IntroductionMost models of critical neuronal networks have in common the theoretical ingredient of separation of time scales, according to which the interval between neuronal avalanches is much longer than their duration. This framework has proven useful for several reasons, among which are the possibilities of learning from an extensive literature on models of self-organized criticality and of comparing results with data obtained from some experimental setups. The brain of a freely behaving animal, however, is often found to operate away from this regime, thereby raising doubts about the relevance of criticality for brain function. In this chapter, two questions aimed at this direction are reviewed. From a theoretical perspective, how could criticality be harnessed by neuronal networks for processing incoming stimuli? From an experimental perspective, how could we know that the brain during natural behavior is critical?In 2003, the idea that the brain (as a dynamical system) sits at (or fluctuates around) a critical point received compelling experimental support from Dietmar Plenz's lab: cortical slices show spontaneous activity in ''bursts'' with the very peculiar feature of not having a characteristic size. As described in the now-classic paper by Beggs and Plenz [1], the probability distributions of size (s) and duration (d) of these neuronal avalanches were experimentally measured and shown to be compatible with power laws, respectively, P(s) ∼ s −3∕2 and P(d) ∼ d −2 . In the following, I will briefly review how this experimental result is connected with the theoretical idea of a critical brain (a much more expanded review can be found e.g., in Ref.[2]).
Phase Transition in a Simple ModelIn order to do that, let us consider a very simple model of activity (e.g., spike) propagation in a network of connected neurons. We will make the drastic simplification that a neuron can be modeled by a finite set of states: if s i (t) = 0, neuron i is in a Criticality in Neural Systems, First Edition. Edited by Dietmar Plenz and Ernst Niebur.