More evidence for chaos in the nervous system can be found at the level of global brain activity. Bizarrely, an apt metaphor for this behavior is an iron slab.6 The electrons it contains can each point in different directions (more precisely, their spins can point). Like tiny magnets, neighboring spins influence each other. When the slab is cold, there is not enough energy to overcome the influence of neighboring spins, and all spins align in the same direction, forming one solid magnet. When the slab is hot, each spin has so much energy that it can shrug off the influence of its neighbor, and the slab’s spins are disordered. When the slab is halfway between hot and cold, it is in the so-called “critical regime.” This is characterized by fluctuating domains of same-spin regions which exhibit the highest possible dynamic correlations—that is, the best balance between a spin’s ability to influence its neighbors, and its ability to be changed.
The critical state can be quite useful for the brain, allowing it to exploit both order and disorder in its computations—employing a redundant network with rich, rapid chaotic dynamics, and an orderly readout function to stably map the network state to outputs. The critical state would be maintained not by temperature, but the balance of neural excitation and inhibition. If the balance is tipped in favor of more inhibition, the brain is “frozen” and nothing happens. If there is too much excitation, it will descend into chaos. The critical point is analogous to an attractor.
But how can we tell whether the brain operates at the critical point? One clue is the structure of the signals generated by the activity of its billions of neurons. We can measure the power of the brain’s electrical activity at different oscillation frequencies. It turns out that the power of activity falls off as the inverse of the frequency of that activity. Once referred to as 1/f “noise,” this relationship is actually a hallmark of systems balanced at their critical point.7 The spatial extent of regions of coordinated neuronal activity also depend inversely on frequency, another hallmark of criticality. When the brain is pushed away from its usual operating regime using pharmacological agents, it usually loses both these hallmarks,8, 9 and the efficiency of its information encoding and transmission is reduced.10