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Ubiquitous bursts of brain waves appear to synchronize disparate and distant memory elements, bringing them together in recollections – ScienceDaily

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A fundamental mystery of the human cerebral cortex is how its 16 billion neurons integrate, or link, the many different kinds of information they encode into a single coherent experience or memory.

Scientists have hypothesized that such binding involves high-frequency oscillations, or “ripples,” that facilitate neural interactions, much like rhythm in music or dance. In an article published on July 7, 2022, PNASresearchers at the University of California San Diego School of Medicine have provided some of the first empirical evidence that such ripples do occur in humans.

“Think about the way you pet your cat: its shape, location, surroundings, color, feel, movement and sound, and your own emotions and actions in response. They’re all tied into a cohesive whole,” said senior author Eric Halgren. Doctor of Philosophy, Professor of Radiology at the University of California, San Diego School of Medicine.

“These different aspects of experience are encoded in locations distributed across the cortical surface of the brain, and experience is served by their spatiotemporal level of firing. The mystery is how the actions in these different places connect.”

Previous studies, mainly in rodents, have found that ripples in another structure, the hippocampus, orchestrate the reproduction of these spatiotemporal patterns during sleep, and are essential for making memories permanent.

Hallgren’s UC San Diego team found that ripples also occur in all areas of the human cerebral cortex, both during wakefulness and during sleep. The pulses were short, lasting about one-tenth of a second, and had a stable, narrow frequency close to 90 cycles per second. The authors estimate that a typical short wave ripple may include approximately 5,000 small modules that become active simultaneously, distributed over the surface of the cortical brain.

This work is part of the doctoral dissertation in neuroscience by first author Charles W. Dickey.

“Remarkably, the ripples appeared simultaneously and synchronized in all lobes and between both hemispheres, even over great distances,” Dickey said. “Cortical neurons increased firing during pulsations, in pulsation rhythm, potentially supporting interactions between distant sites.

“There were more simultaneous instances that preceded successful memory recall. All this suggests that distributed cortical pulsation contributes to the integration of various elements that may constitute a particular experiential memory.’

The researchers found that cortical ripples are often coupled with hippocampal ripples and are embedded in slower oscillations (1 and 12 cycles per second). These slower rhythms are orchestrated by a central structure that controls levels in the cerebral cortex, the thalamus, and modulates neuronal firing, which is necessary for memory consolidation.

“As our experience is organized hierarchically in time, so the rhythms that organize our cortical activity create that experience,” Hallgren said.

The study involved analyzing weekly recordings taken directly from the brains of 18 patients who were followed to determine the location of their epileptic seizures. Ongoing work in Hallgren’s lab demonstrates that neuronal firing patterns in different parts of the cortex are more mutually predictable during collaborative ripples, and collaborative ripples are associated with associating letters to words and meanings to actions.

“Like any other fundamental research that advances our understanding of how the world works, it’s impossible to know what the practical implications will be,” Halgren said. “But I would like to point out that schizophrenia, a common and incurable disease, is characterized by mental fragmentation. Our findings and those of others indicate that a specific type of inhibitory interneuron is critical for ripple generation, and these cells are known to be selective in schizophrenia, as are high-frequency oscillations. We may be a little closer to finding the mechanism of one aspect of this tragic disease.”

Co-authors include: Ilya A. Verzhbinski, Xi Jiang, Burke K. Rosen, Sophie Kayfez, Jerry J. Shea and Sharon Ben-Haim, all of UC San Diego; Brittany Stedelin and Ahmed M. Raslan, Oregon Health & Science University; Emad N. Eskandar, Albert Einstein College of Medicine; Jorge Gonzalez-Martinez, Cleveland Clinic; and Sydney C. Cash, Harvard Medical School.

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