Avalanches of brain co-activation during sleep and wakefulness

BOCACCIO H.; SÁNCHEZ S. M.; DE PINO G.; CASTRO M. N.; LAUFS H.; VILLARREAL M.; TAGLIAZUCCHI E.

CABA

Congreso; Stat Phys 27; 2019

International Union of Pure and Applied Physics (IUPAP)

In the study of consciousness, human sleep provides a relatively easy access to explore the neural changes that underlie the reduction of conscious awareness. The spontaneous activity of the awake brain at different scales shows an avalanche-like behaviour, suggesting that it might operates at criticality. Although many works show brain avalanches for altered states of consciousness none were done in functional Magnetic Resonance Imaging (fMRI). Here, a previously developed point-process in fMRI was applied for the first time in sleep data. For this purpose, we studied resting state data for wakefulness and non-rapid eye movement sleep stages (W, N1, N2, N3) to find differences in large-scale brain dynamics. We defined clusters of contiguous regions that co-activate by a point-process approach (i.e. binarizing the supra-threshold activity). We found a power-law behaviour in the cluster sizes distribution for all stages, reflecting the scale-free properties associated with the critical regime. These distributions show a non-zero probability to find very large clusters, defining avalanches-like behaviour of brain co-activation. We estimated the scaling parameter of power-laws using Maximum Likelihood Estimators (MLE) and made statistical comparisons between stages. Kruskal-Wallis and ANOVA tests show significant differences of scaling parameters between stages (p=0.0012 and p=0.0029, respectively). Wilcoxon and permutation post-hoc tests were used, obtaining only differences between wakefulness and N2 (p=0.0018 and p=0.0015, Bonferroni corrected), which shows that probability to find large clusters is significantly higher in N2 than in W. Similar results was consistently obtained for down-sampled data and for different binarization thresholds in a range. Phase-shuffled data was used as a model without correlations, finding the non-triviality of the results above. While critical avalanche in wakefulness is usually associated to metastability (allowing the existence of a large set of possible configurations to be taken during the wide range of conscious experiences), avalanches in N2 could be related to the phenomenon of evoked neural bistability of slow oscillations (EEG K-complexes), regularly preceded by integrative cortical excitations, which allow large correlations although with a loss of freedom of brain states because of differences in the rules of local dynamics.