When a magnet is heated, it reaches a critical threshold where it loses its magnetization. Known as "criticality," this point of high complexity is achieved when a physical object smoothly transitions from one phase to another.

Figure 1. The brain is in a perfectly balanced state that keeps it from transitioning to a gas or liquid says new research.

Now, a new study from Northwestern University has discovered that the brain's structural features lie near a similar critical point — either at or close to a structural phase transition. Surprisingly, these findings are consistent across brains from humans, mice, and fruit flies, suggesting this phenomenon might be universal. Figure 1 shows the brain is in a perfectly balanced state that keeps it from transitioning to a gas or liquid says new research.

Although researchers are unsure which phases the brain's structure is transitioning between, they believe this new insight could inspire new designs for computational models of the brain's complexity and emergent phenomena. The research was published today (June 10) in Communications Physics, a journal from Nature Portfolio.

"The human brain is one of the most complex systems known, and many properties of the details governing its structure are not yet understood," said István Kovács, the study's senior author from Northwestern. "Several researchers have studied brain criticality in terms of neuron dynamics. But we are examining criticality at the structural level to understand how this underpins the complexity of brain dynamics. This has been a missing piece in our understanding of the brain’s complexity. Unlike in a computer where any software can run on the same hardware, in the brain, the dynamics and the hardware are strongly interrelated."[1]

"The structure of the brain at the cellular level appears to be near a phase transition," said Helen Ansell, the paper's first author from Northwestern. "An everyday example of this is when ice melts into water. It’s still water molecules, but they are undergoing a transition from solid to liquid. We certainly are not saying that the brain is near melting. In fact, we don’t have a way of knowing what two phases the brain could be transitioning between. Because if it were on either side of the critical point, it wouldn’t be a brain."

Kovács is an assistant professor of physics and astronomy at Northwestern's Weinberg College of Arts and Sciences. At the time of the research, Ansell was a postdoctoral researcher in his laboratory; now she is a Tarbutton Fellow at Emory University. While researchers have long studied brain dynamics using functional magnetic resonance imaging (fMRI) and electroencephalograms (EEG), advances in neuroscience have only recently provided massive datasets for the brain’s cellular structure. These data allowed Kovács and his team to apply statistical physics techniques to measure the physical structure of neurons.

For the new study, Kovács and Ansell analyzed publicly available data from 3D brain reconstructions of humans, fruit flies, and mice. By examining the brain at nanoscale resolution, the researchers found the samples exhibited hallmarks of physical properties associated with criticality.[2] "We don't yet know what two phases the brain might be transitioning between. If it were positioned on either side of the critical point, it wouldn't function as a brain."

One such property is the well-known, fractal-like structure of neurons. This nontrivial fractal dimension is an example of a set of observables, known as "critical exponents," that emerge when a system is close to a phase transition.

Brain cells are arranged in a fractal-like statistical pattern at different scales. When zoomed in, the fractal shapes are "self-similar," meaning that smaller parts of the sample resemble the whole. The diverse sizes of various neuron segments also provide a clue. According to Kovács, self-similarity, long-range correlations, and broad size distributions are all signatures of a critical state, where features are neither too organized nor too random. These observations lead to a set of critical exponents that characterize these structural features.

"These are things we see in all critical systems in physics," Kovács said. "It seems the brain is in a delicate balance between two phases."

Kovács and Ansell were amazed to find that all brain samples studied — from humans, mice, and fruit flies — exhibit consistent critical exponents across organisms, meaning they share the same quantitative features of criticality. The underlying, compatible structures among organisms suggest that a universal governing principle might be at play. Their new findings could help explain why brains from different creatures share some fundamental principles.

"Initially, these structures look quite different — a whole fly brain is roughly the size of a small human neuron," Ansell said. "But then we found emerging properties that are surprisingly similar."

"Among the many characteristics that vary across organisms, we relied on statistical physics to identify which measures are potentially universal, such as critical exponents. Indeed, those are consistent across organisms," Kovács said. "As an even deeper sign of criticality, the obtained critical exponents are not independent — from any three, we can calculate the rest, as dictated by statistical physics. This finding opens the way to formulating simple physical models to capture statistical patterns of brain structure. Such models are useful inputs for dynamic brain models and can inspire artificial neural network architectures."

Next, the researchers plan to apply their techniques to emerging new datasets, including larger sections of the brain and more organisms. They aim to determine if this universality will still apply.

The study, "Unveiling universal aspects of the cellular anatomy of the brain," was partially supported by the computational resources at the Quest high-performance computing facility at Northwestern.

Reference:

1. https://news.northwestern.edu/stories/2024/06/brains-structure-hangs-in-a-delicate-balance/
2. https://newatlas.com/biology/brain-phase-transition/

Cite this article:

Gokila G (2024), The Brain's Structure Is Maintained In 'A Fragile Balance.', AnaTechMaz, pp. 266