Awakening latency from sleep for meaningful and non-meaningful stimuli

Influencing dreams through sensory stimulation: A systematic review

Sleep is typically considered a state of disconnection from the environment, yet instances of external sensory stimuli influencing dreams have been reported for centuries. Explaining this phenomenon could provide valuable insight into dreams' generative and functional mechanisms, the factors that promote sleep continuity, and the processes that underlie conscious awareness. Moreover, harnessing sensory stimuli for dream engineering could benefit individuals suffering from dream-related alterations. This PRISMA-compliant systematic review assessed the current evidence concerning the influence of sensory stimulation on sleep mentation. We included 51 publications, of which 21 focused on auditory stimulation, ten on somatosensory stimulation, eight on olfactory stimulation, four on visual stimulation, two on vestibular stimulation, and one on multimodal stimulation. Furthermore, nine references explored conditioned associative stimulation: six focused on targeted memory reactivation protocols and three on targeted lucid reactivation protocols. The reported frequency of stimulus-dependent dream changes across studies ranged from 0 to ∼80%, likely reflecting a considerable heterogeneity of definitions and methodological approaches. Our findings highlight a lack of comprehensive understanding of the mechanisms, functions, and neurophysiological correlates of stimulus-dependent dream changes. We suggest that a paradigm shift is required for meaningful progress in this field.

Adaptive Solutions to the Problem of Vulnerability During Sleep

Sleep is a behavioral state whose quantity and quality represent a trade-off between the costs and benefits this state provides versus the costs and benefits of wakefulness. Like many species, we humans are particularly vulnerable during sleep because of our reduced ability to monitor the external environment for nighttime predators and other environmental dangers. A number of variations in sleep characteristics may have evolved over the course of human history to reduce this vulnerability, at both the individual and group level. The goals of this interdisciplinary review paper are (1) to explore a number of biological/instinctual features of sleep that may have adaptive utility in terms of enhancing the detection of external threats, and (2) to consider relatively recent cultural developments that improve vigilance and reduce vulnerability during sleep and the nighttime. This paper will also discuss possible benefits of the proposed adaptations beyond vigilance, as well as the potential costs associated with each of these proposed adaptations. Finally, testable hypotheses will be presented to evaluate the validity of these proposed adaptations.

Age-Dependent Responsiveness to Smoke Alarm Signals Among Children

OBJECTIVES

Although it has been established that smoke alarms have more difficulty awakening children from sleep than adults, no attempt has been previously made to characterize how smoke alarm responsiveness changes with age during childhood. The objective of this study is to evaluate the age-dependent responsiveness to various smoke alarm signals among children 5 to 12 years old.

METHODS

The effect of age on children's response to 4 types of smoke alarms (human voice, hybrid voice-tone, low-frequency tone, and high-frequency tone) was evaluated using combined data from 3 previous studies.

RESULTS

There were 540 subjects (median age 9 years; 51.7% male). The proportion of children who awakened demonstrated a statistically significant (P < .001) increase of 3.1% to 7.6% for each additional year of age between 5 and 12 years old for the 4 alarm types. Similarly, child age showed a statistically significant (P < .001) effect on the proportion who escaped for each of the 4 alarm types. The proportion of subjects who awakened or escaped did not differ significantly by sex for any of the alarm types. Median time-to-awaken and median time-to-escape decreased with increase in child age for all alarm types.

CONCLUSIONS

This study demonstrates the substantial influence of child age on the effectiveness of audible smoke alarms during childhood. Among 12-year-olds, only 56.3% escaped within 1 minute (and 67.6% within 2 minutes) to a high-frequency tone. However, a hybrid voice-low-frequency tone alarm is >96% effective at awakening and prompting escape within 1 minute among children 9 years and older.

Sensory processing during sleep in Drosophila melanogaster

During sleep, most animal species enter a state of reduced consciousness characterized by a marked sensory disconnect. Yet some processing of the external world must remain intact, given that a sleeping animal can be awoken by intense stimuli (for example, a loud noise or a bright light) or by soft but qualitatively salient stimuli (for example, the sound of a baby cooing or hearing one's own name^1-3). How does a sleeping brain retain the ability to process the quality of sensory information? Here we present a paradigm to study the functional underpinnings of sensory discrimination during sleep in Drosophila melanogaster. We show that sleeping vinegar flies, like humans, discern the quality of sensory stimuli and are more likely to wake up in response to salient stimuli. We also show that the salience of a stimulus during sleep can be modulated by internal states. We offer a prototypical blueprint detailing a circuit involved in this process and its modulation as evidence that the system can be used to explore the cellular underpinnings of how a sleeping brain experiences the world.

Event-related potentials in insomnia reflect altered perception of sleep

STUDY OBJECTIVES

Insomnia is defined by the subjective complaint of poor sleep as well as daytime impairments. Since polysomnography (PSG) typically shows only modest sleep impairment, some still unidentified property of sleep, not mirrored in PSG, may be modified in insomnia.One possible mechanistic hypothesis is that insomnia patients may be more sensitive to inevitably occurring internal or external stimuli during the night, causing brief sleep disruptions then perceived as wake time.

METHODS

Auditory event-related potentials (ERP) to low intensity (50 dB SPL) synthesized guitar tones played continuously throughout two nights of polysomnographically registered sleep were obtained in fifty patients with insomnia disorder (ID, without comorbidities) and 50 age- and sex- matched good sleeper controls (GSC) for each sleep stage and NREM/REM cycle. Phasic and tonic REM were treated as separate stages. Latencies and amplitudes of components P1, N1 and P2 were measured and analyzed by multivariate repeated-measures ANCOVA including effects of group, night, cycle and age.

RESULTS

ID showed reduced P2 amplitudes relative to GSC specifically in phasic REM sleep. The same reduction also correlated with the amount of sleep misperception across groups. Independent component analysis showed a frontal negativity to contribute most to this group difference.

CONCLUSIONS

The present finding can be interpreted as increased mismatch negativity (MMN) in ID, reflecting automated detection of change in the auditory system and a concomitant orienting response. Specifically phasic REM sleep appears to be vulnerable to sensory afferences in ID patients, possibly contributing to the perception of being awake.

Effectiveness of a Voice Smoke Alarm Using the Child's Name for Sleeping Children: A Randomized Trial

OBJECTIVES

To test maternal voice alarm effectiveness under residential conditions and determine whether personalizing the maternal voice alarm message with the child's first name improves effectiveness.

STUDY DESIGN

Using a randomized, nonblinded, repeated measures design, we compared 3 maternal voice smoke alarms with respect to their ability to awaken 176 children 5-12 years old from stage 4 slow-wave sleep and prompt their performance of an escape procedure. A conventional residential high-frequency tone smoke alarm was used as a comparative reference. Children's sleep stage was monitored in a residence-like research setting.

RESULTS

Maternal voice alarms awakened 86%-91% of children and prompted 84%-86% to escape compared with 53% awakened and 51% escaped for the tone alarm. A sleeping child was 2.9-3.4 times more likely to be awakened by each of the 3 voice alarms than the tone alarm. The median time to awaken was 156 seconds for the tone alarm and 2 seconds for each voice alarm. The proportions of children who awakened and escaped differed significantly between the tone alarm and each voice alarm, but no significant differences were found between each pair of the voice alarms, regardless of whether the child's first name was included in the alarm message.

CONCLUSIONS

The maternal voice alarms significantly outperformed the tone alarm under residential conditions. Personalizing the alarm message with the child's first name did not increase alarm effectiveness. These findings have important implications for development of an effective and practical smoke alarm for children.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT01169155.

Sleep Disrupts High-Level Speech Parsing Despite Significant Basic Auditory Processing

The extent to which the sleeping brain processes sensory information remains unclear. This is particularly true for continuous and complex stimuli such as speech, in which information is organized into hierarchically embedded structures. Recently, novel metrics for assessing the neural representation of continuous speech have been developed using noninvasive brain recordings that have thus far only been tested during wakefulness. Here we investigated, for the first time, the sleeping brain's capacity to process continuous speech at different hierarchical levels using a newly developed Concurrent Hierarchical Tracking (CHT) approach that allows monitoring the neural representation and processing-depth of continuous speech online. Speech sequences were compiled with syllables, words, phrases, and sentences occurring at fixed time intervals such that different linguistic levels correspond to distinct frequencies. This enabled us to distinguish their neural signatures in brain activity. We compared the neural tracking of intelligible versus unintelligible (scrambled and foreign) speech across states of wakefulness and sleep using high-density EEG in humans. We found that neural tracking of stimulus acoustics was comparable across wakefulness and sleep and similar across all conditions regardless of speech intelligibility. In contrast, neural tracking of higher-order linguistic constructs (words, phrases, and sentences) was only observed for intelligible speech during wakefulness and could not be detected at all during nonrapid eye movement or rapid eye movement sleep. These results suggest that, whereas low-level auditory processing is relatively preserved during sleep, higher-level hierarchical linguistic parsing is severely disrupted, thereby revealing the capacity and limits of language processing during sleep.SIGNIFICANCE STATEMENT 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 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.

A roadmap for the study of conscious audition and its neural basis

How and which aspects of neural activity give rise to subjective perceptual experience-i.e. conscious perception-is a fundamental question of neuroscience. To date, the vast majority of work concerning this question has come from vision, raising the issue of generalizability of prominent resulting theories. However, recent work has begun to shed light on the neural processes subserving conscious perception in other modalities, particularly audition. Here, we outline a roadmap for the future study of conscious auditory perception and its neural basis, paying particular attention to how conscious perception emerges (and of which elements or groups of elements) in complex auditory scenes. We begin by discussing the functional role of the auditory system, particularly as it pertains to conscious perception. Next, we ask: what are the phenomena that need to be explained by a theory of conscious auditory perception? After surveying the available literature for candidate neural correlates, we end by considering the implications that such results have for a general theory of conscious perception as well as prominent outstanding questions and what approaches/techniques can best be used to address them.This article is part of the themed issue 'Auditory and visual scene analysis'.

Inhibition of Orexin Signaling Promotes Sleep Yet Preserves Salient Arousability in Monkeys

STUDY OBJECTIVES

In addition to enhancing sleep onset and maintenance, a desirable insomnia therapeutic agent would preserve healthy sleep's ability to wake and respond to salient situations while maintaining sleep during irrelevant noise. Dual orexin receptor antagonists (DORAs) promote sleep by selectively inhibiting wake-promoting neuropeptide signaling, unlike global inhibition of central nervous system excitation by gamma-aminobutyric acid (GABA)-A receptor (GABAaR) modulators. We evaluated the effect of DORA versus GABAaR modulators on underlying sleep architecture, ability to waken to emotionally relevant stimuli versus neutral auditory cues, and performance on a sleepiness-sensitive cognitive task upon awakening.

METHODS

DORA-22 and GABAaR modulators (eszopiclone, diazepam) were evaluated in adult male rhesus monkeys (n = 34) with continuous polysomnography recordings in crossover studies of sleep architecture, arousability to a classically conditioned salient versus neutral acoustical stimulus, and psychomotor vigilance task (PVT) performance if awakened.

RESULTS

All compounds decreased wakefulness, but only DORA-22 sleep resembled unmedicated sleep in terms of underlying sleep architecture, preserved ability to awaken to salient-conditioned acoustic stimuli while maintaining sleep during neutral acoustic stimuli, and no congnitive impairment in PVT performance. Although GABAaR modulators induced lighter sleep, monkeys rarely woke to salient stimuli and PVT performance was impaired if monkeys were awakened.

CONCLUSIONS

In nonhuman primates, DORAs' targeted mechanism for promoting sleep protects the ability to selectively arouse to salient stimuli and perform attentional tasks unimpaired, suggesting meaningful differentiation between a hypnotic agent that works through antagonizing orexin wake signaling versus the sedative hypnotic effects of the GABAaR modulator mechanism of action.

Orexin receptor antagonist-induced sleep does not impair the ability to wake in response to emotionally salient acoustic stimuli in dogs

The ability to awaken from sleep in response to important stimuli is a critical feature of normal sleep, as is maintaining sleep continuity in the presence of irrelevant background noise. Dual orexin receptor antagonists (DORAs) effectively promote sleep across species by targeting the evolutionarily conserved wake-promoting orexin signaling pathway. This study in dogs investigated whether DORA-induced sleep preserved the ability to awaken appropriately to salient acoustic stimuli but remain asleep when exposed to irrelevant stimuli. Sleep and wake in response to DORAs, vehicle, GABA-A receptor modulators (diazepam, eszopiclone and zolpidem) and antihistamine (diphenhydramine) administration were evaluated in telemetry-implanted adult dogs with continuous electrocorticogram, electromyogram (EMG), electrooculogram (EOG), and activity recordings. DORAs induced sleep, but GABA-A modulators and antihistamine induced paradoxical hyperarousal. Thus, salience gating studies were conducted during DORA-22 (0.3, 1, and 5 mg/kg; day and night) and vehicle nighttime sleep. The acoustic stimuli were either classically conditioned using food reward and positive attention (salient stimulus) or presented randomly (neutral stimulus). Once conditioned, the tones were presented at sleep times corresponding to maximal DORA-22 exposure. In response to the salient stimuli, dogs woke completely from vehicle and orexin-antagonized sleep across all sleep stages but rarely awoke to neutral stimuli. Notably, acute pharmacological antagonism of orexin receptors paired with emotionally salient anticipation produced wake, not cataplexy, in a species where genetic (chronic) loss of orexin receptor signaling leads to narcolepsy/cataplexy. DORA-induced sleep in the dog thereby retains the desired capacity to awaken to emotionally salient acoustic stimuli while preserving uninterrupted sleep in response to irrelevant 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.

Arousal thresholds during human tonic and phasic REM sleep

The goal of the present study was to investigate arousal thresholds (ATs) in tonic and phasic episodes of rapid eye movement (REM) sleep, and to compare the frequency spectrum of these sub-states of REM to non-REM (NREM) stages of sleep. We found the two REM stages to differ with regard to behavioural responses to external acoustic stimuli. The AT in tonic REM was indifferent from that in sleep stage 2, and ATs in phasic REM were similar to those in slow-wave sleep (stage 4). NREM and REM stages of similar behavioural thresholds were distinctly different with regard to their frequency pattern. These data provide further evidence that REM sleep should not be regarded a uniform state. Regarding electroencephalogram frequency spectra, we found that the two REM stages were more similar to each other than to NREM stages with similar responsivity. Ocular activity such as ponto-geniculo-occipital-like waves and microsaccades are discussed as likely modulators of behavioural responsiveness and cortical processing of auditory information in the two REM sub-states.

Sensory responses during sleep in primate primary and secondary auditory cortex

Most sensory stimuli do not reach conscious perception during sleep. It has been thought that the thalamus prevents the relay of sensory information to cortex during sleep, but the consequences for cortical responses to sensory signals in this physiological state remain unclear. We recorded from two auditory cortical areas downstream of the thalamus in naturally sleeping marmoset monkeys. Single neurons in primary auditory cortex either increased or decreased their responses during sleep compared with wakefulness. In lateral belt, a secondary auditory cortical area, the response modulation was also bidirectional and showed no clear systematic depressive effect of sleep. When averaged across neurons, sound-evoked activity in these two auditory cortical areas was surprisingly well preserved during sleep. Neural responses to acoustic stimulation were present during both slow-wave and rapid-eye movement sleep, were repeatedly observed over multiple sleep cycles, and demonstrated similar discharge patterns to the responses recorded during wakefulness in the same neuron. Our results suggest that the thalamus is not as effective a gate for the flow of sensory information as previously thought. At the cortical stage, a novel pattern of activation/deactivation appears across neurons. Because the neural signal reaches as far as secondary auditory cortex, this leaves open the possibility of altered sensory processing of auditory information during sleep.

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.

Comparison of a personalized parent voice smoke alarm with a conventional residential tone smoke alarm for awakening children

BACKGROUND

Conventional residential tone smoke alarms fail to awaken the majority of children during slow wave sleep. With the objective of identifying a more effective smoke alarm for children, we compared a personalized parent voice smoke alarm with a conventional residential tone smoke alarm, both presented at 100 dB, with respect to their ability to awaken children 6- to 12-years-old from stage 4 sleep and prompt their performance of a simulated self-rescue escape procedure.

METHODS

Using a randomized, nonblinded, clinical research design, a volunteer sample of healthy children 6- to 12-years-old was enrolled in the study. Children were trained how to perform a simulated self-rescue escape procedure when they heard a smoke alarm. Each child's mother recorded a voice alarm message, "First name! First name! Wake up! Get out of bed! Leave the room!" For each child, either the voice or tone smoke alarm was randomly selected and triggered during the first cycle of stage 4 sleep, and then the other alarm was triggered during the second cycle of stage 4 sleep. Children's sleep stage was monitored by electroencephalography, electro-oculography, and chin electromyography. The 4 main outcome measures included the number of children who awakened, the number of children who escaped, the time to awakening, and the time to escape.

RESULTS

Twenty-four children were enrolled. The median age was 9 years, and 11 (46%) were boys. One half of the children received the parent voice alarm first, and one half received the tone alarm first; however, the order that the alarm stimuli were presented was not statistically associated with awakening or escaping. Twenty-three (96%) of the 24 subjects awakened to the parent voice alarm compared with 14 (58%) to the tone alarm. One child did not awaken to either stimulus. Nine children awakened to their parent's voice but not to the tone, whereas none awakened to only the tone and not the voice. Twenty (83%) of the subjects in the parent voice alarm group successfully performed the escape procedure within 5 minutes of alarm onset compared with 9 (38%) in the tone alarm group. The median time to awaken was 20 seconds in the voice alarm group compared with 3 minutes in the tone alarm group. The median time to escape was 38 seconds in the voice alarm group compared with the maximum allowed 5 minutes in the tone alarm group. When exposed to the tone alarm, older children were more likely to awaken and were more likely to escape than younger children. There was no association between child's age and awakening or escaping for children exposed to the parent voice alarm. There was no association between child's gender and awakening or escaping for either alarm type.

CONCLUSIONS

To our knowledge, this study is the first to compare the ability of different types of smoke alarms to awaken children while monitoring sleep stage. The personalized parent voice smoke alarm at 100 dB successfully awakened 96% of children 6- to 12-years-old from stage 4 sleep with 83% successfully performing a simulated self-rescue escape procedure, significantly outperforming the 100-dB conventional residential tone smoke alarm. These findings suggest a clear direction for future research, as well as important fundamental changes in smoke alarm design, that address the unique developmental needs of children. The development of a more effective smoke alarm for use in homes and other locations where children sleep provides an opportunity to reduce fire-related morbidity and mortality among children.

The attention-intention-effort pathway in the development of psychophysiologic insomnia: a theoretical review

Psychophysiologic insomnia (PI) is the most common form of persistent primary insomnia. Its 'behavioral phenotype', comprising elements such as conditioned arousal, sleep-incompatible behavior and sleep preoccupation, has not changed markedly across several generations of diagnostic nosology. Moreover, a substantial outcome literature demonstrates that PI can be treated effectively using a range of psychological interventions. It seems evident that behavioral and cognitive factors play a part. What is less clear is exactly how PI develops and what are its crucial maintaining factors. This paper proposes an explanatory model, that we call the attention-intention-effort pathway. The argument is that sleep normalcy is a relatively automatic process. Consequently, it is vulnerable, and may be inhibited, by focused attention and by direct attempts to control its expression. Drawing upon parallels in the literature on adult psychopathology, and upon recent clinical and experimental studies on insomnia, the evidence for this pathway is considered and a research agenda is outlined. In particular, computerized tests of cognitive bias are seen as offering an objective means of appraising mental processes in insomnia. These may be applied concurrently with somatic measurements in future studies to better understand this common psycho-physiologic condition.

Experimental pain perception remains equally active over all sleep stages

The literature on sensory perception during sleep suggests that light sleep (Stage 2) is more responsive to external sensory stimulation (e.g. sound, electrical shock) than deep sleep (Stages 3 and 4) and REM sleep. The main objective of this study was to characterize the specificity of nociceptive stimulation to trigger sleep arousal-awakening over all sleep stages. Thirteen healthy adults (e.g. without pain or sleep problems; six female and seven male of a mean age of 24.2+/-1.3 years) were included in the study. The responses to noxious intramuscular 5% hypertonic infusion were compared to innocuous vibrotactile and to respective control stimulations: isotonic infusion and auditory stimulations. These stimulations were applied during wakefulness and were repeated during sleep. Polygraphic signals (e.g. brain activity, heart rate) signals were recorded to score sleep arousal over all sleep stages. A subjective assessment of sleep quality was made on next morning. No overnight sensitization or habituation occurred with any of the experimental stimulations. The vibratory-auditory stimulations and the noxious hypertonic infusions triggered significantly (P < 0.05) more awakenings in sleep Stage 2 and in REM than their respective control stimulations. In sleep Stage 2, both vibratory + auditory stimulations and the noxious hypertonic infusions has the same awakening response frequency (approximately 30%), however, with the noxious infusions the response frequency were similar in sleep Stages 3 and 4 (P < 0.05) and in REM (trend). Compared to the baseline night, sleep quality was lower following the night with noxious stimulation (90.1+/-2.7 and 73.3+/-7.4 mm, respectively; P < 0.03. These data suggest that pain during sleep could trigger a sleep awaking response over all sleep stages and not only in light sleep.

EEG slow (approximately 1 Hz) waves are associated with nonstationarity of thalamo-cortical sensory processing in the sleeping human

Intracellular studies reveal that, during slow wave sleep (SWS), the entire cortical network can swing rhythmically between extremely different microstates, ranging from wakefulness-like network activation to functional disconnection in the space of a few hundred milliseconds. This alternation of states also involves the thalamic neurons and is reflected in the EEG by a slow (<1 Hz) oscillation. These rhythmic changes, occurring in the thalamo-cortical circuits during SWS, may have relevant, phasic effects on the transmission and processing of sensory information. However, brain reactivity to sensory stimuli, during SWS, has traditionally been studied by means of sequential averaging, a procedure that necessarily masks any short-term fluctuation of responsiveness. The aim of this study was to provide a dynamic evaluation of brain reactivity to sensory stimuli in naturally sleeping humans. To this aim, single-trial somatosensory evoked potentials (SEPs) were grouped and averaged as a function of the phase of the ongoing sleep slow (<1 Hz) oscillation. This procedure revealed a dynamic profile of responsiveness, which was conditioned by the phase of the spontaneous sleep EEG. Overall, the amplitude of the evoked potential changed sistematically, increasing and approaching wakefulness levels along the negative slope of the EEG oscillation and decaying below SWS average levels along the positive drift. These marked and fast changes of stimulus-correlated electrical activity involved both short (N20) and long latency (P60 and P100) components of SEPs. In addition, the observed short-term response variability appeared to be centrally generated and specifically related to the evolution of the spontaneous oscillatory pattern. The present findings demonstrate that thalamo-cortical processing of sensory information is not stationary in the very short period (approximately 500 ms) during natural SWS.

Semantic analysis of auditory input during sleep: studies with event related potentials

This review summarises the results of event-related potentials studies exploring the extent to which the human brain can extract semantic information from external stimuli during sleep. The persistence of a differential response to the subject's own name, relative to any other proper name, during stage 2 (S2) and paradoxical (REM) sleep (PS) suggests that the brain remains able to discriminate an intrinsically relevant word during these sleep stages. The similarities and the differences between these sleep cognitive responses and the waking P300 are stressed, and the functional significance of this component discussed especially in relation with consciousness and memory of the stimulus. Recent studies of the 'N400' potential evoked by semantically incongruous words, have shown that this component may be also elicited during S2 and PS, indicating preserved detection of semantic discordance during these sleep stages. However, linguistic incongruity appears to be processed in a different manner during PS than during waking, since words devoid of meaning (pseudo-words), which are detected as anomalous and evoke N400 during waking, yielded responses similar to those of congruous words in PS. All these data support the view that some semantic analysis of auditory stimuli remains possible in the human sleeping brain, and warrant further studies to elucidate the extent and limits of these capabilities.

Animal models for information processing during sleep

Information provided by external stimuli does reach the brain during sleep, although the amount of information is reduced during sleep compared to wakefulness. The process controlling this reduction is called 'sensory' gating and evidence exists that the underlying neurophysiological processes take place in the thalamus. Furthermore, it is clear that stimuli given during sleep can alter the functional state of the brain. Two factors have been shown to play a crucial role in causing changes in the sleeping brain: the intensity and the relevance of the stimulus. Intensive stimuli arouse the brain, as well as stimuli having a high informational impact on the sleeping person. The arousal threshold for important stimuli is quite low compared to neutral stimuli. A central question in sleep research is whether associative learning, or in other words the formation of new associations between stimuli, can take place in a sleeping brain. It has been shown that simple forms of learning are still possible during sleep. In sleeping rats, it is proven that habituation, an active, simple form of learning not to respond to irrelevant stimuli, can occur. Moreover, there is evidence for the view that more complex associations can be modulated and newly formed during sleep. This is shown by two experimental approaches: an extinction paradigm and a latent inhibition (pre-exposure) paradigm. The presentation of non-reinforced stimuli during sleep causes slower extinction compared to the same presentation of these stimuli during wakefulness. Consistently, the suppressive capacity of a stimulus in the latent inhibition paradigm is less when previously pre-exposed during sleep, as compared to pre-exposure during wakefulness. Thus, while associative learning is not completely blocked during sleep, aspects of association formation are clearly altered. However, animal studies also clearly indicate that complex forms of learning are not possible during sleep. It is hypothesised that this restriction in information processing during sleep is due to the considerable reduction of incoming information by the sleeping brain. This reduction may serve to protect the sleep process.

Cortical deactivation induced by visual stimulation in human slow-wave sleep

It has previously been demonstrated that sleeping and sedated young children respond with a paradoxical decrease in the blood oxygenation level-dependent (BOLD) functional magnetic resonance imaging (fMRI) signal in the rostro-medial occipital visual cortex during visual stimulation. It is unresolved whether this negative BOLD response pattern is of developmental neurobiological origin particular to a given age or to a general effect of sleep or sedative drugs. To further elucidate this issue, we used fMRI and positron emission tomography (PET) to study the brain activation pattern during visual stimulation in spontaneously sleeping adult volunteers. In five sleeping volunteers fMRI studies confirmed a robust signal decrease during stimulation in the rostro-medial occipital cortex. A similar relative decrease at the same location was found during visual stimulation and polysomnographically verified slow-wave sleep in a separate group of six subjects using H(2)(15)O PET measures of the regional cerebral blood flow (rCBF). This decrease was more rostro-dorsal compared to the relative rCBF increase along the calcarine sulcus found during visual stimulation in the awake state. This study reconfirms the previously described paradoxical stimulation-correlated negative BOLD signal change in the rostro-medial occipital cortex, expanding this response mode to an age spectrum ranging from the newborn to the adult. Further, the use of complementary brain mapping techniques suggests that this decrease was secondary to a relative rCBF decrease. Possible mechanisms for the paradoxical response pattern during sleep include an active inhibition of the visual cortex or a disruption of an energy-consuming process.

Corticothalamic resonance, states of vigilance and mentation

During various states of vigilance, brain oscillations are grouped together through reciprocal connections between the neocortex and thalamus. The coherent activity in corticothalamic networks, under the control of brainstem and forebrain modulatory systems, requires investigations in intact-brain animals. During behavioral states associated with brain disconnection from the external world, the large-scale synchronization of low-frequency oscillations is accompanied by the inhibition of synaptic transmission through thalamocortical neurons. Despite the coherent oscillatory activity, on the functional side there is dissociation between the thalamus and neocortex during slow-wave sleep. While dorsal thalamic neurons undergo inhibitory processes due to the prolonged spike-bursts of thalamic reticular neurons, the cortex displays, periodically, a rich spontaneous activity and preserves the capacity to process internally generated signals that dominate the state of sleep. In vivo experiments using simultaneous intracellular recordings from thalamic and cortical neurons show that short-term plasticity processes occur after prolonged and rhythmic spike-bursts fired by thalamic and cortical neurons during slow-wave sleep oscillations. This may serve to support resonant phenomena and reorganize corticothalamic circuitry, determine which synaptic modifications, formed during the waking state, are to be consolidated and generate a peculiar kind of dreaming mentation. In contrast to the long-range coherent oscillations that occur at low frequencies during slow-wave sleep, the sustained fast oscillations that characterize alert states are synchronized over restricted territories and are associated with discrete and differentiated patterns of conscious events.

Neuronal phenomena associated with vigilance and consciousness: from cellular mechanisms to electroencephalographic patterns

The neuroanatomical substrates controlling and regulating sleeping and waking, and thus consciousness, are located in the brain stem. Most crucial for bringing the brain into a state conducive for consciousness and information processing is the mesencephalic part of the brain stem. This part controls the state of waking, which is generally associated with a high degree of consciousness. Wakefulness is accompanied by a low-amplitude, high-frequency electroencephalogram, due to the fact that thalamocortical neurons fire in a state of tonic depolarization. Information can easily pass the low-level threshold of these neurons, leading to a high transfer ratio. The complexity of the electroencephalogram during conscious waking is high, as expressed in a high correlation dimension. Accordingly, the level of information processing is high. Spindles, and alpha waves in humans, mark the transition from wakefulness to sleep. These phenomena are related to drowsiness, associated with a reduction in consciousness. Drowsiness occurs when cells undergo moderate hyperpolarizations. Increased inhibitions result in a reduction of afferent information, with a lowered transfer ratio. Information processing subsides, which is also expressed in a diminished correlation dimension. Consciousness is further decreased at the onset of slow wave sleep. This sleep is controlled by the medullar reticular formation and is characterized by a high-voltage, low-frequency electroencephalogram. Slow wave sleep becomes manifest when neurons undergo a further hyperpolarization. Inhibitory activities are so strong that the transfer ratio further drops, as does the correlation dimension. Thus, sensory information is largely blocked and information processing is on a low level. Finally, rapid eye movement sleep is regulated by the pontine reticular formation and is associated with a "wake-like" electroencephalographic pattern. Just as during wakefulness, this is the expression of a depolarization of thalamocortical neurons. The transfer ratio of rapid eye movement sleep has not yet been determined, but seems to vary. Evidence exists that this type of sleep, associated with dreaming, with some kind of perception and consciousness, is involved in processing of "internal" information. In line with this, rapid eye movement sleep has higher correlation dimensions than slow-wave sleep and sometimes even higher than wakefulness. It is assumed that the "near-the-threshold" depolarized state of neurons in the thalamus and cerebral cortex is a necessary condition for perceptual processes and consciousness, such as occurs during waking and in an altered form during rapid eye movement sleep.

Sleep, brain activation and cognition

Some data have shown the presence of time-of-day effects in learning processes. We explore here whether the same phenomenon occurs during the night and how it relates to REM sleep. In an initial approach to the question, this paper points out the relationships between: 1) REM sleep and brain activation, and 2) REM sleep and information processing. The data are discussed in terms of a REM sleep implication on information processing and we examine the possibility of modifying this processing by acting on REM sleep.

Autonomic responses to meaningful and non-meaningful auditory stimuli in coma

In a group of poisoned patients, the lowest MCS (Munich Coma Scale) vigilance grade at which patients responded when called by their names was determined. In 12 patients at this vigilance grade, the extent to which the meaning of the names contributed to their reactions was investigated. Their first name and a control stimulus (name backwards) were repeatedly presented via headphones. Stimulus intensity was 90 dB(A). During stimulus presentation, changes in heart rate, finger pulse amplitude, and electrodermal activity were recorded. A comparison between responses under both stimulus conditions revealed that the first reactions of poisoned patients to their names are primarily elicited by the physical properties of the stimulus.

On the function of sleep

It is proposed that sleep serves the function of maintaining immobility in animals at times when immobility is an optimum behavioural survival strategem. Sleep is of great evolutionary age and is always carefully tailored to the life-style of each animal; abundant in some species and absent from others. The survival advantages conferred on an animal by the power of sleep to schedule behaviour effectively may justify its existence and persistence in the evolution of species.