Late component variants of single auditory evoked responses during NREM sleep stage 2 in man

Structure of Spectral Composition and Synchronization in Human Sleep on the Whole Scalp: A Pilot Study

We used numerical methods to define the normative structure of the different stages of sleep and wake (W) in a pilot study of 19 participants without pathology (18-64 years old) using a double-banana bipolar montage. Artefact-free 120-240 s epoch lengths were visually identified and divided into 1 s windows with a 10% overlap. Differential channels were grouped into frontal, parieto-occipital, and temporal lobes. For every channel, the power spectrum (PS) was calculated via fast Fourier transform and used to compute the areas for the delta (0-4 Hz), theta (4-8 Hz), alpha (8-13 Hz), and beta (13-30 Hz) bands, which were log-transformed. Furthermore, Pearson's correlation coefficient and coherence by bands were computed. Differences in logPS and synchronization from the whole scalp were observed between the sexes for specific stages. However, these differences vanished when specific lobes were considered. Considering the location and stages, the logPS and synchronization vary highly and specifically in a complex manner. Furthermore, the average spectra for every channel and stage were very well defined, with phase-specific features (e.g., the sigma band during N2 and N3, or the occipital alpha component during wakefulness), although the slow alpha component (8.0-8.5 Hz) persisted during NREM and REM sleep. The average spectra were symmetric between hemispheres. The properties of K-complexes and the sigma band (mainly due to sleep spindles-SSs) were deeply analyzed during the NREM N2 stage. The properties of the sigma band are directly related to the density of SSs. The average frequency of SSs in the frontal lobe was lower than that in the occipital lobe. In approximately 30% of the participants, SSs showed bimodal components in the anterior regions. qEEG can be easily and reliably used to study sleep in healthy participants and patients.

Consistent sequential activity across diverse forms of UP states under ketamine anesthesia

During slow-wave sleep, the neocortex shows complex, self-organized spontaneous activity. Similar slow-wave oscillations are present under anesthesia where massive, persistent network activity (UP states) alternates with periods of generalized neural silence (DOWN states). To investigate the neuronal activity patterns occurring during UP states, we recorded simultaneously from populations of cells in neocortical layer V of ketamine/xylazine-anesthetized rats. UP states formed a diverse class. In particular, simultaneous-onset UP states were typically accompanied by sharp field potentials and 10-14 Hz modulation, and were often grouped in a 3 Hz ('delta') pattern. Longer, slow-onset UP states did not exhibit 10-14 Hz modulation, and showed a slow propagation across recording electrodes ('traveling waves'). Despite this diversity, the temporal patterns of spiking activity were similar across different UP state types. Analysis of cross-correlograms revealed conserved temporal relationships among neurons, with each neuron having specific timing during UP states. As a group, putative interneurons were most active at the beginning of UP states and putative pyramidal cells were active uniformly throughout the duration of UP states. These results show that UP states under ketamine anesthesia have a stable, fine-structured firing pattern despite a large variability in global structure.

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.

The effects of sleep stages and time of night on NREM sleep ERPs

Event-related potential (ERP) is one of the best techniques for studying information processing during sleep because it does not require behavioral responses or consciousness awareness. Several ERP components have been identified during non-rapid eye movement (NREM) sleep, but the associated underlying processes of these waveforms remain unclear. The present study examines the effect of sleep stage and time of night on the NREM ERPs to further understand these processes. An oddball paradigm was conducted in 11 healthy subjects to elicit ERPs throughout the night. Polysomnographic recordings were also applied to identify sleep stages. The results showed that P220, N350, and P900 decreased during the second half of the night, when the NREM sleep drive is partially satiated. This finding is consistent with the notion that the NREM ERPs reflect an inhibitory process associated with sleep drive. P220 and P900 were also found to increase as subjects entering deep sleep. However, the N350 was not affected by the deepening of sleep and peaked earlier during stage 1 sleep. Although these components are all related to the process for sleep preservation, the N350 may be more associated with sleep-wake transition and the P220 and P900 with the process of deepening of sleep.

The impact of slow wave sleep proximity on evoked K-complex generation

During human stage 2 non-rapid eye movement (NREM) sleep, spontaneous K-complexes are more likely to occur prior to transitions to stage 3 or stage 4 sleep (referred to as slow wave sleep or SWS) compared to transitions to REM sleep, suggesting that the K-complex may be the 'forerunner' of SWS. The present study investigated the impact of SWS or REM sleep proximity on the probability of evoking a K-complex (pKC) during stage 2 and on components of the NREM sleep averaged evoked potential. Ten subjects spent three nights in the laboratory. On either the second or third night tones were presented continuously during sleep. Evoked K-complexes and sleep-evoked potentials were assessed for the 10 min of stage 2 prior to SWS (SWS-10) or REM (REM-10) sleep episodes as well as for all of SWS. pKC did not differ between SWS-10 (0.88) and SWS (0.91) but was significantly larger in SWS-10 than REM-10 (0.63). Amplitude effects were seen for the P2, N350, P900 NREM sleep-evoked potential components but not for the K-complex related N550. In each case where amplitude effects were found, SWS-10 was larger than REM-10. No latency differences were seen between conditions for the earlier components (P2, N350) however, both N550 and P900 were significantly shorter during SWS-10 compared to REM-10. These results are consistent with previous spontaneous K-complex studies and are supportive of a relationship between the K-complex and delta activity. They also indicate that stage 2 may consist of a continuum of microstates between SWS and REM sleep that are indicative of different brain stem, diecephalic and cortical patterns of activation.

K-complex, a reactive EEG graphoelement of NREM sleep: an old chap in a new garment

This review summarises data gathered on the KC phenomenon over the past 70 yr. The following issues are discussed: definitions, morphology and topography of KC, the regular participation in NREM sleep, elicitability features of evoked KC, autonomic and motor concomitants, relationship of KC with information processing during NREM sleep, relationship of KC and deltas of NREM sleep, and relationship of KC with sleep cyclicity. KC is a complex multifunctional phenomenon of the sleeping brain involved in information processing and defence against the arousal effect of sensory stimuli. To put the old chap in a new garment, the relationship of KC with synchronisation-type and desynchronisation-type micro-arousals, and the 'cyclic alternating pattern', will be discussed, with an emphasis on the sleep-protecting role of KC and synchronisation-type answers in sleep regulation executed by phasic events. Lastly, the role of KC providing gating functions in idiopathic generalized epilepsies and other, different, sleep disorders are characterised. A 'theoretical epilogue' is appended to show some system theoretical and regulational aspects.

Attention to external stimuli during wakefulness and sleep: Evoked 40-Hz response and N350

Changes of two components of the auditory event-related potential, the evoked 40-Hz response and the N350, were studied during different stages of wakefulness and sleep. The evoked 40-Hz response has been proposed to represent an attention-modulating mechanism; the N350 seems to reflect an inhibitory process associated with reduced information processing. Because recent literature suggests that both components reflect opposite mechanisms, an inverse relationship was expected. Ten participants were presented with tone pips while reading, lying awake in bed, and during light sleep, slow wave sleep, and REM sleep. A significant evoked 40-Hz response was observed during reading and lying awake in bed. N350 was evident in all conditions and, as expected, peaked during light sleep. The hypothesized inverse relationship was confirmed. These findings support former proposals that relate both components to attention. Possibly, the N350 is most pronounced during the sleep transition period because its inhibitory processes have to prevail over the attentional mechanisms (40-Hz response) to permit sleep onset.

Event-related potentials during forced awakening: a tool for the study of acute sleep inertia

Sleep propensity and sleep inertia were assessed in 43 patients with excessive daytime sleepiness (EDS) and 21 sleep-deprived controls, using a forced awakening test under continuous electroencephalographic (EEG) recording. Event-related potentials (ERPs) were first obtained in waking, while participants performed a target detection auditory task. Subjects were then allowed to take a nap with lights off and sleep latency was calculated. After 3 min of continuous sleep, frequent and rare tones were suddenly presented again (and ERPs recorded) in a forced awakening condition, which was repeated a second time if patients fell asleep. ERPs in pre-nap wakefulness did not differ in patients and controls. On forced awakening, almost half (48%) of EDS patients retained morphologically normal ERPs, but showed a significant delay of P300 relative to waking. In the other half of the patients (and none of the controls), the N200/P300 complex to targets was replaced on forced awakening by high-amplitude negative waves ('sleep negativities'). Single subject analysis showed that 65% of patients had abnormal responses during forced awakening (significant P3 delay or sleep negativities), while only three of them (7%) had abnormal ERPs on wakefulness. The presence of sleep negativities was associated with shorter sleep latencies and increased target detection errors on forced awakening. Sleep negativities were more prevalent in narcolepsy and idiopathic hypersomnia than in EDS associated to psychiatric disorders. By combining sleep latency and ERP measures, the forced awakening test provided a robust and relatively rapid tool (45-60 min) to evaluate both sleep propensity and sleep inertia within a single recording session. The test allows each subject to act as his/her own control, thus increasing sensitivity. In the present series, it proved to be much more discriminative than waking ERPs alone to demonstrate specific abnormalities in patients complaining of excessive daytime sleepiness.

Evoked potential components unique to non-REM sleep: relationship to evoked K-complexes and vertex sharp waves

Following the loss of wakeful consciousness, the averaging of responses to stimuli produce evoked potential waveforms with prominent components either unique to or greatly enhanced by non-REM sleep. In the sleep onset periods (stage 1) these are the P2 and N350. Following the establishment of stable sleep (stage 2 and SWS), the N550 and P900 are also prominent. Investigation of the EEG associated with individual responses indicates that a good proportion of stimuli elicit, K-complexes or vertex sharp waves (VSWs) and occasionally will elicit both. Recent work has indicated that the N550 in the averaged response is due to the presence of K-complexes and that the N350 is at least largely due to the presence of VSWs. The large size of these grapho-elements indicates that they are probably produced by a synchronized discharge of multiple neural units. Both are readily observed in the absence of external stimulation and occur as normal components of sleep, indeed the K-complex is used as one of the identifying features of the onset of stable non-REM sleep. The present review details the investigation of these features and their associated evoked potential components, in terms of stimulus features, brain states associated with their production, their scalp topography, and changes as a function of age.

Auditory information processing during human sleep as revealed by event-related brain potentials

The main goal of this review is to elucidate up to what extent pre-attentive auditory information processing is affected during human sleep. Evidence from event-related brain potential (ERP) studies indicates that auditory information processing is selectively affected, even at early phases, across the different stages of sleep-wakefulness continuum. According to these studies, 3 main conclusions are drawn: (1) the sleeping brain is able to automatically detect stimulus occurrence and trigger an orienting response towards that stimulus if its degree of novelty is large; (2) auditory stimuli are represented in the auditory system and maintained for a period of time in sensory memory, making the automatic-change detection during sleep possible; and (3) there are specific brain mechanisms (sleep-specific ERP components associated with the presence of vertex waves and K-complexes) by which information processing can be improved during non-rapid eye movement sleep. However, the remarkably affected amplitude and latency of the waking-ERPs during the different stages of sleep suggests deficits in the building and maintenance of a neural representation of the stimulus as well as in the process by which neural events lead to an orienting response toward such a stimulus. The deactivation of areas in the dorsolateral pre-frontal cortex during sleep contributing to the generation of these ERP components is hypothesized to be one of the main causes for the attenuated amplitude of these ERPs during human sleep.

Alpha burst activity during human REM sleep: descriptive study and functional hypotheses

OBJECTIVES

Spectral power contribution in the range of alpha activity is a well-known electrophysiological feature of human REM sleep, which could be caused by the spontaneous bursts of alpha activity not associated with arousals that usually appear during this brain state. The present study was undertaken to determine the density of alpha bursts during tonic and phasic (oculomotor) REM periods for each REM cycle. In addition, this phasic brain event was also described from a spectral and topographical point of view.

METHODS

Ten healthy right-handed subjects (5 females) aged 19-25 years (mean 22.9 years, SD 2.6) participated in the present study. Each selected subject filled in a daily sleep log for 2 weeks before the experimental night to provide information on all 3 salient aspects of sleep pattern, sleep experience and sleep effects.

RESULTS

The results revealed that transient REM-alpha bursts, which lasted about 3 s and were accompanied by no increase in the EMG amplitude, appeared in all subjects who participated in this study, showing a higher density in the third and fourth REM cycle during phasic in comparison with tonic periods. The bandpass filtered signals showed the highest spectral contribution for the slower alpha components (8-9 Hz), the occipital scalp regions being the main generator source of this brain activity.

CONCLUSIONS

The authors hypothesize that REM-alpha bursts may work as micro-arousals (or incomplete arousals) facilitating the brain connection with the external world in this cerebral state, whereas REM-alpha arousals - usually longer and accompanied by changes in the EMG amplitude - generate a shift of brain state associated with sleep fragmentation (complete arousal).

Structure of the auditory evoked magnetic fields during sleep

We studied the effects of sleep on auditory evoked magnetic fields following pure tone stimulation applied to the right ear of 10 healthy normal volunteers to investigate the changes in the processing of auditory perception in the primary auditory cortex. Dual 37-channel biomagnetometers were used to record auditory evoked magnetic fields over the bilateral temporal lobes in response to presented tones. Auditory evoked magnetic fields were compared for three stimulus frequencies (250, 1000 and 4000 Hz) and three sleep stages (awake state, sleep stages 1 and 2). Four main components, M50, M100, M150 and M200, were identified with latencies of approximately 50, 100, 150 and 200 ms, respectively. The latency of each component had a tendency to be prolonged with the depth of sleep stage in all frequencies. The amplitude ratios of the early-latency components (M50 and M100) showed a tendency of reduction compared with the same components in the awake state. By contrast, the amplitude ratios of the long-latency components (M150 and M200) were significantly enhanced with an increase in the sleep stage compared with the same components in the awake state. The equivalent current dipoles of all components in all conditions were detected at the superior temporal cortex (the primary auditory cortex). As for the changes in the equivalent current dipole location of each component, the equivalent current dipole was detected in the more posterior and medial region in responses to the high-frequency tone (1000 and 4000 Hz) compared with those to 250 Hz tone stimulation. Although the equivalent current dipoles of the early-latency components (M50 and M100) were in regions more anterior and superior compared to those in the awake state, there was no consistent tendency of changes in equivalent current dipole locations between each sleep stage in the late-latency components (M150 and M200). These findings are probably due to the difference in generating mechanisms between the early- and late-latency components.

Auditory event-related potentials to semantic priming during sleep

The present study uses the N400 component of event-related potentials (ERPs) as a processing marker of single spoken words presented during sleep. Thirteen healthy volunteers participated in the study. The auditory ERPs were registered in response to a semantic priming paradigm made up of pairs of words (50% related, 50% unrelated) presented in the waking state and during sleep stages II, III-IV and REM. The amplitude, latency and scalp distribution parameters of the negativity observed during stage II and the REM stage were contrasted with the results obtained in the waking state. The 'N400-like' effect elicited in these stages of sleep showed a mean amplitude for pairs of unrelated words significantly greater than for related pairs and an increment of latency. These results suggest that during these sleep stages a semantic priming effect is maintained actively although the lexical processing time increases.

Distributions and sources of magnetoencephalographic K-complexes

Whole-head magnetoencephalographic (MEG) and midline electroencephalographic (EEG) signals were simultaneously recorded from 6 subjects during drowsiness and sleep to define the topography and source distribution of K-complexes. In light sleep, K-complexes were also triggered by infrequent tones. Distributions of spontaneous and triggered magnetic K-complexes did not differ systematically, nor did those evoked by right- and left-ear stimuli, but there were large intra- and interindividual differences. Minimum-norm estimates and current dipoles were used to characterize the source currents. Current direction and distribution varied remarkably between the K-complexes appearing in similar situations. In one subject, most K-complexes were adequately modelled with two current dipoles which were situated in the left and right inferior parietal lobes. In other subjects, the current distributions were more complex, suggesting several brain regions to be active during one K-complex; the dominant foci were in frontal and parietal lobes. Our results suggest that the K-complex is not a stereotyped response of the cortex to internal or external stimuli, comparable to evoked responses, but a diffuse and variable cortical reaction during which large areas of cortex may be active.

Sensory processing during early and late nocturnal sleep

The present experiments in 10 healthy men compared auditory evoked potentials (AEPs) and heart rate (HR) indicators of stimulus processing during early and late phases of nocturnal stage 2 sleep. Definition of early and late sleep relied on endocrine pituitary-adrenal secretory activity which is known to be inhibited during early nocturnal sleep but sharply increases during late sleep. AEPs and HR responses were recorded to trains of 10 tone pips (1000 Hz; interstimulus interval 15 s; intertrain interval > 3 min). On one night, tone pips were presented in the first part of sleep, on the other night tone presentation took place in the second part, with the order of conditions balanced across subjects. Amplitudes of N150 and N550 components of the AEP, and of acceleratory and deceleratory HR responses, were higher during the first than second part of nocturnal sleep (P < 0.05). Moreover, habituation of P240 and N550 amplitudes was slower during the first than second part of sleep (P < 0.05). In supplementary experiments, AEP and HR responses to the same stimuli did not differ between the first and second part of the night when subjects were waking during stimulation. Results indicate a reduced inhibitory control over cortical stimulus processing during early nocturnal sleep. This diminished inhibition of cortical processing together with other concomitant changes during early sleep (such as the enhanced inhibition of pituitary-adrenal secretion) may reflect a coordinated regulatory function of sleep possibly mediated by hippocampal mechanisms.

Sequential analysis of the brain's transfer properties during consecutive REM episodes

Classical analysis of the spontaneous sleep EEG has revealed alterations of REM sleep in psychiatric diseases and under the influence of drugs. In order to elucidate possible functional differences between different REM episodes even in healthy subjects we investigated in 10 volunteers the transfer properties of the brain by measuring auditory (AEP) and visual evoked potentials (VEP) from scalp positions Fz, Cz and Pz during the night. According to linear system theory we computed the so-called amplitude-frequency characteristics (AFC) from averaged AEPs and VEPs during the first and each of the following 3 REM episodes. These functions describe the relationship between the input and output of the investigated system. A 3-factorial analysis of variances with the independent factors frequency band, REM episode and electrode position revealed a statistically significant main effect for the factor REM episode under auditory stimulation (P = 0.05), whereas no significant main effect for REM episode was found under visual stimulation (P = 0.88). Applying a 2-factorial analysis of variance with the independent factors REM episode and electrode position in the case of auditory stimulation we could demonstrate a statistically significant main effect (P = 0.029) for the factor REM episode in the beta range (12.5-20 Hz). A subsequent analysis of contrasts revealed that the first REM episodes could be differentiated from each other. For auditory stimulation the beta resonance during the first REM episode appears enhanced compared to each of the later REM episodes. These findings point to a functional difference of the brain's transfer functions between the first and the 3 following REM episodes, indicating different information processing during consecutive paradoxical sleep.

Auditory event-related potentials to deviant stimuli during drowsiness and stage 2 sleep

Twelve subjects were tested using a 3-tone auditory oddball paradigm consisting of a standard 1000 Hz tone (P = 80%) and two deviants, namely, a 1200 Hz tone and a 2000 Hz tone (both P = 10%). Testing took place in 3 conditions: (1) attend, in which the subject had to count one of the deviant tones; (2) ignore, in which the subject read a book; and (3) sleep, in which the subject was encouraged to go to sleep during presentation of the tones. In the awake conditions stimulus deviance elicited mismatch negativity (MMN) and P3. During drowsiness, no separate mismatch negativity (MMN) could be detected, but the 2000 Hz tone evoked a broad fronto-central early negative deflection, suggesting an overlap of N1 and MMN. In the same condition, P210, N330 and P430 appeared, all being sensitive to magnitude of deviance. During stage 2, the P210, N330 and P430 amplitudes increased, most notably to the large deviant. These data indicate that differential processing of auditory inputs is maintained during drowsiness and stage 2 sleep, but do not support the notion that MMN or P3 activity comparable to the waking state occurs to oddball stimuli during this stage. It is hypothesised that during light sleep, scanning of the environment is performed by a different system than in the awake state and that during drowsiness a gradual switch between these two systems takes place.

K-complex evoked in NREM sleep is accompanied by a slow negative potential related to cognitive process

Evoked cortical responses to two kinds of auditory stimuli (rare and frequent) were analyzed to determine whether or not a K-complex evoked in stage 2 of NREM sleep is accompanied by some endogenous cognitive components of the event-related potential. All the 7 subjects examined in this sleep state failed to provide the correct behavioral response to auditory stimuli, but a K-complex was evoked more frequently by rare stimuli than by frequent stimuli. EEG segments in stage 2 were averaged separately according to the presence or absence of K-complexes emerging just after the stimulation. In cases where K-complexes did not emerge, a long-lasting negative potential of relatively low voltage appeared in the difference wave, which was obtained by subtracting the averaged EEG for frequent stimuli from that for rare stimuli. In cases where K-complexes emerged, a similar long-lasting negative potential of large amplitude appeared in the difference wave. These data may indicate that a K-complex evoked by an external stimulus is accompanied by a potential related to a cognitive process, which appears with greater amplitude in cases where a K-complex is evoked.

Is the appearance of mismatch negativity during stage 2 sleep related to the elicitation of K-complex?

There is no convincing evidence for the occurrence of mismatch negativity (MMN) elicited by infrequent deviant tones in a homogenous tone stream during sleep in adult humans. Also the data presented here failed to show an MMN during any stage of sleep when event-related potentials (ERPs) were averaged across all trials of the same sleep stage. The aim of the study was to determine whether the MMN appearance during sleep is related to the variations in microstates of sleep that differ in terms of stimulus elicited phasic EEG events. The focus was on stage 2 sleep. The single responses to a deviant tone were classified into 3 types during stage 2 prior to averaging ERPs. These 3 response types included K-complex, other phasic EEG events and no visually discernible phasic EEG events. The results showed that an MMN-like deflection indeed appeared during stage 2 but only when the deviant tone also elicited a K-complex. This type of deflection was not seen when the deviant tone was presented without the intervening standard tones. This supports the hypothesis that a true MMN to the deviant tone was seen during stage 2 sleep preceding a K-complex.

Endogenous components of event-related potential appearing during NREM stage 1 and REM sleep in man

Information processing in the brain during sleep was studied by analyzing the evoked cortical response to auditory stimulations presented in the odd-ball paradigm. Eight subjects were examined in different sleep stages. The subjects could provide the correct behavioral response to the auditory stimulation by pressing a key button in the light part of stage 1 of NREM sleep, just succeeding to the waking state, but none of the subjects could give the correct behavioral response in the other sleep stages. In the deep part of stage 1 of NREM sleep and REM sleep, a cortical potential corresponding to P300, the endogenous component of the event-related potential (ERP) recorded in the waking state, was recorded in 6 of the 8 subjects in spite of the absence of the behavioral response. In stages 2, 3 and 4 of NREM sleep, emergence of this endogenous component of ERP could not be confirmed. The present findings provide electrophysiological evidence indicating that selective information processing corresponding to sensory discrimination of auditory stimuli is actively performed in stage 1 of NREM sleep and REM sleep.

Effects of rate of tone-pip stimulation on the evoked K-Complex

The effects of rate of presentation on the evoked K-Complex were examined. In different conditions, brief duration tone pips were presented every 5, 10 or 30 s. Trials were sorted into those on which a K-Complex was or was not elicited and averaged separately. The evoked K-Complex consisted of two different negative components peaking at approximately 350 and 550 ms (respectively), followed by a positive component peaking at approximately 900 ms. K-Complexes were elicited more often in Stage 2 early and 2 late than in slow-wave sleep although the differences were not significant. K-Complexes were elicited more often in the Long ISI compared to the Short and Medium ISI conditions. When a K-Complex was evoked, the amplitude of N350 and N550 was greater for the Long ISI than for the Short or Medium ISI conditions. P900 amplitude remained invariant regardless of the ISI. A micro-analysis was carried out whenever 3 consecutive occurring K-Complexes were elicited. During the Short and Medium ISI conditions, N350 and N550 following the second and third occurrence of the K-Complex were significantly attenuated compared to the first occurrence. There was no difference in N350 and N550 amplitudes among the 3 consecutive occurrences during the Long ISI condition. The decay in amplitude over consecutive occurrences of the K-Complex might be a reflection of either habituation or refractory processes.

ERP and behavioral changes during the wake/sleep transition

Event-related potentials (ERPs) following infrequent and frequent stimuli were studied as subjects moved from wakefulness to sleep. Subjects were instructed to respond to the infrequent "target" stimuli (attend condition) or to ignore the stimuli (ignore condition). Parietal P300, prominent following target ERPs in wakefulness under the attend condition, disappeared in association with reduced behavioral responsiveness and emergence of a central negativity (N350). The N350 and preceding and following positivities (P220 and P450) became the dominant feature of both target and nontarget ERPs under both attend and ignore conditions. The P220-N350-P450 complex was larger and peak latencies were shorter under the attend condition. Peak amplitudes tended to be larger following targets, especially under the attend condition. The findings suggest that, although the processes underlying P300 are less likely to be engaged, processing of stimulus deviance and task relevance continues in sleepiness and sleep, and is reflected by variance in N350 and related activity.

Reptilian waking EEG: slow waves, spindles and evoked potentials

Signal spectral analysis procedures were used to compute the power spectrum of Gallotia galloti lizards EEG at different (5-35 degrees C) body temperatures. EEG power spectra were mainly characterized by a low frequency peak between 0.5 and 4 Hz which was present at the different body temperatures. A second spectral peak, corresponding to spindles of similar pattern to the sleep spindles of mammals, also appears in the spectra. The peak frequency of the spindles increased with the body temperature. Flash evoked potentials were characterized by a slow triphasic component upon which a spindle was superimposed, adopting a morphology similar to the K complexes of mammalian sleep. The characteristics of this EEG and evoked potentials support the hypothesis of homology between the waking state of the reptiles and the slow wave sleep of mammals.

Arousals without awakening--dynamic aspect of sleep

K complex as an elementary form of arousal was investigated by evoked K complexes. The components building up evoked K complexes and topographical sleep-level differences and influence of the kind of cognitive elaboration were analysed. Evidences are presented supporting that K complex represents a cascade of events originating from sources of different topography and probably by a different kind of elaboration activated in a certain order requested by the nature and context of the eliciting stimuli. The power spectra of evoked arousals--including K complexes--were investigated. The poststimulus spectral pattern is characterized by a short initial power elevation and a following reduction of all frequency bands except a simultaneous but prolonged (5-20 s) and strong (50%) power reduction at the 13-14 Hz sigma spindle band. This phenomenon seems to be a common feature in different stages of slow wave sleep. This stimulus-related microstate could serve as a transitory stand-by state ready to reach higher arousal rapidly while maintaining the continuity of sleep; hence, the inhibition of spindle activity could provide a phasically improved thalamocortical sensory inflow after environmental stimuli. A microstructural scheme of arousals without awakening has been delineated.

The influence of lorazepam medication upon the transfer properties of the brain during sleep in man

In order to get better insight into the principles of information processing by the brain during sleep and its alterations under the influence of drugs we applied some tools from linear system theory to sleep EEG data. We investigated late components of auditory and visually evoked potentials (AEPs and VEPs) during different sleep stages and calculated from these the so-called amplitude-frequency characteristics (AFC). The main advantage of this analysis is that it enables one to detect functional differences during different sleep stages. This information can hardly be obtained by conventional spectral analysis. The result of our investigation was that the transfer properties of the brain during sleep were extremely different and that lorazepam medication not only resulted in quantitative alterations of the sleep profile but mainly in highly significant alterations of the functional properties of sleep.

Auditory event-related potentials during stage 2 NREM sleep in humans

Event-related potential (ERP) recordings were used to investigate the nature of auditory stimulus evaluation during stage 2 sleep. Frequent and rare stimuli, differing in intensity and frequency, were presented to six adult subjects while awake and asleep. The latency and voltage distribution of one of the long-latency components evoked during sleep resembled the P3 component evoked while awake. However, it was attenuated in voltage and superimposed on N3, a large late negative component, most probably the slow potential of the K complex. The identification of a P3-like potential during sleep suggests that the P3 potential is not solely a marker of active cognitive processes, but contains a small component which reflects automatic, pre-attentive evaluation of deviant stimuli.

Long latency evoked potential components in human slow wave sleep

The wave morphology of long latency (more than 200 msec) components of auditory evoked potentials of healthy human adults was analysed in different slow wave sleep stages and at different electrode positions. The results indicated that the marked negative deflection consisted of 2 distinct components with 300 and 550 msec latency and with central and frontal amplitude maxima, respectively. Very long latency components (N1500 and P1900) were observed with monotonously increasing amplitudes parallel to the deepening of sleep. The contribution of the large amplitude, transient evoked slow waves to the SWS EEG and the general role of information processing in the regulation of sleep are briefly discussed.

The P300 component in sleep

The present investigation utilized the P300 component of the auditory evoked potential as an index of information processing (discrimination) in sleep. Auditory evoked potentials were recorded to target and nontarget stimuli during sleep stages 3/4, 2 and REM under two probability conditions. Corresponding "nontone" waveforms were generated in each sleep stage, representing averaged EEG activity with no tones presented. Target P300 amplitude was higher than both corresponding "nontone" targets and tone nontargets. Probability did not affect the target-nontarget relationship. Latency of P300 increased and amplitude decreased from wakefulness through sleep; however, neither amplitude nor latency differed among sleep stages. Amplitude and latency of N200 increased during sleep. While N200 amplitude was highest in Stage 3-4, N200 latency did not differ among sleep stages. These findings suggest that the P300 recorded in sleep indexes similar cognitive processes as the P300 recorded in wakefulness. That P300 as well as N200 latency increased in sleep suggests that processes indexed by these components may slow during sleep.