Brain ‘ripples’ really do exist, explaining how memories form in humans

In a breakthrough study, scientists have found how memories are formed. Scientists from the University of California San Diego School of Medicine discovered brain “ripples” help bind information across the human cortex.

It’s the first empirical evidence that these so-called ripples occur in people.

“Think about the experience of petting your cat: its form, location, surroundings, color, feel, movement, and sound, plus your own responding emotions and actions. They are all bound together in a coherent whole,” says Dr. Eric Halgren, study senior author and professor of radiology at University of California San Diego School of Medicine, in a statement. “These different aspects of the experience are encoded in locations distributed across the cortical surface of the brain, and the experience is sub-served by their spatiotemporal firing pattern. The mystery has been how activities in those different locations get connected.”

Within the human cortex, 16 billion neurons bind the many different kinds of information they encode into a single coherent memory. In previous studies in rodents, researchers found ripples in the hippocampus, which “organize the replay of these spatiotemporal patterns during sleep, and this is essential for making memories permanent.”

However, the UC San Diego scientists found these ripples also happened in all areas of the human cortex, in waking as well as sleep. The ripples only lasted one-tenth of a second and had a frequency close of 90 cycles per second. A typical ripple event is believed to involve nearly 5,000 small modules becoming active instantaneously and distributed across the cortical surface.

“Remarkably, the ripples co-occurred and synchronized across all lobes and between both hemispheres, even at long distances,” says Charles Dickey, study first author. “Cortical neurons increased firing during ripples, at the ripple rhythm, potentially supporting interaction between distant locations. There were more co-occurrences preceding successful memory recall. All of which suggests that distributed, cortical co-ripples promote the integration of different elements that may comprise a particular experiential memory.”

Researchers analyzed the brains of 18 patients who were being monitored to locate the origin of their epileptic seizures. During the week-long analysis, researchers found that cortical ripples were often coupled with hippocampal ripples and embedded in slower oscillations — one and 12 cycles per second. “These slower rhythms are orchestrated by a central structure controlling cortical activity levels, the thalamus, and modulate neuronal firing, which is needed for memory consolidation,” the media release reads.

“Like any other basic research that increases our understanding of how the world works, it is impossible to know what its practical implications will be,” explains Halgren. “But I would note that schizophrenia, a common and incurable disease, is characterized by mental fragmentation. Our findings and those of others indicate that a particular type of inhibitory interneuron is crucial for the generation of ripples, and these cells are known to be selectively affected by schizophrenia, as are high frequency oscillations. Perhaps we are a little closer to finding a mechanism for one aspect of this tragic disease.”

The study was published in the journal PNAS.

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