Researchers observe new complexity of traveling brain waves in memory circuits
UC San Francisco researchers have observed a new feature of neural activity in the hippocampus – the brain’s memory center – that may explain how this vital region of the brain combines a diverse range of inputs into memories on several levels that can be recalled later.
Using a special “micro-grid” recording device developed by colleagues at Lawrence Livermore National Laboratory (LLNL), UCSF researchers were able to measure hippocampal activity in study participants undergoing surgery to treat severe epilepsy. They found that brain waves traveled back and forth through this structure, integrating messages from different areas of the brain, and showed for the first time what scientists could only speculate before.
“Brain recordings are an important part of the direction of epilepsy surgery,” said Edward Chang, MD, PhD, chair of the department of neurological surgery and lead author of the study, which appears May 12 in Nature communications. “The new high density electrode grid technology used here allowed us to see a new property of hippocampal activity that was previously unknown.”
Chang specializes in treating epilepsy with brain surgery, in which the hippocampus, a long structure located deep in the brain in an area called the temporal lobe, is exposed and sometimes totally or partially removed. The hippocampus can be a source of seizures for people with epilepsy and is one of the first areas of the brain affected by Alzheimer’s disease.
Previous studies had suggested that waves of activity in the hippocampus only travel in one direction: from the back, which encodes most of the physical location information, to the front, which encodes most of the physical location. emotional information. For Jon Kleen, MD, PhD, lead author of the study and assistant professor of neurology at the Weill Institute for Neurosciences, this one-way trip was not enough to explain how this small region of the brain manages to connect several types of information. to form A memory.
As an example, he says, imagine you lost your keys in Times Square. “You remember the spatial aspect” where “- Times Square – but you also remember the emotional feeling” Ack, I lost my keys! “”, Did he declare. To process a memory, Kleen noted, there must be a way to integrate many parts of a memory together. To do this, he speculated, it would make sense for brainwaves to travel through multiple routes to process information.
Custom electrode array provides two-dimensional view of brain waves
In an effort to test this hypothesis, Chang and Kleen teamed up with Razi Haque, head of the Implantable Microsystems group at LLNL, to develop a device that could provide a high-resolution two-dimensional image of neural activity. Haque helped create a device smaller than a penny, containing 32 electrodes spaced 2mm apart in a flexible polymer that could conform to the shape of the hippocampus.
During the surgery, Chang gently placed the electrode array directly on the hippocampi of six different surgical patients to monitor electrical activity while the patients rested. Using algorithms such as machine learning to analyze the data, the team found that not only do brain waves move along both the hippocampus, but the directions in which they move can be predicted. .
The team also found that sometimes waves of two different frequencies were present at once, moving in different directions and potentially carrying different information. This discovery provides new insight into how the hippocampus can integrate information from multiple areas of the brain into detailed memories.
Wave direction changes with cognitive activity
Two of the patients were awake and interacting during surgery. Kleen was able to show them pictures of common objects, such as a dog, and ask them to remember the word for it. Electrode data showed that as a patient remembered the word, activity cycles consistently moved from the back of the hippocampus to the front. Seconds later, the activity cycles changed, traveling in the opposite direction. “The direction of wave movement can be a biomarker reflecting the cognitive process the patient is engaged in at that time,” Kleen said.
These early observations are just the tip of the iceberg, he said. The next steps are to make observations with an even higher resolution electrode array and to observe neural activity in patients performing more complex cognitive tasks. Ultimately, he hopes the information gained can lead to treatments using deep brain stimulation to improve neurostimulatory therapies that have been very successful in epilepsy.
“The purpose of this research is to accelerate our understanding of how the hippocampus works, so that we can remedy the damage we see in patients with epilepsy and Alzheimer’s disease,” Kleen said. “If we find that in some patients the waves are not moving properly, we can design more sophisticated stimulation regimens that may be more effective in preventing seizures or restoring cognition.”
The co-authors of the article include Jason Chung and Kristin Sellers, PhD, of UCSF, as well as other researchers at LLNL. The project was funded by NINDS R25NS070680 and K23NS110920 (JK) grants; R01-DC012379, R00-NS065120, and partially funded by DARPA grant DP2-OD00862. For more authors and funding, please see the article.
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