Correlations in the brain

Neuronal avalanches are a novel mode of spontaneous brain activity, experimentally found in vitro and in vivo, which exhibits a robust scaling behaviour. They suggest that the brain operates close to a critical point, as evidenced by the absence of a characteristic size in the phenomenon. The temporal organization of neuronal avalanches can be characterized by the distribution of waiting times between successive events. Experimental measurements in the rat cortex in vitro exhibit a non-monotonic behavior, not usually found in other natural processes. Numerical simulations provide evidence that this behavior is a consequence of the alternation between states of high and low activity, leading to a dynamic balance between excitation and inhibition. During these different states, both the single neuron behavior and the network excitability level, keeping memory of past activity, are tuned by homeostatic mechanisms. Moreover, by systematically removing smaller avalanches from the experimental time series we evidence the characteristic periodicity of θ and β/γ oscillations, which derive from the temporal organization of avalanches of different sizes: Large avalanches occur at low frequency triggering cascades of smaller avalanches, which occur at higher frequency.This behavior is also detected at a larger scale, i.e., on fMRI and MEG data from resting patients. Indeed, by monitoring temporal correlations we confirm that the system is able to self-regulate the activity level tuning the size of successive events according to their temporal distance.