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'Princess Leia' brainwaves help store memories
Salk Institute scientists discovered these circular "Princess Leia" oscillations, which they describe in the journal eLife as waves, forming each night from associations made between aspects of that day's experiences.
Short-term memory is stored in an area of the brain called the hippocampus. Long-term memories, however, are encoded in the neocortex. The transfer of memories from the hippocampus to the neocortex is called memory consolidation, and happens while we sleep.
Sleep spindles — a type of brain wave pattern that occurs in the earliest stages of non-REM sleep — help consolidate memories. Non-rapid eye movement (NREM) sleep has little or no eye movement as the mind is more organized. The differences between REM and NREM is believed to influence memory formation. Previous studies showed that the more sleep spindles a brain exhibits overnight, the more numbers a person will remember the next day. But, exactly how sleep spindles related to memory was unclear. Scientists had been limited in the past to detecting only one spindle at a time as only one electrode could be placed in one area of the brain at a time.
Sejnowski and Muller wanted to get a broader picture, so turned to large-scale recordings known as intracranial electrocorticograms (ECoGs), that measure activity in many areas of the brain all at once. Patients with epilepsy often have ECoG arrays temporarily implanted in their brains in order to locate where their epileptic seizures origniate. So, scientists collected and studied data from five such patients on their healthy, seizure-free nights.
When they crunched the ECoG data from each night, researchers were in for a surprise — sleep spindles weren't peaking simultaneously everywhere in the cortex. Instead, oscillations were sweeping in circular patterns around and around the neocortex, peaking in one area, and then — a few milliseconds later — an adjacent area.
"We think that this brain activity organization is letting neurons talk to neurons in other areas," says Muller. "The time scale that these waves travel is at the same speed it takes neurons to communicate with each other."
Throughout the night, researchers observed the same rotating patterns, each lasting about 70 milliseconds — repeated hundreds and hundreds of times in a matter of hours.
"If we understand how memories are being linked in the brain, we could potentially come up with methods for disrupting memories after trauma," says Sejnowski. "Also, disorders like schizophrenia, affect sleep spindles. So this is really an interesting topic of study as well."
Other researchers on the study were Dominik Koller of the Salk Institute; Giovanni Piantoni and Sydney S. Cash of Massachusetts General Hospital; and Eric Halgren of the University of California San Diego.
The work and the researchers involved were supported by grants from the National Institutes of Health, Howard Hughes Medical Institute, the Swartz Foundation and the Office of Naval Research.
About the Salk Institute for Biological Studies
(A) Spikes emitted from region A will arrive at B with a delay of 20 milliseconds (top).
(B) In contrast, if spindles are spatio-temporally organized, EPSPs from region A
will align with spikes in region B.
Image Credit: Salk Institute for Biological Studies;
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Rotating waves over five spindle oscillation cycles.