Animal models for information processing during sleep

Sensory gating and gaining in sleep: the balance between the protection of sleep and the safeness of life (a review)

Sleep is a brain state characterised by a low vigilance level and diminished consciousness. Reaction to and processing of external stimuli is attenuated in sleep. During sleep, the reticular thalamic nucleus reduces the flow of sensory activity to the cerebral cortex through inhibition of the thalamus. This sensory gating process facilitates sleep. After reaching the afferent layers of primary cortex, the reduced sensory flow is adjusted, gained, and processed within various cortical layers before being transferred by the corticofugal system back to appropriate subdivisions of the thalamus as feedback. Thalamic subdivisions then dispatch this sensory information to related areas of the cerebral cortex, where it is (sub)consciously perceived. When necessary, a sleeping individual can be awakened by a wake-up call, either by stimuli indicating danger, or by personally meaningful stimuli. It is safe for a sleeping individual that it can be aroused when necessary. Evidently, there are two processes by which the brain adjusts the response to sensory stimuli before entering (sub)consciousness. Firstly 'sensory gating', a process favourable to the maintenance of sleep by reducing the sensory input to the brain through the reticular thalamic nucleus and secondly 'sensory gaining', a process implying that the gained preserved sensory input is continuously analysed by the corticofugal system to detect dangerous and relevant environmental elements, indispensable for safeness and well-being of the sleeper.

Altered EEG power spectrum, but not sleep-wake architecture, in HCN1 knockout mice

Sleep is a complex biological state characterized by large populations of neurons firing in a rhythmic or synchronized manner. HCN channels play a critical role in generating and sustaining synchronized neuronal firing and are involved in the actions of anaesthetics. However, the role of these channels in sleep-wakefulness per se has yet to be studied. We conducted polysomnographic recordings of Hcn1 constitutive knockout (Hcn1 KO) and wild-type (WT) mice in order to investigate the potential role of HCN1 channels in sleep/wake regulation. EEG and EMG data were analysed using the Somnivore™ machine learning algorithm. Time spent in each vigilance state, bout number and duration, and EEG power spectral activity were compared between genotypes. There were no significant differences in the time spent in wake, rapid eye movement (REM) or non-REM (NREM) sleep between Hcn1 KO and WT mice. Wake bout duration during the inactive phase was significantly shorter in Hcn1 KO mice whilst no other bout parameters were affected by genotype. Hcn1 KO mice showed a reduction in overall EEG power which was particularly prominent in the theta (5-9 Hz) and alpha (9-15 Hz) frequency bands and most evident during NREM sleep. Together these data suggest that HCN1 channels do not play a major role in sleep architecture or modulation of vigilance states. However, loss of these channels significantly alters underlying neuronal activity within these states which may have functional consequences.

Automatic Sensory Predictions: A Review of Predictive Mechanisms in the Brain and Their Link to Conscious Processing

The human brain has the astonishing capacity of integrating streams of sensory information from the environment and forming predictions about future events in an automatic way. Despite being initially developed for visual processing, the bulk of predictive coding research has subsequently focused on auditory processing, with the famous mismatch negativity signal as possibly the most studied signature of a surprise or prediction error (PE) signal. Auditory PEs are present during various consciousness states. Intriguingly, their presence and characteristics have been linked with residual levels of consciousness and return of awareness. In this review we first give an overview of the neural substrates of predictive processes in the auditory modality and their relation to consciousness. Then, we focus on different states of consciousness - wakefulness, sleep, anesthesia, coma, meditation, and hypnosis - and on what mysteries predictive processing has been able to disclose about brain functioning in such states. We review studies investigating how the neural signatures of auditory predictions are modulated by states of reduced or lacking consciousness. As a future outlook, we propose the combination of electrophysiological and computational techniques that will allow investigation of which facets of sensory predictive processes are maintained when consciousness fades away.

Functional connectivity of thalamus in children with primary nocturnal enuresis: results from a resting-state fMRI study

Primary nocturnal enuresis (PNE) is characterized by a low cure rate and a high reoccurrence rate, since its underlying mechanism remains unclear. Based on the recent studies that thalamus plays an important role in waking up a sleeping person, here we further investigate the functional connectivity (FC) information between thalamus and other brain regions, in order to make better understanding of the PNE's pathogenesis. In this study, we enrolled 30 children diagnosed with PNE and 30 typically developing children that are age and sex matched, the thalamus-based FC estimates were extracted at the resting-state. Experiments showed that for children with PNE, there were four brain regions found with a reduced connection efficiency with thalamus, that were cerebellum posterior lobe, frontal lobe, parietal lobe and precentral gyrus. It can be concluded that these relevant regions might induce an arousal disorder, and therefore further lead to PNE. This finding also provides a new insight in the pathophysiology of PNE.

Diminished Auditory Responses during NREM Sleep Correlate with the Hierarchy of Language Processing

Natural sleep provides a powerful model system for studying the neuronal correlates of awareness and state changes in the human brain. To quantitatively map the nature of sleep-induced modulations in sensory responses we presented participants with auditory stimuli possessing different levels of linguistic complexity. Ten participants were scanned using functional magnetic resonance imaging (fMRI) during the waking state and after falling asleep. Sleep staging was based on heart rate measures validated independently on 20 participants using concurrent EEG and heart rate measurements and the results were confirmed using permutation analysis. Participants were exposed to three types of auditory stimuli: scrambled sounds, meaningless word sentences and comprehensible sentences. During non-rapid eye movement (NREM) sleep, we found diminishing brain activation along the hierarchy of language processing, more pronounced in higher processing regions. Specifically, the auditory thalamus showed similar activation levels during sleep and waking states, primary auditory cortex remained activated but showed a significant reduction in auditory responses during sleep, and the high order language-related representation in inferior frontal gyrus (IFG) cortex showed a complete abolishment of responses during NREM sleep. In addition to an overall activation decrease in language processing regions in superior temporal gyrus and IFG, those areas manifested a loss of semantic selectivity during NREM sleep. Our results suggest that the decreased awareness to linguistic auditory stimuli during NREM sleep is linked to diminished activity in high order processing stations.

Neural Dynamics of Emotional Salience Processing in Response to Voices during the Stages of Sleep

Sleep has been related to emotional functioning. However, the extent to which emotional salience is processed during sleep is unknown. To address this concern, we investigated night sleep in healthy adults regarding brain reactivity to the emotionally (happily, fearfully) spoken meaningless syllables dada, along with correspondingly synthesized nonvocal sounds. Electroencephalogram (EEG) signals were continuously acquired during an entire night of sleep while we applied a passive auditory oddball paradigm. During all stages of sleep, mismatch negativity (MMN) in response to emotional syllables, which is an index for emotional salience processing of voices, was detected. In contrast, MMN to acoustically matching nonvocal sounds was undetected during Sleep Stage 2 and 3 as well as rapid eye movement (REM) sleep. Post-MMN positivity (PMP) was identified with larger amplitudes during Stage 3, and at earlier latencies during REM sleep, relative to wakefulness. These findings clearly demonstrated the neural dynamics of emotional salience processing during the stages of sleep.

Vascular compliance limits during sleep deprivation and recovery sleep

STUDY OBJECTIVES

Our previous studies showed that evoked hemodynamic responses are smaller during wake compared to sleep; suggesting neural activity is associated with vascular expansion and decreased compliance. We explored whether prolonged activity during sleep deprivation may exacerbate vascular expansion and blunt hemodynamic responses.

DESIGN

Evoked auditory responses were generated with periodic 65 dB speaker clicks over a 72-h period and measured with cortical electrodes. Evoked hemodynamic responses were measured simultaneously with optical techniques using three light-emitting diodes, and a photodiode.

SETTING

Animals were housed in separate 30×30×80 cm enclosures, tethered to a commutator system and maintained on a 12-h light/dark cycle. Food and water were available ad libitum.

PATIENTS OR PARTICIPANTS

Seven adult female Sprague-Dawley rats.

INTERVENTIONS

Following a 24-h baseline recording, sleep deprivation was initiated for 0 to 10 h by gentle handling, followed by a 24-h recovery sleep recording. Evoked electrical and hemodynamic responses were measured before, during, and after sleep deprivation.

MEASUREMENTS AND RESULTS

Following deprivation, evoked hemodynamic amplitudes were blunted. Steady-state oxyhemoglobin concentration increased during deprivation and remained high during the initial recovery period before returning to baseline levels after approximately 9-h.

CONCLUSIONS

Sleep deprivation resulted in blood vessel expansion and decreased compliance while lower basal neural activity during recovery sleep may allow blood vessel compliance to recover. Chronic sleep restriction or sleep deprivation could push the vasculature to critical levels, limiting blood delivery, and leading to metabolic deficits with the potential for neural trauma.

K-complexes are not preferentially evoked to combat sounds in combat-exposed Vietnam veterans with and without post-traumatic stress disorder

The primary objective was to compare the evoked K-complex response to salient versus non-salient auditory stimuli in combat-exposed Vietnam veterans with and without post-traumatic stress disorder (PTSD). Three categories of auditory stimuli (standard 1000Hz tones, trauma-related combat sounds, and affectively neutral environmental sounds) were presented during stage 2 sleep utilizing an oddball paradigm with probabilities of occurrence of 60%, 20% and 20% respectively. Twenty-four combat-exposed Vietnam veterans, 14 with PTSD and 10 without PTSD were studied in a sleep laboratory at the National Center for PTSD in Menlo Park, CA. While significantly fewer K-complexes overall were elicited in patients, there were no differences in the proportion of K-complexes elicited by tones and combat stimuli within either group. Patients produced significantly more K-complexes to neutral stimuli than to tone or combat stimuli. Examination of the N550 component of the evoked K-complex revealed significantly longer latencies in the patient group. Across the entire sample, N550 latencies were longer for combat stimuli relative to tone neutral stimuli. There were no group or stimulus category differences for N550 amplitude. The results suggest that salient information, as defined by trauma-related combat sounds, did not preferentially elicit K-complexes in either the PTSD group or the control group, suggesting that K-complexes function to protect sleep more than to endogenously process meaningful stimuli.

State-dependent changes in cortical gain control as measured by auditory evoked responses to varying intensity stimuli

STUDY OBJECTIVES

Auditory evoked potential (AEP) components correspond to sequential activation of brain structures within the auditory pathway and reveal neural activity during sensory processing. To investigate state-dependent modulation of stimulus intensity response profiles within different brain structures, we assessed AEP components across both stimulus intensity and state.

DESIGN

We implanted adult female Sprague-Dawley rats (N = 6) with electrodes to measure EEG, EKG, and EMG. Intermittent auditory stimuli (6-12 s) varying from 50 to 75 dBa were delivered over a 24-h period. Data were parsed into 2-s epochs and scored for wake/sleep state.

RESULTS

All AEP components increased in amplitude with increased stimulus intensity during wake. During quiet sleep, however, only the early latency response (ELR) showed this relationship, while the middle latency response (MLR) increased at the highest 75 dBa intensity, and the late latency response (LLR) showed no significant change across the stimulus intensities tested. During rapid eye movement sleep (REM), both ELR and LLR increased, similar to wake, but MLR was severely attenuated.

CONCLUSIONS

Stimulation intensity and the corresponding AEP response profile were dependent on both brain structure and sleep state. Lower brain structures maintained stimulus intensity and neural response relationships during sleep. This relationship was not observed in the cortex, implying state-dependent modification of stimulus intensity coding. Since AEP amplitude is not modulated by stimulus intensity during sleep, differences between paired 75/50 dBa stimuli could be used to determine state better than individual intensities.

Cortical evoked responses associated with arousal from sleep

STUDY OBJECTIVE

To determine if low-level intermittent auditory stimuli have the potential to disrupt sleep during 24-h recordings, we assessed arousal occurrence to varying stimulus intensities. Additionally, if stimulus-generated evoked response potential (ERP) components provide a metric of underlying cortical state, then a particular ERP structure may precede an arousal.

DESIGN

Physiological electrodes measuring EEG, EKG, and EMG were implanted into 5 adult female Sprague-Dawley rats. We delivered auditory stimuli of varying intensities (50-75 dBa sound pressure level SPL) at random intervals of 6-12 s over a 24-hour period. Recordings were divided into 2-s epochs and scored for sleep/wake state. Following each stimulus, we identified whether the animal stayed asleep or woke. We then sorted the stimuli depending on prior and post-stimulus state, and measured ERP components.

RESULTS

Auditory stimuli did not produce a significant increase in the number of arousals compared to silent control periods. Overall, arousal from REM sleep occurred more often compared to quiet sleep. ERPs preceding an arousal had decreased mean area and shorter N1 latency.

CONCLUSION

Low level auditory stimuli did not fragment animal sleep since we observed no significant change in arousal occurrence. Arousals that occurred within 4 s of a stimulus exhibited an ERP mean area and latency had features similar to ERPs generated during wake, indicating that the underlying cortical tissue state may contribute to physiological conditions required for arousal.

How does the pitch and pattern of a signal affect auditory arousal thresholds?

How arousal thresholds vary with different sounds is a critical issue for emergency awakenings, especially as sleepers are dying in fires despite having a working smoke alarm. Previous research shows that the current high-pitched (3000+ Hz) smoke alarm signal is significantly less effective than an alternative signal, the 520 Hz square wave, in all populations tested. However, as the number of sounds tested has been small further research is needed. Here we measured auditory arousal thresholds (AATs) across signals with a range of characteristics to determine the most effective waking signal. Thirty-nine young adults participated over three nights. In Part A, nine signals were presented in stage 4 sleep with ascending decibel levels. Signals were short beeps in the low- to mid-frequency range with different spectral complexities: square waves, pure tones, whoops and white noise. Part B manipulated temporal patterns, inserting silences of 0, 10 and 21 s after each 12 s of beeps. It was found that the low-frequency (400 and 520 Hz) square waves yielded significantly lower AATs than the alternatives. A trend was found across the three temporal manipulations, with a 10 s intervening silence showing some advantage. These findings support earlier research indicating that the best sound for awakening from deep sleep is a low-frequency square wave. It is argued that the signal with the lowest response threshold when awake may be the same as the most arousing signal when asleep, especially where the sleeper processes the signal as meaningful.

Mechanisms underlying state dependent surface-evoked response patterns

Cortical evoked response potentials (ERPs) display a rich set of waveforms that are both context and state dependent. However, the mechanisms that underlie state dependent ERP patterns are unclear. Determining those mechanisms through analysis of single trial ERP waveform signatures may provide insight into the regulation of cortical column state and the roles that sleep plays in cortical function. We implanted rats with electroencephalogram (EEG) and electromyogram (EMG) electrodes to record ERPs and to assess sleep/wake states continuously during 1-2 s random auditory clicks. Individual cortical auditory ERPs were sorted into one of eight behavioral states, and fell into three categories based on amplitude and latency characteristics. ERPs within waking and rapid eye movement (REM) sleep were predominantly low amplitude and short latency. Approximately 50% of ERPs during light quiet sleep (quiet sleep 1 and quiet sleep 2) exhibited low amplitude, short latency responses, and the remaining ERPs had high amplitude, long latency responses. This distribution was characteristic of EEG fluctuations during low frequency delta waves. Significantly more individual ERPs showed very low amplitudes during deep quiet sleep (quiet sleep 3 and quiet sleep 4), resulting in a lower average ERP. These results support the hypothesis that evoked response amplitudes and waveform patterns follow specific EEG patterns. Since evoked response characteristics distribute differently across states, they could aid our understanding of sleep mechanisms through state-related and local neural signaling.

Anticipatory cortical activation precedes auditory events in sleeping infants

Background

Behavioral studies have shown that infants can form associations between environmental events and produce anticipatory actions for the predictable event, but the neural mechanisms for the learning and anticipation of events in infants are not known. Recent neuroimaging studies revealed that the association cortices of infants show activation related to auditory-stimulus discrimination and novelty detection during sleep. In the present study, we expected that when an auditory cue (beeps) predicted an auditory event (a female voice), specific regions of the infant cortex would show anticipatory activation before the event onset even while sleeping.

Methodology/Principal Findings

We examined the cortical activation of 3-month-old infants during delays between the cue and the event by using multi-channel near-infrared spectroscopy. To investigate spatiotemporal changes in cortical activation over the experimental session, we divided the session into two phases (early and late phase) and analyzed each phase separately. In the early phase, the frontal regions showed activation in response to the cue that was followed by the event compared with another cue that was not followed by any event. In the late phase, the temporoparietal region, in addition to the frontal region, showed prominent activation in response to the cue followed by the event. In contrast, when the cue was followed by an event and no-event in equal proportions, cortical activation in response to the cue was not observed in any phase.

Conclusions

Sleeping 3-month-old infants showed anticipatory cortical activation in the temporoparietal and frontal regions only in response to the cue predicting the event, suggesting that infants can implicitly form associations between temporally separated events and generate the anticipatory activation before the predictable event. Furthermore, the different time evolution of activation in the temporoparietal and frontal regions suggests that these regions may be involved in different aspects of learning and predicting future events.

Neural representations during sleep: From sensory processing to memory traces

In the course of a day, the brain undergoes large-scale changes in functional modes, from attentive wakefulness to the deepest stage of sleep. The present paper evaluates how these state changes affect the neural bases of sensory and cognitive representations. Are organized neural representations still maintained during sleep? In other words, despite the absence of conscious awareness, do neuronal signals emitted during sleep contain information and have a functional relevance? Through a critical evaluation of the animal and human literature, neural representations at different levels of integration (from the most elementary sensory level to the most cognitive one) are reviewed. Recordings of neuronal activity in animals at presentation of neutral or significant stimuli show that some analysis of the external word remains possible during sleep, allowing recognition of behaviorally relevant stimuli. Event-related brain potentials in humans confirm the preservation of some sensory integration and discriminative capacity. Behavioral and neuroimaging studies in humans substantiate the notion that memory representations are reactivated and are reorganized during post-learning sleep; these reorganisations may account for the beneficial effects of sleep on behavioral performance. Electrophysiological results showing replay of neuronal sequences in animals are presented, and their relevance as neuronal correlates of memory reactivation is discussed. The reviewed literature provides converging evidence that structured neural representations can be activated during sleep. Which reorganizations unique to sleep benefit memory representations, and to what extent the operations still efficient in processing environmental information during sleep are similar to those underlying the non-conscious, automatic processing continually at work in wakefulness, are challenging questions open to investigation.

Is the failure to detect stimulus deviance during sleep due to a rapid fading of sensory memory or a degradation of stimulus encoding?

The mismatch negativity (MMN) is thought to reflect the outcome of a system responsible for the detection of change in an otherwise repetitive, homogenous acoustic environment. This process depends on the storage and maintenance of a sensory representation of the frequently presented stimulus to which the deviant stimulus is compared. Few studies have been able to record the MMN in non-rapid eye movement (NREM) sleep. This pattern of results might be explained by either a rapid fading of sensory memory or an inhibition of stimulus input prior to entry into the cortical MMN generator site. The present study used a very rapid rate of presentation in an attempt to capture mismatch-related negativity prior to the fading of sensory memory. Auditory event-related potentials were recorded from 12 subjects during a single sleep period. A 1000 Hz standard stimulus was presented every 150 ms. At random, on 6.6% of the trials, the standard was changed to either a large 2000 Hz or a small 1100 Hz deviant. In wakefulness, the large deviant elicited an extended negativity that was reduced in amplitude following the presentation of the small deviant. This negativity was also apparent during REM sleep following the presentation of the large deviant. These deviant-related negativities (DRNs) were probably a composite of N1 and MMN activity. During NREM sleep (stage 2 and slow-wave sleep), only the large deviant continued to elicit a DRN. However this DRN might be overlapped by the initial activity of a component that is unique to sleep, the N350. There was little evidence of the DRN or the MMN during sleep following the presentation of the small deviant. A rapid rate of presentation, therefore, does not preserve the MMN following small deviance within sleep. It is possible that inhibition of sensory input occurs before entry into the MMN generating system in the temporal cortex.

Fear conditioning-induced plasticity in auditory thalamus and cortex: To what extent is it expressed during slow-wave sleep?

After fear conditioning to a tone, rats received nonawakening presentations of the tone alone during slow-wave sleep (SWS) episodes. Multiunit activity was recorded in the medial part of the medial geniculate (MGm) and in the primary auditory cortex (ACx). Although tone-evoked responses were increased in MGm and ACx during the 3 conditioning sessions, group data failed to show any significant changes during SWS. Nonetheless, the few recordings (5/29) that exhibited the strongest conditioned responses during wakefulness expressed enhanced responding during SWS. Compared with previous data obtained in MGm during paradoxical sleep, associative plastic changes were less easily expressed during SWS. These results are discussed with regard to functional changes that occur in the thalamocortical system across vigilance states.

Event-related potential measures of the inhibition of information processing: II. The sleep onset period

The loss of consciousness during the sleep onset period is associated with dramatic changes in information processing. Human event-related potentials (ERPs) reflect these changes. Short- and mid-latency ERPs are only minimally affected by sleep onset. On the other hand, long-latency ERPs are very much affected. A negative wave, N1, peaking at approximately 100 ms gradually decreases in amplitude until it reaches baseline level during definitive stage 2 sleep. The changes in N1 are especially apparent when the subject no longer signals awareness of the external stimulus or when stage 1 is dominated by theta activity in the EEG. The positive peaks, P1 and P2, peaking at approximately 50 and 180 ms, respectively, may appear to increase in amplitude (i.e. also be less negative). A long-lasting processing negativity (PN) may overlap and summate with these peaks during the waking state. During sleep onset, the PN dissipates, thus explaining the apparent positive baseline shift in the ERP waveform. In an oddball task, when an alert and awake subject detects a rare, relevant stimulus, a large positive wave, P300, maximum over parietal areas of the scalp, is observed. This P300 is, however, widely dispersed and can be observed over frontal areas of the scalp. When the subject no longer signals detection of this target stimulus, P300 can no longer be recorded. During stage 1, the parietal P300 remains large, providing the subject overtly detects the target. The amplitude of the frontal aspect of P300 is much reduced as response times slow. This may reflect deactivation of the frontal lobes during the sleep onset period. The infrequent change of an otherwise rapidly presented homogenous train of stimuli is associated with another long-lasting negativity, the mismatch negativity (MMN). The MMN also decreases in amplitude during the sleep onset period, reaching baseline level during definitive sleep. The vertex sharp wave (VSW) becomes apparent during the sleep onset period. Associated with the VSW is a late negative ERP, sometimes called the sleep N2 or the N350, peaking between 300 and 350 ms. It is unique to the sleep onset and sleep periods, becoming very large during stage 1-theta or when the subject no longer shows signs of awareness of the external stimulus.

Mismatch negativity (MMN): an objective measure of sensory memory and long-lasting memories during sleep

Sleep, unlike wakefulness, facilitates the internal stimulus generation and hinders the processing of external stimulation. Nevertheless, evidence yielded by physiological studies in animals and event-related potential (ERP) studies in humans suggest that basic functions of the central auditory system are still preserved during sleep. This review is focused on the automatic change-detection function of the auditory system as revealed by a negative ERP component called mismatch negativity (MMN). MMN mainly originates in the auditory cortex, although it also receives an important contribution from subcortical areas (especially at thalamic level), as well as frontal areas. We discuss recent experiments supporting the use of MMN as an objective measure of sensory memory and long-lasting memories not only during wakefulness, but also during sleep. The outcome of the activation of MMN generating system during sleep highly differs from that in waking, especially when there is no previous information about the stimulus sequence in the neuronal network as a result of learning. We discuss these differences in MMN generation in terms of a dynamicist view of the brain that emphasizes the importance of the integration between bottom-up and top-down influences on sensory processing, independently of the processing level in the auditory hierarchy.