The rat brain provides even more evidence that our brain is operating near the tipping point

The human brain does not seem to have much in common with how the water freezes or turns into gas. But over the past decade, more and more evidence has shown that the brain, as a system, functions in the same way as water, which approaches the critical point of a phase transition. Now, a team of Brazilian scientists has discovered additional evidence in the rat's brain that this may well be the case. The team described its findings in a recent article published Letters of physical examination.

The notion of "self-organized criticality" goes back to a historical document in 1987, when the Danish physicist Per Bak concluded that the exquisite order of nature resulted from a kind of phase transition. This precise moment of transition is commonly called a "tipping point" or "critical point".

A brain is the thing

As a general rule, a classical phase transition occurs only when the temperature and pressure are perfectly suitable for a given system. Self-organized criticality occurs spontaneously as a result of many local interactions between the many elements of a system, such as millions of grains of sand that go from the top to the bottom of an hourglass. The pile grows grain by grain until it becomes unstable enough for the next grain to fall into an avalanche. The base of the pile expands, restoring stability, and the pile begins again, until the pile of sand reaches the critical point again. These avalanches follow a so-called "law of power", that is, the smaller ones occur more often than the larger ones.

Bak expanded his ideas into a 1996 book, How does nature work, applying the notion of self-organized criticality to earthquakes, financial markets, traffic jams, biological evolution, how galaxies are distributed in the universe and the brain. In fact, he bored a group of neuroscientists in 1999 by happily informing them that he understood the fundamentals of how the brain works. Just like the pile of sand in an hourglass, he proposed that the brain falters at the same critical point, a balance obtained by avalanches of electrical activity between neurons at all scales of size. In other words, he proposed that electrical activity be subject to a power law. The idea is that this critical state is a kind of "Goldilocks Zone" that optimizes the transfer and processing of information in the brain while maintaining stability.

True, the brain is not a heap of highly idealized sand, but the concept makes sense. The brain is a complex system that must adapt to changing circumstances. Why, then, would he not have put mechanisms in place to maintain an ideal balance between order and disorder, so to speak? "A brain that is not critical is a brain that does exactly the same thing every minute or, at the other extreme, is so chaotic that it does a completely random thing, whatever the circumstances, "said Dante Chialvo, UCLA neurophysiologist and the National Council of Scientific and Technical Research of Argentina declared memorablely to Quanta in 2014." It's the brain of an idiot. "

Meaning nothing?

All this was purely theoretical – and quite controversial – until 1992, when Swedish scientists conducted experiments with rice grains pressed between glass plates, adding grains one by one and recording any avalanches to the camera. As Bak had predicted, there were more avalanches smaller than larger, in accordance with the law of indicator power. In 2003, Indiana University biophysicist, John Beggs, and several of his colleagues were the first to demonstrate that slices of cortex from a brain showed evidence of a relationship between exhibitors. Add to this evidence from other studies using magnetoencephalography, electroencephalographic (EEG) recordings of interactions between individual neurons and computer simulations with fMRI imaging data of the resting state of the brain. "Hypothesis could just have a case.

The most important problem is what might be called a "focus" problem: critical states in nature are usually fragile, in the sense that a slight disturbance will move it away from the exact critical point. And since the brain is a particularly noisy system, there would be many such disturbances. That's why Beggs and his colleague at Indiana University, Gerardo Ortiz, have since said that the brain may have floated nearrather than precisely at, the critical point – consider it more of a "near critical" phase space than a traditional tipping point. Similarly, Viola Priesemann of the Max Planck Institute for Dynamics and Self-organization in Göttingen argued that the brain operates in a slightly subcritical area.

Not everyone is convinced. Alain Destexhe, theoretical neuroscientist at the National Center for Scientific Research (CNRS) in France, is among the most virulent critics of the brain operating in a critical state. Among other concerns, power law distributions do not provide sufficient evidence of criticality, as other processes in nature can mimic that signature. In a 2017 article, he and his co-author, Jonathan Touboul, proposed that the true sign of criticality is the critical relationship between the exhibitor and the exhibitor.

"It's amazing"

In addition, if the brain is operating close to criticality, what kind of phase transition could occur? This is the question of Antonio Fontenele and Mauro Copelli – both of the Universidade Federal de Pernambuco in Brazil – and various colleagues have tried to answer their experiences. They monitored the brain of the laboratory rats as they moved asleep (thanks to a little anesthesia) and awake (technically synchronized and unsynchronized states, respectively) and concluded that the rodent brain presented all they cycled between sleep and wakefulness. More precisely, the researchers saw avalanche peaks following a power distribution.

In other words, "the transition is between an unsynchronized firing and a synchronized trigger, this transition point is indeed essential and the brain hovers around it," said Beggs, who did not not participate in the experiments. "They had avalanches too, and they satisfied the relationship between exhibitors – for me, it's amazing."

Beggs was particularly impressed by the second figure in this last article, which shows all the data points that follow the relationship between exponents, evidence of more solid criticality than a distribution to the law of power alone. This figure includes data from multiple experiments involving anesthetized rats, free-motion mice, anesthetized macaque monkeys, and even turtle brains. This is consistent with an article last year reporting the recording of neuronal activity in the zebrafish brain, which showed similar signs of near-criticality flutter.

"This shows that a variety of quite different species, under different conditions, all line up near the critical point," Beggs said. "Usually, with biological data, you expect a cloud of points that can show a rough correlation, here you get something that is noticeably different: there is an order, which suggests that different brains revolve around criticality, independently many biological details. " This should delight physicists, who love universality – that is, when an equation can capture the large-scale behavior of a complex system like the brain – and irritate them. biologists. However, "it is very difficult to dismiss the idea of ​​criticality in the cortex, since the data fit fairly well between species," Beggs said.

DO I: Letters of physical examination, 2019. 10.1103 / PhysRevLett.122.208101 (About DOIs).