Making waves may be key to making memories

G. Bard Ermentrout, Distinguished University Professor of Mathematics in the Kenneth P. Dietrich School of Arts and Sciences, working with a team led by University of Chicago, has shown distinct patterns in waves of brain activity carry information when forming and recalling memories.

These waves form different shapes - spirals or concentric waves, for instance - and move in different directions based on what a person is doing. They vary from person to person and according to task, show up as short but stable bursts of neural activity and flexibly change their shape for encoding different types of behavior, such as memory encoding and retrieval.

Researchers have known that there are waves of activity in the brain, "but here we were able to show that there are actually different spatial patterns in these waves," said senior author Joshua Jacobs, a professor of neurology at the University of Chicago and senior author of the study, which was published in the journal Nature Communications.

Previously, researchers thought brain wave patterns to be more like standing waves, or oscillations. In the 1990s, however, people began looking into these patterns theoretically, computationally and mathematically, Ermentrout said: "In recent years people have begun to realize that activity in the brain is not synchronous oscillations, but it's organized into various types of waves."

A natural question arises: "Do these waves mean anything? Could they be associated with any behavior or anything like that?"

To find out, Ermentrout's collaborators turned to the gold standard of measuring electrical activity in the human brain: people preparing for surgery to manage severe, drug-resistant epilepsy. During the presurgical period, patients are implanted with electrodes in their brain to measure brain activity and help doctors pinpoint to the region responsible for epileptic seizures.

While the subjects were hospitalized with the electrodes implanted, Jacobs and his team worked with them to perform a series of memory tasks testing word recall and spatial ability. One task, a "treasure hunt" meant to focus on spatial memory, resembled a video game where the patients navigated an environment and had to remember where different objects were located. In a second task, the patients memorized a series of English 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. Electrodes can capture those signals at a particular time, such as the moment a person is trying to encode or recall a word. Each electrode covers about 1 square centimeter of brain surface and can detect activity in up to 1 million brain cells. Combined, the data collected by the electrodes can help paint a picture of neuronal activity as it occurs across several regions of the brain.

Anup Das, a postdoctoral fellow in Jacobs' lab, analyzed the data captured by the electrodes and found distinct types of wave patterns; some basic shapes traveled in a straight line in one direction. Some curled into spirals. Some propagated like outward sources like waves in a pond after a rock has been tossed in. Others behaved like a sink, moving in the opposite direction closing in on a point at the center.

The group analyzed the data and found they were able to match specific patterns to patients' actions at the time the signals were recorded.

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 remained remarkably consistent per individual. So much so that the researchers were able to decode the behavior from the wave shape alone about 70% of the time (compared to chance at 50%).

In one instance, Ermentrout said, "Das was able to determine that the subject had been looking at the English letter 'H' when a pattern appeared," simply by looking at the waves of neural activity.

Several features of the traveling waves affected how well patients did on their memory tests, including waves' directions and strength, Ermentrout said.

"Active neural activity can ride these waves like a surfboard," he said. "If the wave isn't strong enough to propagate, the information wouldn't get where it needs to go." In the case of memory that means an event or a word wouldn't be encoded in the first place. This finding in particular may present paths to treatments for people with disorders of memory.

"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," Jacobs said. "And then if we understand where they're coming from mechanistically, then we can potentially apply brain stimulation in a way to strengthen them, which will enhance memory performance in patients. This will help us build brain-computer interfaces or transcranial magnetic stimulation therapies to enhance memory for patients with cognitive decline."

Matt Wood contributed; read the original press release from the University of Chicago. Image courtesy of Nature Communications, CC BY-NC-ND 4.0.