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Probing the E/I balance in large scale stimulus-evoked and resting-state activity of the human brain
Oshrit Arviv1 and Abraham Goldstein2 and Oren Shriki3
1The Gonda Multidisciplinary Brain Research Center, Bar-Ilan University, Ramat Gan, Israel 2Department of Psychology, Bar-Ilan University, Ramat Gan, Israel 3Department of Cognitive and Brain Sciences, Ben-Gurion University, Beer Sheva, Israel

In recent years, several experimental studies suggested that the cortex at resting-state operates at proximity to critical dynamics. These dynamics allow neuronal activity to propagate over long distances without premature termination or explosive growth and reflect a subtle balance of excitatory and inhibitory forces. Here, using Magnetoencephalography (MEG), we examine whether stimulus-evoked activity can also be regarded as E/I balanced at these spatial and temporal scales. Additionally, we investigate the relation between the dynamics associated with maintaining E/I balance in resting-state activity to that in stimulus-evoked activity. We found that the recorded cortical activity is critical and organizes as neuronal avalanches at both resting-state and stimulus-evoked activities. Moreover, a significantly high intra-subject similarity between avalanche size distributions at both cognitive states was found, suggesting that the distribution captures specific features of the individual brain dynamics. When comparing different subjects, a higher inter-subject consistency was found for stimulus-evoked activity than for resting-state. This was expressed by the distance between avalanche size distributions of different participants, and was supported by the spatial spreading of the avalanches involved. During the course of stimulus-evoked activity, we demonstrate time-locked fluctuations in the gain of the neuronal system, implying short time-scale deviations from precise balance. Nonetheless, over longer time-scales, the overall E/I balance in stimulus-evoked activity is retained. Comparing the spatial organization of resting and evoked activity suggests a switch between task-negative (default mode) and task-positive networks. Overall, this study offers a novel outlook on evoked activity, and proposes that the human brain operates near an optimal dynamical regime for information processing also when evoked by stimuli.