Critical thinking: How networks in the brain may be optimally organized

January 16th, 2024

Written by: Joseph Stucynski

If you’ve ever stared at falling sand in an hourglass, you might have noticed that when it accumulates on the pile below, every so often it will trigger an avalanche. Most of the time the avalanches are tiny and quick, just involving the top sand rolling lower, but sometimes the weight of the sand overcomes friction of the pile and a large chunk of sand fractures off and slides lower. It turns out that this seemingly simple sand pile is a mathematically interesting system that exhibits a trait called self-organizing criticality.¹ The term ‘criticality’^1,2 in this case refers to the fact the sand pile is constantly balancing at a critical transition point between stability (the sand pile building up), and chaos (sand avalanches triggering).  Notably, criticality doesn’t just apply to sand, it also may be an important part of how your brain works,^2,3,4,5 and new research shows that sleep might be very important for maintaining this property!⁶

The edge of chaos

The idea that the brain, with its billions of neurons, operates at the edge of chaos might sound crazy at first, but it turns out that operating on this knife’s edge of criticality allows a system to process information at optimum efficiency.^3,4,5,6 And since the brain needs to perform so many tasks, computations, and behaviors, it also needs to organize and activate its systems in an efficient manner. 

Like avalanches on the sand pile, the firing of neurons throughout the brain seems to follow the mathematical criteria for criticality. That is, small numbers of neurons fire more frequently in mini-avalanches of activity, but the larger the number of neurons that fire together, the more rare those neural avalanches are. For those interested, this is called a power law.^3,4,5 In this way, the brain maintains balance between too few neurons firing, and too many at once, both of which may prevent the brain from functioning well. 

In a related way, the brain must also maintain balance between order and chaos. If neurons fire too chaotically it results in a sub-critical state, and it is harder for the brain to process information, like when a person is under the influence of certain drugs or anesthetics.⁶ But if neurons fire in perfect order across the brain in a super-critical state, you can end up with an epileptic seizure.³ By operating in a critical state at the transition point between order and chaos, the brain processes information efficiently to deal with an ever-changing environment.

Admittedly, neuroscientists don’t all agree that the brain meets the definition for criticality, but the field is growing due to a steady trickle of evidence and a dedicated field of researchers. But still, what does all this mean for you? What does it mean for your brain to be at criticality and could you even feel when it’s not? As it turns out, one of the reasons you need to sleep at night may be to maintain this criticality.

Sleep is critical

As you go about your day and the refreshing effects of a good night’s sleep wash away, you may feel like your brain is slipping further from an optimal state. In a recent paper, neuroscientists tested whether your brain moves increasingly farther away from criticality during the day, and if sleep can help restore this critical state for the next day.⁷

To do this, the neuroscientists recorded from many neurons in the brains of rats while they were awake and asleep. The team then measured mathematical aspects of criticality to compare the rats’ awake brains to their sleeping brains. They found that while their brains’ ‘closeness to criticality’ changed from moment to moment, in general their brains were farther from criticality the longer they were awake.⁷ Likewise, their brains were closer to criticality during sleep and closest just before they woke up after long durations of sleep. In other words, the longer the rats were awake, the further their brains were from criticality, while sleep reset their brains to a critical state.

While the authors of this study did not investigate exactly how sleeping moves the brain closer to a critical state, their findings present a provocative new view of the central function of sleep and why it is so important to maintaining brain health and function. Ultimately, this represents another piece of evidence suggesting that criticality is a core operating principle of how the brain works – just something to remember the next time you find yourself exhausted at the end of a workday feeling like you need to take a nap.

References

1. Abelian sandpile model, Wikipedia. https://en.wikipedia.org/wiki/Abelian_sandpile_model
2. Ouellette, J. Sand pile model of the mind grows in popularity. Scientific American, 2014. https://www.scientificamerican.com/article/sand-pile-model-of-the-mind-grows-in-popularity/
3. O’Byrne, J., and Jerbi, K. How critical is brain criticality? Trends in Neurosciences, 2022. https://www.cell.com/trends/neurosciences/fulltext/S0166-2236(22)00164-3
4. Beggs, J.M., Timme, N. Being critical of criticality in the brain. Frontiers in Psychology, 2012. https://www.frontiersin.org/articles/10.3389/fphys.2012.00163/full
5. Cocchi, L., Gollo, L.L., Zalesky, A., Breakspear, M. Criticality in the brain: A synthesis of neurobiology, models and cognition. Progress in Neurobiology, 2017. https://www.sciencedirect.com/science/article/pii/S0301008216301630
6. Maschke, C., O’Byrne, J., Colombo, M.A., Boly, M., Gosseries, O., Laureys, S., Rosanova, M., Jerbi, K., Blain-Moraes, S. Criticality of resting-state EEG predicts perturbational complexity and level of consciousness during anesthesia. BioRxiv, 2023. https://www.biorxiv.org/content/10.1101/2023.10.26.564247v1    
7. Xu, Y., Schneider, A., Wessel, R., Hengen, K.B. Sleep restores an optimal computational regime in cortical networks. Nature Neuroscience, 2024. https://www.nature.com/articles/s41593-023-01536-9  

Cover photo by Nathan Dumlao on Unsplash