Criticality between cortical states

Since the first measurements of neuronal avalanches, the critical brain hypothesis has gained traction. However, if the brain is critical, what is the phase transition? For several decades it has been known that the cerebral cortex operates in a diversity of regimes, ranging from highly synchronous states (e.g. slow wave sleep, with higher spiking variability) to desynchronized states (e.g. alert waking, with lower spiking variability). Here, using independent signatures of criticality, we show that a phase transition occurs in an intermediate value of spiking variability. The critical exponents point to a universality class different from mean-field directed percolation (MF-DP). Importantly, as the cortex hovers around this critical point, it follows a linear relation between the avalanche exponents that encompasses previous experimental results from different setups and is reproduced by a model.