Coherence analysis of the human sleep electroencephalogram (EEG) was used to investigate relations between brain regions. In all-night EEG recordings from eight young subjects, the temporal evolution of power and coherence spectra within and between cerebral hemispheres was investigated from bipolar derivations along the antero-posterior axis. Distinct peaks in the power and coherence spectra were present in NREM sleep but not in REM sleep. They were situated in the frequency range of sleep spindles (13–14 Hz), alpha band (9–10 Hz) and low delta band (1–2 Hz). Whereas the peaks coincided in the power and coherence spectra, a dissociation of their temporal evolution was observed. In the low delta band, only power but not coherence showed a decline across successive NREM sleep episodes. Moreover, power increased gradually in the first part of a NREM sleep episode, whereas coherence showed a rapid rise. The results indicate that the intrahemispheric and interhemispheric coherence of EEG activity attains readily a high level in NREM sleep and is largely independent of the signal amplitude.
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"In this light, alpha activity provides a potent window onto the instantaneous responsiveness of the sleeping brain. Future research should investigate the extent to which other features of EEG dynamics, such as spectral coherence , , cross-frequency phase synchrony , , or nested oscillations ,  offer useful information about empirical measures of sleep depth. "
[Show abstract][Hide abstract] ABSTRACT: The neural correlates of the wake-sleep continuum remain incompletely understood, limiting the development of adaptive drug delivery systems for promoting sleep maintenance. The most useful measure for resolving early positions along this continuum is the alpha oscillation, an 8-13 Hz electroencephalographic rhythm prominent over posterior scalp locations. The brain activation signature of wakefulness, alpha expression discloses immediate levels of alertness and dissipates in concert with fading awareness as sleep begins. This brain activity pattern, however, is largely ignored once sleep begins. Here we show that the intensity of spectral power in the alpha band actually continues to disclose instantaneous responsiveness to noise--a measure of sleep depth--throughout a night of sleep. By systematically challenging sleep with realistic and varied acoustic disruption, we found that sleepers exhibited markedly greater sensitivity to sounds during moments of elevated alpha expression. This result demonstrates that alpha power is not a binary marker of the transition between sleep and wakefulness, but carries rich information about immediate sleep stability. Further, it shows that an empirical and ecologically relevant form of sleep depth is revealed in real-time by EEG spectral content in the alpha band, a measure that affords prediction on the order of minutes. This signal, which transcends the boundaries of classical sleep stages, could potentially be used for real-time feedback to novel, adaptive drug delivery systems for inducing sleep.
"This finding was observed in all sleep stages including REM sleep. Interestingly, the interhemispheric coherence of slow waves was stable across sleep episodes, whereas SWA showed a marked decrease (Achermann and Borbely, 1998b). The contribution of the corpus callosum for coherent activity is illustrated by the reduced interhemispheric coherence during NREM sleep in congenitally acallosal children (Koeda et al., 1995; Kuks et al., 1987) and adults (Nielsen et al., 1993), as well as in adults with callosotomy (Montplaisir et al., 1990) and mice with total callosal agenesis (Vyazovskiy et al., 2004). "
[Show abstract][Hide abstract] ABSTRACT: Sleep studies often observe differences in slow wave activity (SWA) during non-rapid eye movement sleep between subjects. This study investigates to what extent these absolute differences in SWA can be explained with differences in grey matter volume, white matter volume or the thickness of skull and outer liquor rooms. To do this, we selected the 10-min interval showing maximal SWA of 20 young adult subjects and correlated these values lobe-wise with grey matter, skull and liquor thickness and globally with white matter as well as segments of the corpus callosum. Whereas grey matter, skull thickness and liquor did not correlate significantly with maximal slow wave activity, there were significant correlations with the anterior parts of the corpus callosum and with one other white matter region. In contrast, electroencephalogram power of higher frequencies correlates positively with grey matter volumes and cortical surface area. We discuss the possible role of white matter tracts on the synchronization of slow waves across the cortex.
Full-text · Article · Mar 2011 · Journal of Sleep Research
") in the specific domain of state-related interhemispheric variations. 3. Finally, the present results extend, in terms of ''directionality ,'' several previous lines of evidence: 1) the reduction of spontaneous neural activity in the splenium of the corpus callosum across sleep stages in animals (Berlucchi 1965); 2) the reduction in transcallosal inhibition especially from right to left motor areas across sleep stages in humans (Bertini et al. 2004); 3) evidence of slow sleep EEG oscillations as traveling waves having intra-and interhemispheric directions of propagation (Massimini et al. 2004); 4) the drop in functional interhemispheric connectivity during early sleep stages (Massimini et al. 2005); 5) the modulation of interhemispheric EEG coherence across wake--sleep stages in normal subjects (Nielsen et al. 1990; Guevara et al. 1995; Achermann and Borbely 1998) and in patients with partial resection or agenesis of the corpus callosum (Montplaisir 1990; Nielsen et al. 1993); and 6) the decrement of regional cerebral blood flow in the left parietal area during slow-wave sleep (Kajimura et al. 1999). "
[Show abstract][Hide abstract] ABSTRACT: Neuroscientists' efforts to better understand the underlying processes of human consciousness are growing in a variety of multidisciplinary approaches. Relevant within these are the studies aimed at exploring the physiological substratum of the propagation and reduction of cerebral-namely, corticocortical-communication flows. However, the preferential direction of the information flow between brain hemispheres is as yet largely unknown. It is the aim of the present research to study the communication flows between brain hemispheres, their directionality, and their regional variations across wake-sleep states. A second aim is to investigate the possibility of an association between different brain rhythms and different preferred directions of the information flow. Scalp electroencephalograms (EEGs) were recorded in 10 normal volunteers from wakefulness to early sleep stages (viz., resting wakefulness, sleep stages 2 and 4, and rapid eye movement [REM] of the first sleep cycle). EEG rhythms of interest were delta (1-4 Hz), theta (5-7 Hz), alpha (8-11 Hz), sigma (12-15 Hz), and beta (16-30 Hz). The direction of the interhemispheric information flow was evaluated by computing directed transformation function from these EEG rhythms. Interhemispheric directional flows varied as a function of the state of consciousness (wake and early sleep stages) and in relation to different cerebral areas. Across wake to sleep states, we found that delta and beta rhythms convey interhemispheric signals with opposite directions: preferred right to left hemisphere direction for delta and left to right for beta rhythms. A log correlation confirmed that the trend of low to high EEG frequencies-traditionally associated with an increasing state of vigilance-was significantly related to the direction of the communication flow from the left to right hemisphere. This evidence might open the way for a variety of research lines on different psychophysiological and pathological conditions.