By Mauro Copelli (Recife, Brazil).
The idea that the brain operates near a second-order phase transition has gained traction in the last two decades. Since the first experimental observation of neuronal avalanches in vitro, a putative connection has been established between brain dynamics and a critical branching process or, more generally, any process belonging to the mean-field directed percolation – MF-DP – universality class. These processes are marked by a phase transition between an active and an absorbing state. We have observed different signatures of criticality in cortical extracellular recordings of urethane-anesthetized rats, including avalanche, maximum entropy and complexity analyses. In this experimental setup, the brain spontaneously drifts between asynchronous and synchronous cortical states. Scaling relations in avalanche analysis yielded critical exponents different from those of MF-DP, suggesting a different universality class. By employing two different models belonging to the MF-DP universality class and treating the simulated data exactly like experimental data, we reproduced most of the experimental results. We found that subsampling the model and adjusting the time bin used to define avalanches are sufficient ingredients to change the apparent exponents of the critical point (while satisfying scaling relations). Moreover, experimental results are only reproduced within a narrow range in parameter space around the phase transition.