Abstract
Neuronal avalanches are a hallmark feature of critical dynamics in the brain. While the theoretical framework of a critical branching processes is generally accepted for describing avalanches during ongoing brain activity, there is a current debate about the corresponding dynamical description during stimulus-evoked activity. As the brain activity evoked by external stimuli considerably varies in magnitude across time, it is not clear whether the parameters that govern the neuronal avalanche analysis (a threshold or a temporal scale) should be adaptively altered to accommodate these changes. Here, the relationship between neuronal avalanches and time-frequency representations of stimulus-evoked activity is explored. We show that neuronal avalanche metrics, calculated under a fixed threshold and temporal scale, reflect genuine changes in the underlying dynamics. In particular, event-related synchronization and de-synchronization are shown to align with variations in the power-law exponents of avalanche size distributions and the branching parameter (neural gain), as well as in the spatio-temporal spreading of avalanches. Nonetheless, the scale-invariant behavior associated with avalanches is shown to be a robust feature of healthy brain dynamics, preserved across various periods of stimulus-evoked activity and frequency bands. Taken together, the combined results suggest that throughout stimulus-evoked responses the operating point of the dynamics may drift within an extended-critical-like region.
Highlights
Criticality, a state found in complex systems situated at the edge of a phase transition, is marked by dynamical properties that lack any distinctive spatial or temporal scale
In order to assess the sensitivity of the neuronal avalanche analysis to evoked responses and its relations to temporal and spectral features, we analyzed MEG data collected from a group of subjects (n = 21, age = 22.6 ± 3.0 years) preforming a visual task of face perception[17]
In Arviv et al.[17], we showed that the avalanche size distributions of both cognitive states for relatively long segments (1 sec trials) closely resemble each other for each individual subject, and that their characteristics align with near-critical dynamics
Summary
Criticality, a state found in complex systems situated at the edge of a phase transition, is marked by dynamical properties that lack any distinctive spatial or temporal scale. Stimulus-evoked activity was characterized by a tendency for larger size avalanches and temporary deviations from the predictions of a critical branching process This suggests that sensory adaptation and other compensating mechanisms may be involved in maintaining criticality over long temporal scales. Yu et al.[19] suggested that the proximity to a critical branching process is maintained all throughout stimulus-evoked activity (even on relatively short temporal scales of 100 or 400 msec), much like in the mean-field description of ongoing brain activity. We suggest that changes in the underlying dynamics of stimulus-evoked responses may shift the operating point of the neural system within an extended critical-like region (Griffiths phase)[21,22,23] This may cause changes in the corresponding power-law exponents, while maintaining scale-free statistics
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