Abstract
BackgroundIt has previously been reported that EEG sigma (10–15 Hz) activity during sleep exhibits infraslow oscillations (ISO) with a period of 50 s. However, a detailed analysis of the ISO of individually identified sleep spindles is not available. New methodWe investigated basic properties of ISO during baseline sleep of 34 healthy young human participants using new and established methods. The analyses focused on fast sleep spindle and sigma activity (13–15 Hz) in NREM stage 2 and slow wave sleep (SWS). To describe ISO in sigma activity we analyzed power of power of the EEG signal. For the study of ISO in sleep spindle activity we applied a new method in which the EEG signal was reduced to a spindle on/off binary square signal. Its spectral properties were contrasted to that of a square signal wherein the same spindles and also the inter spindle intervals were permutated randomly. This approach was validated using surrogate data with imposed ISO modulation. ResultsWe confirm the existence of ISO in sigma activity albeit with a frequency below the previously reported 0.02 Hz. These ISO are most prominent in the high sigma band and over the centro-parieto-occipital regions. A similar modulation is present in spindle activity. ISO in sleep spindles are most prominent in the centro-parieto-occipital regions, left hemisphere and second half of the night independent of the number of spindles. ConclusionsThe comparison of spectral properties of binary event signals and permutated event signals is effective in detecting slow oscillatory phenomena.
Highlights
Brain activity is characterized by a multitude of time scales ranging from tens of milliseconds up to hours (Buzsáki, 2006)
We aimed to study the infraslow oscillations (ISO) in both (i) sigma and (ii) automatically detected sleep spindles across NREM stage 2 and slow wave sleep (SWS) and the extent to which they are modulated by brain topography and sleep history
Due to the different mechanisms underlying fast and slow sleep spindles, their contrasting prevalences in the two sleep stages (NREM stage2 and SWS) (Yordanova et al, 2017) and the prominence of high sigma ISO we focused our study on fast spindles
Summary
Brain activity is characterized by a multitude of time scales ranging from tens of milliseconds (gamma oscillations) up to hours (e.g. ultradian and circadian modulation) (Buzsáki, 2006). Sleep spindles are phasic oscillatory events corresponding to the sigma band (10–15 Hz) in the human sleep EEG (De Gennaro and Ferrara, 2003; Bódizs et al, 2009) They represent the most salient feature of NREM stage 2 sleep and are closely associated with both physiological and cognitive aspects of sleep (Purcell et al, 2017; Dijk et al, 1993; Bódizs et al, 2005). ISO in the brain are present at both cortical and subcortical level and in different species It has recently been shown in both mice and humans that activity in most frequency bands, but primarily in sigma, is modulated at a frequency of 0.02 Hz (Lecci, 2017; Lecci et al, 2017). Conclusions: The comparison of spectral properties of binary event signals and permutated event signals is effective in detecting slow oscillatory phenomena
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