Scale-free behavioral dynamics directly linked with scale-free cortical dynamics

Naturally occurring body movements and collective neural activity both exhibit complex dynamics, often with scale-free, fractal spatiotemporal structure. Scale-free dynamics of both brain and behavior are each associated with functional benefits to the organism. Despite their similarities, scale-free brain activity and scale-free behavior have been studied separately, without a unified explanation. Here we show that scale-free dynamics of behavior and certain subsets of cortical neurons are strongly related. Surprisingly, the scale-free neural subsets exhibit stochastic winner-take-all competition with other neural subsets. This observation is inconsistent with current theory of critical phenomena in neural systems. We develop a computational model which incorporates known cell-type-specific circuit structure, explaining our findings with a new type of criticality. Our results establish neural underpinnings of scale-free behavior and clear behavioral relevance of scale-free neural activity. Popular Summary Over the past 2 decades many measurements of brain activity have revealed a peculiar property; the activity is statistically similar when examined at different scales. This self-similar, fractal nature of brain activity has attracted the interest of neuroscientists and physicists alike. In physics, fractal structure shows up in certain non-living systems when they are operating near the tipping point of a phase transition (e.g. liquid water turning into gaseous vapor). This led to the criticality hypothesis, which posits that the brain may also operate near the tipping point of a phase transition, i.e. near criticality. Initially the criticality hypothesis was very exciting at a basic level - physical principles of phase transitions promised to help us understand how the brain works. However, a important question naturally arose and remains unresolved. What do the scale-free, fractal fluctuations of brain activity have to do with the behavior of the organism? Is it just background noise in the brain or is there something fractal-like about how we behave and interact with the world? Here we show for the first time that scale-free fluctuations of brain activity are directly related to moment-to-moment fluctuations in behavior. In fact, we show that both the brain activity and mouse behavior have very similar fractal structure. These results take the criticality hypothesis to a new level of biological and, perhaps, psychological relevance. Scale-free brain dynamics are not just background noise; they correspond to the dynamics of behavior. ### Competing Interest Statement The authors have declared no competing interest.