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. 2012 Apr 10;109(15):5856-61.
doi: 10.1073/pnas.1111133109. Epub 2012 Mar 26.

Hierarchical clustering of brain activity during human nonrapid eye movement sleep

Mélanie Boly  1 , Vincent Perlbarg, Guillaume Marrelec, Manuel Schabus, Steven Laureys, Julien Doyon, Mélanie Pélégrini-Issac, Pierre Maquet, Habib Benali
Affiliations

Hierarchical clustering of brain activity during human nonrapid eye movement sleep

Mélanie Boly et al. Proc Natl Acad Sci U S A. .

Abstract

Consciousness is reduced during nonrapid eye movement (NREM) sleep due to changes in brain function that are still poorly understood. Here, we tested the hypothesis that impaired consciousness during NREM sleep is associated with an increased modularity of brain activity. Cerebral connectivity was quantified in resting-state functional magnetic resonance imaging times series acquired in 13 healthy volunteers during wakefulness and NREM sleep. The analysis revealed a modification of the hierarchical organization of large-scale networks into smaller independent modules during NREM sleep, independently from EEG markers of the slow oscillation. Such modifications in brain connectivity, possibly driven by sleep ultraslow oscillations, could hinder the brain's ability to integrate information and account for decreased consciousness during NREM sleep.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
(A) The six networks extracted during wakefulness. (B) Levels of brain hierarchical integration. (C) Increases in functional clustering ratio in the brain and the six networks (all significant with a probability >0.95). Networks: dATT, dorsal attentional; DM, default mode; EC, executive control; MOT, sensorimotor; SAL, salience; VIS, visual.
Fig. 2.
Fig. 2.
Brain connectivity modularity indexes (group mean result ± SDs) during wakefulness and during NREM sleep. The modularity index, calculated by using the Louvain algorithm, quantifies the degree to which a system may be subdivided into clearly delineated clusters.
Fig. 3.
Fig. 3.
Within-network key node density maps (quantifying the integration between each ROI and the rest of the network) during wakefulness and NREM sleep. The values were sorted for the two conditions to allow one to compare the relative importance of different nodes in each network. Values are displayed on a scatter plot to allow comparison between wakefulness and NREM sleep. The lengths of ellipsoid axes correspond to condition-specific group results SDs. Color codes correspond to the areas for which the rank in the hierarchy is the same (black), higher (blue), or lower (red) during NREM sleep compared with wakefulness. Please refer to Table S1 for ROI coordinates and name abbreviations.
See this image and copyright information in PMC

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