![]() ![]() For this reason, the proposed spatiotemporal arousal process, described below, also amounts to a generative mechanism underlying BOLD FC. We propose that this topographically organized modulation is the predominant physiological process that is reflected in spontaneous, spatially patterned fluctuations in the BOLD signal. Accordingly, the proposed spatiotemporal process comprises large-scale, topographically organized patterns of excitability (involving coordinated metabolic and electrophysiological changes) that evolve over tens of seconds in parallel with arousal. 1A), which systematically vary along the unimodal-transmodal axis in their relation to the immediate sensory environment ( 11). These different topographies should be organized according to the major functional systems of the brain ( Fig. ![]() Likewise, we hypothesize that different phases within a canonical arousal cycle are associated with different topographies of enhanced excitability. Global brain states and behavior vary according to the phase of ongoing arousal fluctuations ( 2). Theoryįigure 1 illustrates our proposed framework, which casts infra-slow arousal fluctuations as a spatiotemporal cycle that endogenously regulates brain-wide physiology ( Fig. In sum, our results suggest that infra-slow arousal waves are a physiologically integrative process supporting an intrinsic spatiotemporal organization of brain-wide excitability. We characterize several additional features of these waves that, together, offer a parsimonious account for many spatiotemporal features of spontaneous BOLD signal fluctuations, including large-scale FC structure. We provide novel support for each of these predictions, presenting converging evidence across multiple human fMRI datasets, multiple indices of arousal, and hemisphere-wide electrocorticography (ECoG) in macaque monkeys. Fourth, similarly organized traveling waves should also be apparent in electrophysiological recordings. Third, these phase shifts should systematically vary along the principal, unimodal-transmodal axis of FC ( 11). Second, regional phase shifts of the BOLD signal, relative to physiological indices of arousal, should be organized according to FC network identity. First, BOLD signal fluctuations throughout the brain should be coherent with arousal fluctuations. These findings appeal to a broader literature implicating an endogenous, infra-slow (<~0.1 Hz) neuromodulatory process that temporally organizes brain-wide function in relation to arousal ( 7, 8).įour major predictions follow from this traveling wave model. Recently, massively parallel neural recordings have demonstrated that these ongoing arousal fluctuations account for a substantial fraction of variability in single-unit firing rates throughout the brain ( 5, 6). Thus, in awake rodents, fluctuations in physiological (e.g., pupil size) and behavioral (e.g., locomotor activity) variables over tens of seconds are correlated with changes in global brain state, indexed by neural oscillations, incidence of sharp-wave ripples, or the extracellular environment ( 2– 4). Accumulating evidence indicates that global brain function is also temporally structured in relation to these arousal fluctuations ( 2). This regulation is supported by autonomic arousal fluctuations that coordinate body-wide physiology in relation to anticipated behavioral demands, e.g., cycling between “fight-or-flight” versus “rest-and-digest” modes ( 1). Organisms continuously regulate multiple physiologic variables. These findings suggest that traveling waves spatiotemporally pattern brain-wide excitability in relation to arousal. Last, we demonstrate similar, cortex-wide propagation of neural activity measured with electrocorticography in macaques. We show that these waves can parsimoniously account for many features of spontaneous fMRI signal fluctuations, including topographically organized functional connectivity. Here, using fMRI in humans, we show that ongoing arousal fluctuations are associated with global waves of activity that slowly propagate in parallel throughout the neocortex, thalamus, striatum, and cerebellum. However, a unifying physiological account of this structure has so far been lacking. The correlation structure (“functional connectivity”) of these fluctuations recapitulates the large-scale functional organization of the brain. ![]() We hypothesize that these waves are the predominant physiological process reflected in spontaneous functional magnetic resonance imaging (fMRI) signal fluctuations. We propose and empirically support a parsimonious account of intrinsic, brain-wide spatiotemporal organization arising from traveling waves linked to arousal. ![]()
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