Nicholas G. Hatsopoulos, PhD, Professor of Organ Biology and Anatomy at the University of Chicago, has long had an interest in space. Specifically, the physical space occupied by the brain.
“In our heads, the brain is completely crumpled up. If you flattened the human cortex into a single 2D sheet, it would cover two and a half square feet of space – roughly the size of four sheets of paper. You would think that the brain would use all this space to organize patterns of activity, but aside from the fact that one part of the brain controls the arm and another the leg, we have largely ignored how the brain might use this spatial organization.
Now in a new study published Jan. 16 Proceedings of the National Academy of Sciences, Hatsopoulos and his team have found evidence that the brain does indeed use the spatial organization of high-frequency propagating waves of neural activity during movement.
The presence of propagating waves of neural activity is well known, but they have traditionally been associated with an animal’s general behavioral state (such as being awake or asleep). This study is the first evidence that spatially organized recruitment of neural activity across the motor cortex can provide information about details of a planned movement.
The team hopes the work will help how researchers and engineers decipher motor information to build better brain-machine interfaces.
To conduct the study, the researchers recorded the activity of multi-electrode arrays implanted in the primary motor cortex of macaque monkeys while the monkeys performed a task that required them to move a joystick. Then they looked for wave-like patterns of activity, particularly those with high amplitude.
“We focused on the high-frequency band signals because they provide rich information, ideal spatial range, and easy signal acquisition in each electrode,” said Wei Liang, first author of the study and a graduate student in the Hatsopoulos lab.
They found that these propagating waves, made up of the activity of hundreds of neurons, traveled in different directions across the cortical surface depending on which way the monkey pushed the joystick.
“It’s like a series of falling dominoes,” said Hatsopoulos. “Any wave patterns we’ve seen in the past didn’t tell us what the animal was doing, it was just going to happen. It’s very exciting because now we’re looking at this expanding wave pattern and showing the direction.” Wave goes tells you something about what the animal is going to do.”
The results offer a new perspective on cortical function. “This shows that space matters,” said Hatsopoulos. “Rather than just paying attention to what populations of neurons are doing and what matters to them, we see that there are spatially organized patterns that carry information. It’s a completely different way of thinking about things.”
The research was challenging as they examined the activity patterns of individual movements rather than averaging the records over repeated trials, which can be quite noisy. The team was able to develop a computational method to clean up the data to bring clarity to the recorded signals without losing important information.
“If you average across studies, you’re missing out on information,” Hatsopoulos said. “If we’re going to implement this system as part of a brain-machine interface, we can’t do any averaging attempts – your decoder needs to do this on the fly, while motion is occurring, for the system to work effectively.”
Knowing that these waves carry information about movement opens the door to a new dimension in understanding how the brain moves the body, which in turn may provide additional information for the computer systems that will control the brain-machine interfaces of the future.
“The spatial dimension has been largely ignored so far, but it’s a new angle we can use to understand cortical function,” Hatsopoulos said. “When trying to understand the calculations that the cortex performs, we should consider how the neurons are arranged spatially.”
Future studies will examine whether similar wave patterns can be seen in more complicated movements, such as B. sequential movements versus simple point-to-point reaching, and whether or not wave-like electrical stimulation of the brain can affect the monkey’s movement.
The study “Propagating spatiotemporal Activity Patterns across Macaque Motor Cortex Carry kinematic information” was supported by the National Institutes of Health (R01 NS111982). Other authors are Karthikeyan Balasubramanianb and Vasileios Papadourakis from the University of Chicago.