It’s shape, not just wiring: How brain structure influences function
For over 100 years, scientists have agreed that discrete collections of brain cells fire off signals to other brain areas through a series of interconnected fibers. In a new study, researchers applied a mathematical model to brain activity and found that brain function may have more to do with shape than connectivity.
First appearing in the early 1970s, neural field theory (NFT) combines an understanding of brain anatomy and physiology with math to model large-scale brain activity. Previous predictions using NFT have suggested that the shape of the brain may be more fundamental to its functioning than the neuronal connections between regions. Now, researchers from Monash University in Melbourne, Australia, have tested those predictions to see whether they’re true.
“We’re trying to change how we view the brain,” said James Pang, the lead author of the study. “The traditional approach in neuroscience is that every brain process is attributed to just one particular region, in a very local part of your brain, but it’s only recently become possible to study the entire brain at once in living humans, with advances in technologies like MRI.”
The researchers examined more than 10,000 functional magnetic resonance imaging (fMRI) scans of people at rest and while they performed tasks. After reconstructing the scans to create a ‘brain map’ for each individual, the researchers discovered a link between the brain’s function and its shape or geometry.
“The close link between geometry and function is driven by wave-like activity propagating throughout the brain, just as the shape of a pond influences the ripple patterns that are formed by a falling pebble,” said Alex Fornito, one of the study’s co-authors.
These waves are called ‘eigenmodes’, a physics term that denotes a system’s natural or preferred vibration such that various parts move together at the same frequency. The researchers liken it to plucking a violin string.
“The best way to understand what eigenmodes are is to think of a violin,” Pang said. “Every time you pluck its string, it vibrates with the same pattern, and this pattern corresponds to the notes that you hear. The preferred patterns of vibration are the eigenmodes of the string.”
The vibration patterns they observed were either localized or covered the entire brain and were related to brain activity.
“There are many eigenmodes, and every single eigenmode encodes a different frequency,” said Pang. “All of them can combine in various different ways to support whatever brain processes that you can think of. It means the idea that only certain neurons in a certain part of the brain are working when you do something is probably inaccurate, because other parts of the brain are also contributing.”
Comparing eigenmodes related to brain shape – that is, its size, shape and contours – with those obtained from the connections between neurons, the researchers found that brain shape eigenmodes might have a stronger influence on brain activity. Their findings overturn over 100 years of accepted science about the brain’s workings.
“Just as the resonant frequencies of a violin string are determined by its length, density and tension, the eigenmodes of the brain are determined by its structural, physical, geometric and anatomical properties,” said Pang.
The researchers believe their findings could be used to more easily predict patterns of brain activity in disease states simply by looking at shape.
“The work opens opportunities to understand the effects of diseases like dementia and stroke by considering models of brain shape, which are far easier to deal with than models of the brain’s full array of connections,” Pang said. “People with dementia, for example, have atrophy [shrinkage] in certain parts of the brain, which may change the activity they can support.”
The researchers acknowledge that their study raises two issues that require further consideration.
“First is, we move away from the usual view that mapping the complex array of cells and their connections in the brain is necessary to understand how it works,” said Pang. “Instead, we show that brain shape may actually be more important. We’re still getting our heads around what that means for understanding brain function. The second problem concerns applications. The approach creates new opportunities for brain mapping in different species, through development and aging, and in different brain disorders, leading to new research directions not just for us at Monash, but people everywhere.”
The study was published in the journal Nature.
Source: Monash University