Topographical characteristics of slow wave activities during the transition from wakefulness to sleep

Abstract

Objectives: This study examined the topographical characteristics of slow wave(delta and theta) activities during the transition from wakefulness to sleep, using topographic mapping of EEG power and coherence.

Methods: Sonography of nocturnal sleep was recorded on 10 male subjects. Each recording, from ‘lights-off’ to 5 min after the appearance of the first sleep spindles, was analyzed. Typical EEG patterns during the transition from wakefulness to sleep were classified into 9 stages (EEG stages).

Results and conclusions: Topographic maps of coherence in delta-and theta-band activities demonstrated that the synchronous component at the anterior-central areas of the scalp appeared corresponding with increasing power. The populations of high coherence pairs among total pairs were computed for each band and each EEG stage to examine the regional differences of EEG. The populations of the delta-band activity increased clearly from EEG stage 6 in the anterior-central areas. The populations of the theta-band activity increased clearly from EEG stage 7 in the anterior-central areas. These results suggest that the dominant synchronous component of slow wave activities during the transition from wakefulness to sleep increased as a function of EEG stages in the anterior-central areas and increased clearly after the appearance of vertex sharp waves.

Introduction

Examining the relationships between physiological changes and behavioral, subjective changes that occur during the transition from wakefulness to sleep will facilitate understanding of brain function during wakefulness and sleep. The brain undergoes meaning functional modifications that are reflected in electroencephalogram (EEG) activity and its own metabolic rate during sleep (Guevara et al., 1995). Because EEG variations during the transition from wakefulness to sleep were remarkable, early investigators (Davis et al., 1938, Gibbs and Gibbs, 1950, Shiotsuki et al., 1954) noted the characteristics of the waveforms and frequencies of EEG during the transition from wakefulness to sleep and classified them. However, using standard sleep criteria (Rechtschaffen and Kales, 1968), most EEG patterns during the transition from wakefulness to sleep have been combined into standard sleep stage 1.

Studies (Hori et al., 1990, Hasan and Broughton, 1994) that examined the topographical behavior of EEG activity in the sleep onset period (SOP) indicated that such topograms were useful for illustrating the characteristics of EEG changes that the early investigators noted on qualitative EEG studies (Davis et al., 1938, Gibbs and Gibbs, 1950, Foulkes and Vogel, 1965, Dement and Kleitman, 1957). When the topographical behavior of EEG in the SOP is of interest, however, the standard sleep stage criteria, especially for sleep stage 1, are too vague to define when the convergence of behavioral, subjective and polygraphical measures in the SOP is being studied.

Therefore, to examine the EEG variations of the SOP, typical EEG patterns during the waking-sleeping transition period were classified into 9 EEG stages (Hori et al., 1991, Hori et al., 1994, Tanaka et al., 1996). Nine EEG stages are effective for detailing the sleep stage 1 latency of the multiple sleep latency test (MSLT) or of the repeated test of sustained wakefulness (RTSW). It excels in the sleepiness measuring method which use of stage 1 latency as the main index. Murphy et al. (1999) investigated the utility of EEG stage (the Hori 9-stage sleep onset scoring systems) in intentional sleep onset, unintentional sleep onset, narcolepsy and insomnia. And they reported that EEG stage has been shown to be a useful and sensitive method for evaluating the sleep onset period. Recently, we examined the topographical characteristics of 9 EEG stages during the transition from wakefulness to sleep (Tanaka et al., 1995, Tanaka et al., 1997). However in these studies, typically only one quantitative EEG technique was used. Therefore, it would be necessary to examine coherence analyses of the EEG during the transition from wakefulness to sleep, which is a measure of the linear correlation between two EEG derivations. The neurophysiological modifications occurring during sleep are restricted not only to local changes but also to functional differentiation between different cortical sites, as revealed by coherence analyses between two EEG signals (Shaw, 1984). Coherence is thought to be a measure of the functional and structural connectivity between two brain sites (Ford et al., 1986). Recently, decreases in alpha and beta power and decreases in alpha and beta coherence between the frontal and occipital brain sites were observed during the transition from wakefulness to sleep (Wright et al., 1995). This study used EEG power, inter-hemispheric and intra-hemispheric coherence (16 pairs) to examine spatio-temporal changes of EEG activity during transition to sleep. Although these studies were informative, the coherence analyses employed a limited number of scalp electrodes. Even if the temporal sequence were thoroughly examined, the spatial variations of those EEG patterns during the transition from wakefulness to sleep were not well considered. Furthermore, delta-band activity was not examined in detail. The EEG power density in a low frequency range corresponding to slow wave band (1–7 Hz) is an indicator of a progressively declining process during sleep (Borbely et al., 1981), and it is suggested that the slow wave band activity could be used as the parameter to describe the transition process from wakefulness to sleep. The present study focused on the slow wave band (delta-theta) activities, and examined the topographic characteristics in slow wave activities during the transition from wakefulness to sleep, using topographic mapping of EEG power and EEG coherence (66 pairs) corresponding to 9 EEG stages.