There are many layers to the human brain. From the wrinkled exterior to the darkest depths, scientists are trying to understand them all. But in delving into the brain’s intricate neural circuits, they seem to have overlooked patterns of activity swirling on the surface.
A team of fluid physicists from the University of Sydney in Australia and Fudan University in China discovered brain signals rippling across the outer layer of the brain’s neural tissue, the cerebral cortex, on scans of the brains of 100 young adults. Signals naturally arranged as spirals, such as whirlpools in an emptying bath or whirlwinds of turbulent air.
“Understanding how the spirals relate to cognitive processing could significantly improve our understanding of brain dynamics and functions,” say senior author Pulin Gong, a physicist at the University of Sydney.
The cortex is the wrinkled outer layer of neuron-dense tissue that folds into the two hemispheres of the brain, responsible for computing complex cognitive functions such as language and storing memories.
Neuroscientists have focused primarily on bottom-up mapping of brain activity to understand the inner workings of regions such as the cortex: imaging cells to determine how they communicate as networks that give rise to their function.
In an exciting twist, the team analyzed brain imaging data collected as part of the Human Connectome project using methods best known to fluid physicists who study complex wave patterns in turbulent flows.
Functional MRI scans produce image data showing when and where the brain ‘lights up’ in a burst of activity, flooded with oxygenated blood. The spiral patterns identified in the data resemble kaleidoscopic waves or, simplified to directional vortices, circular pressure lines on a weather map.
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“These spiral patterns exhibit intricate and complex dynamics, moving across the brain surface as they rotate around central points known as phase singularities,” explains Gong.
“Just as vortices work in turbulence, the spirals engage in intricate interactions and play a critical role in organizing the brain’s complex activities,” he said. supposes.
Rhythmic spiral waves have been detected before neural circuitsrun through local brain regions that process sensory input, such as the visual, auditory, and somatosensory cortices.
These advancing waves are intriguing enough at the cellular level, especially when you consider how turbulent vortices have also been observed elsewhere in the body and in nature: in suspensions of swimming bacteria, biochemical signaling of the heart and in the membranes of living cells.
But how vortex-like waves might occur in the cerebral cortex as a whole hadn’t been studied until now, creating a gap in understanding how brain functions in each of the regions might be interconnected.
The spiral waves appeared to span multiple networks of interconnected cells and were located in precise anatomical locations, suggesting they may play a role in coordinating brain activity.
This working theory was tested with additional analyses, revealing that the brain spirals changed direction to adjust brain activity during language processing and working memory tasks such as listening to stories and answering math problems.
“An important feature of these brain spirals is that they often arise at the boundaries that separate different functional networks in the brain,” explains Yiben Xu, Physics graduate student at the University of Sydney.
At those locations, the researchers think the rotating spirals could act as a gate, allowing brain activity to flow to another region when the spins are oppositeor like a wall that blocks it if they rotate in the same direction.
“Due to their rotational motion, they effectively coordinate the flow of activity between these networks,” Xu suggests.
The findings fit into an alternative theory of how complex brain functions arise from the activity of individual cells firing away. The theory suggests that wave-like patterns of brain activity are sculpted by the shape of the brain itself — its folds, grooves and contours — rather than its interconnections.
Neurobiologist Kentaroh Takagaki of Tokushima University in Japan, who was not involved in the work, say Gong and colleagues’ results also “provide a strong counterpoint” to the columnar hypothesis of the brain, which describes how the cortex is organized into columns of neurons that process information in blocks.
However, the fMRI images used in the study only captured slow-moving waves of brain activity, so more research is needed to see if similar patterns emerge in faster brainwave oscillations and in higher-resolution scans.
“By unraveling the mysteries of brain activity and uncovering the mechanisms that govern coordination, we are moving closer to unlocking the full potential of understanding cognition and brain function,” Gong say.
The study is published in Nature human behavior.