lapses, the selective spiking responses of individual MTL neurons are attenuated, delayed, and lengthened. These ‘neuronal lapses’ are evident on a trial-by-trial basis when comparing the slowest behavioral PVT reaction times to the fastest. In addition, during these lapses, LFPs exhibit a relative local increase in ‘sleep-like’ slow/theta activity that invades the activity of the awake brain. Our results show that cognitive lapses involve local state-dependent changes in neuronal activity already present in the MTL.
Why does slow wave activity (SWA) increase after extended wakefulness? it is due to neuronal “fatigue” or increased neuronal synchrony? We forced neurons in the mouse cortex to fire at high levels for 6 h in 2 different conditions: during active wake with exploration and during sleep, using local optogenetic stimulation. We find that neurons need more time OFF only after sustained firing in wake, suggesting that fatigue due to sustained firing alone is unlikely to account for the increase in SWA that follows sleep deprivation.
A long held hypothesis postulated that brain waves called sleep spindles inhibit transmission of information from thalamus to cortex. In here we disputed this claim by showing neurons in the primary auditory cortex (first station after the thalamus) respond similarly to sounds, regardless of the occurrence of sleep spindles.
Are rapid eye movements (REMs) in sleep associated with visual-like activity, as during wakefulness? Here we examined single-unit activities and intracranial EEG across the human MTL and neocortex during sleep and wakefulness, and during visual stimulation with fixation. During both sleep and wakefulness, REM onsets are associated with distinct intracranial potentials, reminiscent of PGO waves. Individual neurons, especially in the MTL, exhibit reduced firing rates before REMs as well as transient increases in firing rate immediately after, similar to activity patterns observed upon image presentation during fixation without eye movements. Our results suggest that REMs during sleep rearrange discrete epochs of visual-like processing as during wakefulness.
Sleep entails a disconnection from the external environment. By and large, sensory stimuli do not trigger behavioral responses and are not consciously perceived as they usually are in wakefulness. Traditionally, sleep disconnection was ascribed to a thalamic “gate,” which would prevent signal propagation along ascending sensory pathways to primary cortical areas. Here, we compared single-unit and LFP responses in core auditory cortex as freely moving rats spontaneously switched between wakefulness and sleep states. Despite robust differences in baseline neuronal activity, both the selectivity and the magnitude of auditory-evoked responses were comparable across wakefulness, NREM and REM sleep. The processing of deviant tones was also compared in sleep and wakefulness using an oddball paradigm. Robust stimulus-specific adaptation (SSA) was observed following the onset of repetitive tones, and its strength was comparable across vigilance states. Thus, responses in core auditory cortex are preserved across sleep states, suggesting that evoked activity in primary sensory cortices is driven by external physical stimuli with little modulation by vigilance state. We suggest that sensory disconnection during sleep occurs at a stage later than primary sensory areas.
Sleep spindles are an EEG hallmark of NREM sleep and are believed to mediate many sleep-related functions, from memory consolidation to cortical development. Spindles differ in location, frequency, and association with slow waves, but whether this heterogeneity may reflect different physiological processes and potentially serve different functional roles remains unclear. Here we used a unique opportunity to record intracranial depth EEG and single-unit activity in multiple brain regions of neurosurgical patients to better characterize spindle activity in human sleep. We find that spindles occur across multiple neocortical regions, and less frequently also in the parahippocampal gyrus and hippocampus. Most spindles are spatially restricted to specific brain regions. In addition, spindle frequency is topographically organized with a sharp transition around the supplementary motor area between fast (13-15 Hz) centroparietal spindles often occurring with slow-wave up-states, and slow (9-12 Hz) frontal spindles occurring 200 ms later on average. Spindle variability across regions may reflect the underlying thalamocortical projections. We also find that during individual spindles, frequency decreases within and between regions. In addition, deeper NREM sleep is associated with a reduction in spindle occurrence and spindle frequency. Frequency changes between regions, during individual spindles, and across sleep may reflect the same phenomenon, the underlying level of thalamocortical hyperpolarization.
The most prominent EEG events in sleep are slow waves, reflecting a slow (<1 Hz) oscillation between up and down states in cortical neurons. It is unknown whether slow oscillations are synchronous across the majority or the minority of brain regions–are they a global or local phenomenon? To examine this, we recorded simultaneously scalp EEG, intracerebral EEG, and unit firing in multiple brain regions of neurosurgical patients. We find that most sleep slow waves and the underlying active and inactive neuronal states occur locally. Thus, especially in late sleep, some regions can be active while others are silent. We also find that slow waves can propagate, usually from medial prefrontal cortex to the medial temporal lobe and hippocampus. Sleep spindles, the other hallmark of NREM sleep EEG, are likewise predominantly local. Thus, intracerebral communication during sleep is constrained because slow and spindle oscillations often occur out-of-phase in different brain regions.
In an awake state, neurons in the cerebral cortex fire irregularly and EEG recordings display low-amplitude, high-frequency fluctuations. During sleep, neurons oscillate between ‘on’ periods, when they fire as in an awake brain, and ‘off’ periods, when they stop firing altogether and the EEG displays high-amplitude slow waves. However, what happens to neuronal firing after a long period of being awake is not known. Here we show that in freely behaving rats after a long period in an awake state, cortical neurons can go briefly ‘offline’ as in sleep, accompanied by slow waves in the local EEG. Neurons often go offline in one cortical area but not in another, and during these periods of ‘local sleep’, the incidence of which increases with the duration of the awake state, rats are active and display an ‘awake’ EEG. However, they are progressively impaired in a sugar pellet reaching task. Thus, although both the EEG and behaviour indicate wakefulness, local populations of neurons in the cortex may be falling asleep, with negative consequences for performance.
Dreams are a remarkable experiment in psychology and neuroscience, conducted every night in every sleeping person. They show that the human brain, disconnected from the environment, can generate an entire world of conscious experiences by itself. Content analysis and developmental studies have promoted understanding of dream phenomenology. In parallel, brain lesion studies, functional imaging and neurophysiology have advanced current knowledge of the neural basis of dreaming. It is now possible to start integrating these two strands of research to address fundamental questions that dreams pose for cognitive neuroscience: how conscious experiences in sleep relate to underlying brain activity; why the dreamer is largely disconnected from the environment; and whether dreaming is more closely related to mental imagery or to perception.