By
Matt Wood
Director of Communications, Biological Sciences Division
A new study from the University of Chicago used direct recordings of brain activity from human participants to show how “traveling waves” of activity spread across different regions of the brain to coordinate memory formation and recall. The activity forms distinct patterns, such as spirals and concentric waves, that change depending on what the person was doing, including remembering individual items.
“We’ve known for many years that oscillations or waves of activity happen in the human brain, but here we were able to show that there are actually different spatial patterns of these waves,” said Joshua Jacobs, PhD, Professor of Neurology at UChicago and senior author of the study, which was published in Nature Communications.
A fingerprint of patterns
The researchers used recordings of brain activity from patients with drug-resistant epilepsy who have had electrodes implanted in the brain (typically 100-150 for each person) to help doctors determine the cause of their seizures. During the time the patients were hospitalized while the electrodes were implanted, Jacobs and his team worked with them to perform a series of memory tasks on a laptop.
One task, a “treasure hunt” meant to focus on spatial memory, was like a video game where they navigated an environment and had to remember where different objects were located. In the second task, the patients memorized a series of letters and then had to remember which letters they saw in subsequent sequences.
Brain cells communicate with each other in part by producing electrical signals. When a person thinks about something, like remembering a location or letter, the electrodes capture this activity. Each electrode is about one centimeter square in size and can detect activity in up to one million brain cells each. Combined, the data collected by the electrodes can help paint a picture of neuronal activity as it occurs throughout different regions of the brain.
Anup Das, PhD, a postdoctoral researcher who led the study, used AI, machine learning, and signal processing algorithms to analyze this data. Several distinct patterns emerged. Some waves traveled in straight lines in one direction, or planes, while others formed curling spirals. Some waves propagated in concentric circles from a single source, radiating outward like a broadcast or moving inward like a draining sink.
When Das and study collaborator Bard Ermentrout, PhD, Distinguished University Professor of Mathematics at the University of Pittsburgh, studied these patterns, they were able to map them back to specific activities the patients were doing. The patterns differed among individuals. For example, the act of remembering a location in the treasure hunt task might generate a spiral wave for one person, while it produced a radiating source for another. But the patterns were remarkably consistent per individual. The researchers were able to decode the behavior from the wave shape alone about 70% of the time (compared to chance at 50%).
“We saw that there is a specific shape associated with a specific type of behavior,” Das said. “These shapes are not random. I like to think of it as a fingerprint for each person.” In addition to distinguishing broad cognitive states such as memory encoding and recall, the waves also changed their shape depending on the specific letter (“B”, “G”, etc.) the participant was memorizing. “This suggests that brain waves can also encode item specific, fine-grained details in addition to broad events,” Das said.
Reverse engineering patterns
Since the shape of the waves for different activities varied from person to person, the researchers aren’t yet sure if there is a purpose or function for each pattern. Jacobs speculates that it might have to do with information flow, such as a straight wave moving in one direction to communicate directly with an adjacent brain region. The spirals might help with spatial memory consolidation, like remembering where objects are located, by repeating the same activity over the same area of the cortex repeatedly.
The researchers hope that by understanding these different patterns, they can reverse engineer the activity and use the same patterns with technology like brain computer interfaces or transcranial magnetic stimulation to enhance memory for patients with cognitive decline.
“If a traveling wave is relevant for a certain kind of memory encoding, then if you want to help a person remember things better, maybe you can apply brain stimulation that strengthens that kind of traveling wave,” Jacobs said. “Hopefully, by characterizing patterns like this, then we can go to the next step and use mathematical models of each of these waves to understand where they are coming from. And then if we understand where they're coming from mechanistically, then we can potentially apply brain stimulation in a way to strengthen them.”
The study, “Planar, Spiral, and Concentric Traveling Waves Distinguish Behavioral States in Human Memory,” was supported by the National Science Foundation. Erfan Zabeh from Columbia University was an additional author.
