Abstract
The existence of two different types of sleep spindles (slow and fast) is well-established, according to their topographical distribution at scalp- and cortical-level. Our aim was to provide a systematic investigation focused on the temporal evolution of sleep spindle sources during non-rapid eye movement (NREM) sleep. Spindle activity was recorded and automatically detected in 20 healthy subjects. Low resolution brain electromagnetic tomography (LORETA) was applied for the EEG source localization. Aiming to evaluate the time course of the detected slow and fast spindle sources, we considered the first four NREM sleep cycles and divided each cycle into five intervals of equal duration. We confirmed the preferential localization in the frontal (Brodmann area 10) and parietal (Brodmann area 7) cortical regions, respectively for slow (11.0–12.5) and fast (13.0–14.5) spindles. Across subsequent NREM sleep episodes, the maximal source activation remained systematically located in Brodmann area 10 and Brodmann area 7, showing the topographical stability of the detected generators. However, a different time course was observed as a function of the type of spindles: a linear decrease across subsequent cycles was found for slow spindle but not for fast spindle source. The intra-cycle variations followed a “U” shaped curve for both spindle source, with a trough around third and fourth interval (middle part) and the highest values at the beginning and the end of the considered temporal window. We confirmed the involvement of the frontal and parietal brain regions in spindle generation, showing for the first time their changes within and between consecutive NREM sleep episodes. Our results point to a correspondence between the scalp-recorded electrical activity and the underlying source topography, supporting the notion that spindles are not uniform phenomena: complex region- and time-specific patterns are involved in their generation and manifestation.
Highlights
Sleep spindles are transient oscillatory activity that appears in electroencephalography (EEG) during non-rapid eye movement (NREM) sleep
We considered current source density (CSD) as dependent variable, and it was computed separately for the slow and fast spindle sources
Multiple sources were identified for fast spindles, mostly restricted and situated in the parietal areas, with the superior parietal lobule (BA7) as the most active region
Summary
Sleep spindles are transient oscillatory activity that appears in electroencephalography (EEG) during NREM sleep. The physiological function of these oscillatory patterns has not yet been completely elucidated The evidence regarding their role in sleep maintenance appears controversial 2016), whereas it is well-known their critical role in learning and memory consolidation (Diekelmann and Born, 2010; Cox et al, 2014) These phenomena are initiated in the reticular nucleus of the thalamus and the reciprocal interactions between specific regions of the cortex shape their duration and amplitude (Steriade and Llinás, 1988; McCormick and Bal, 1997; Ujma et al, 2019). The former is prevalent in the anterior cortical regions, while the latter spreads mainly in central and posterior areas (Werth et al, 1996; Zeitlhofer et al, 1997)
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