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Date: [2023-02-03 Fri]

Critical Brain Hypothesis

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The ability of the phenomenon of criticality to explain the sudden emergence of new properties in complex systems has fascinated scientists in recent decades. When systems are balanced at their “critical point,” small changes in individual units can trigger outsized events, just as falling pebbles can start an avalanche. That abrupt shift in behavior describes the phase changes of water from ice to liquid to gas, but it’s also relevant to many other situations, from flocks of starlings on the wing to stock market crashes. In the 1990s, the physicist Per Bak and other scientists suggested that the brain might be operating near its own critical point. Ever since then, neuroscientists have been searching for evidence of fractal patterns and power laws at work in the brain’s networks of neurons. What was once a fringe theory has begun to attract more mainstream attention, with researchers now hunting for mechanisms capable of tuning brains toward criticality.

1. Relevant Articles

1.1. Your Brain Is On the Brink of Chaos - Nautilus

Article Link: https://nautil.us/your-brain-is-on-the-brink-of-chaos-234981/

In one important way, the recipient of a heart transplant ignores its new organ: Its nervous system usually doesn’t rewire to communicate with it. The 40,000 neurons controlling a heart operate so perfectly, and are so self-contained, that a heart can be cut out of one body, placed into another, and continue to function perfectly, even in the absence of external control, for a decade or more. This seems necessary: The parts of our nervous system managing our most essential functions behave like a Swiss watch, precisely timed and impervious to perturbations. Chaotic behavior has been throttled out.

Or has it? Two simple pendulums that swing with perfect regularity can, when yoked together, move in a chaotic trajectory. Given that the billions of neurons in our brain are each like a pendulum, oscillating back and forth between resting and firing, and connected to 10,000 other neurons, isn’t chaos in our nervous system unavoidable?

Among many scientists, too, there is a great deal of resistance to the idea that chaos is at work in biological systems. Many intentionally preclude it from their models. It subverts computationalism, which is the idea that the brain is nothing more than a complicated, but fundamentally rule-based, computer.

Chaos seems unqualified as a mechanism of biological information processing, as it allows noise to propagate without bounds, corrupting information transmission and storage.

At the same time, chaos has its advantages. On a behavioral level, the arms race between predator and prey has wired erratic strategies into our nervous system.

A moth sensing an echolocating bat, for example, immediately directs itself away from the ultrasound source.

Chaos is not the same as disorder.

Chaotic systems can also exhibit stable patterns called “attractors” that emerge to the patient observer. Over time, chaotic trajectories will gravitate toward them. Because chaos can be controlled, it strikes a fine balance between reliability and exploration. Yet because it’s unpredictable, it’s a strong candidate for the dynamical substrate of free will.

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.

1.2. Your Brain Operates at the Edge of Chaos. Why That's Actually a Good Thing - CNET

Article Link: https://www.cnet.com/science/biology/features/your-brain-operates-at-the-edge-of-chaos-why-thats-actually-a-good-thing/

Your brain is constantly perched on the edge of chaos. And it's not because you're behind on 47 laptop updates or obsessing over that typo in an email you sent your boss.

No, because even at your most zen, your 86 billion brain cells strut along a tightrope between calm and catastrophe; serenity and disarray; order and disorder

By studying the minds of monkeys, turtles, fish and, of course, humans, scientists have delved into this perilous place. It's known as the "edge of chaos" or less dramatically, the "critical point".

A research paper published last month in Physical Review Letters, for instance, states the critical point offers brains a "desirable trade-off between linearity, optimal for information storage, and nonlinearity, required for computation."

Say your brain wants to signal for you to do something, like open up Seamless, the food delivery app.

Let's assume your brain goes with one. One neuron talks to two neurons, which talk to four, and so on, like a gossip train. Soon, all your neurons are on high alert about your Seamless quest, or as Beggs puts it, the network "blows up really quickly.

This is called supercritical behavior, and it's usually way too much stimulation.

And in fact, supercriticality is thought to be associated with chronic seizures, or epilepsy.

What about two? One neuron sends information to half a neuron, which sends it to a fourth…then to an eighth… and the signal pretty much "dies out,"

This is called subcritical behavior, and wouldn't effectively pass the Seamless message along.

Our final path is three. One neuron shares information with "about" one neuron, which shares information with another, and the signal easily goes from point A to point B. This is good. This is called critical behavior.

Consider the stock market, which also stands at the critical point

Always, some people want to sell and others want to buy. It's an almost exactly balanced duality, which is why markets are usually pretty steady. But what if something catastrophic occurs, like a global pandemic? Or a war?

People would panic, and as most financial gurus would agree, they'd start to sell. That'd spur massive market fluctuations.

Think of these "fluctuations" as a signature of edge-of-chaos systems.

By contrast, some things follow what's called Gaussian distribution, aka a standard bell curve, which doesn't lead to those fluctuations. Human height is a good example of something with Gaussian distribution. If we were to map the height of every person in the world, we'd rarely see anyone fall far from the average. No massive fluctuations.

Now, you might be thinking, why wouldn't the brain prefer a calmer, Gaussian lifestyle? The edge-of-chaos world seems highly risky. Well, there are a wealth of benefits to teetering between order and disorder, too.

Edge-of-chaos systems have a super-duper-ultra-wide range within which to work, thanks to the whole fluctuation thing.

"you could have information passed from one part of the brain to the next … and perhaps even go through the entire brain"

If the brain followed a Gaussian distribution, by contrast, it'd sort of be restricted when communicating stuff. It couldn't reach neurons far across the organ during frightening situations

Human brains are constantly bombarded with external stimuli like people talking, bustling street noise and even the warmth of holding a coffee cup. This means neurons turn on and off constantly. Because of such flux, it's unrealistic for the brain to be at the ideal critical point all the time.

After running neurological experiments on mice, they saw that if the animals' brains couldn't access the critical point, they optimized themselves by following what's called the Wisdom line, or simply, the second best choice.

Beggs calls this phenomenon "quasi-criticality."

If you walk away with only one message, I hope it's this one: The next time you're stressed out, if someone asks how you're doing and you say, "I'm on the edge of chaos," you're not being dramatic at all. You're being scientifically accurate.

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