For over 100 years, scientists have thought that the brain activity patterns that define human consciousness arose from how different brain regions communicate with each other through trillions of cellular connections.
Now, by examining more than 10 000 different maps of human brain activity, Monash University-led researchers found that the overall shape of a person’s brain has a much greater influence on thought and behaviour than its neuronal connectivity. This may sound like the old pseudoscience of phrenology, which based theories of personality and cognition on the shape of the head and its bumps.
Not so for this study, which combines approaches from physics, neuroscience and psychology to overturn the century-old model revolving around complex brain connectivity, instead revealing a relationship between brain shape and activity. The researchers published their ground-breaking findings in the journal Nature.
Lead author Dr James Pang said the findings were significant because they greatly simplified the way that we can study how the brain functions, develops and ages.
“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,” Dr Pang said.
“We have long thought that specific thoughts or sensations elicit activity in specific parts of the brain, but this study reveals that structured patterns of activity are excited across nearly the entire brain, just like the way in which a musical note arises from vibrations occurring along the entire length of a violin string, and not just an isolated segment,” he said.
Using magnetic resonance imaging (MRI), the researchers studied eigenmodes, which are the natural patterns of vibration or excitation in a system, where different parts of the system are all excited at the same frequency. Eigenmodes are normally used in areas such as physics to study physical systems only recently have they been applied to studying brain.
Their study focused on developing the optimal way to construct the eigenmodes of the brain.
“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, but which specific properties are most important has remained a mystery,” said co-lead author, Dr Kevin Aquino, of BrainKey and The University of Sydney.
‘Like the shape of a drum influences the sounds that it can make’
The team, led by Professor Alex Fornito, compared how well eigenmodes derived from models of brain shape could account for different patterns of activity as opposed to eigenmodes from models of brain connectivity.
“We found that eigenmodes defined by brain geometry – its contours and curvature – represented the strongest anatomical constraint on brain function, much like the shape of a drum influences the sounds that it can make,” said Fornito.
“Using mathematical models, we confirmed theoretical predictions that 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 wave ripples that are formed by a falling pebble,” he said.
“These findings raise the possibility of predicting the function of the brain directly from its shape, opening new avenues for exploring how the brain contributes to individual differences in behavior and risk for psychiatric and neurological diseases.”
The research team found that, across over 10 000 MRI activity maps, obtained as people performed different tasks developed by neuroscientists to probe the human brain, activity was dominated by eigenmodes with spatial patterns that have very long wavelengths, extending over distances exceeding 40 mm.
“This result counters conventional wisdom, in which activity during different tasks is often assumed to occur in focal, isolated areas of elevated activity, and tells us that traditional approaches to brain mapping may only show the tip of the iceberg when it comes to understanding how the brain works,” Dr Pang said.
Source: MedicalXpress
