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'Princess Leia' brainwaves help store memories

Every night while you sleep, electrical waves of brain activity circle around each side of your brain, tracing a pattern that — were it on the surface of your head — might look like the twin hair buns of Star Wars' Princess Leia.

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.

"The scale and speed of Princess Leia waves in the cortex are unprecedented, this discovery advances Brain Research through Advancing Innovative Neurotechnologies (BRAIN) Initiative."

Terrence Sejnowski PhD, Director, Salk Computational Neurobiology Laboratory.

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.

"For a long time, neuroscience researchers recorded activity at one point in the brain at a time, putting many data points together without seeing the whole picture simultaneously,"
adds Lyle Muller, a Salk research associate and first author of the new work. Scientists had long believed each sleep spindle oscillation peaked at the same time everywhere in the neocortex — like a light bulb flashing on and off.

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.

Why would different areas of the neocortex need to communicate to store memories? Because, one single memory is composed of many parts (smell, sound, sight) each stored in a different area of the cortex. A memory is consolidated, Muller and Sejnowski believe, by waves of circular sleep spindles forming links between these aspects of a single memory.

"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."

Information in a computer is quantified by the number of bits that can be stored and recovered. An important question about the brain is how much information can be stored at a synapse through synaptic plasticity, which depends on the history of probabilistic synaptic activity. The strong correlation between size and efficacy of a synapse allowed us to estimate the variability of synaptic plasticity. In an EM reconstruction of hippocampal neuropil we found single axons making two or more synaptic contacts onto the same dendrites, having shared histories of presynaptic and postsynaptic activity. The spine heads and neck diameters, but not neck lengths, of these pairs were nearly identical in size. We found that there is a minimum of 26 distinguishable synaptic strengths, corresponding to storing 4.7 bits of information at each synapse. Because of stochastic variability of synaptic activation the observed precision requires averaging activity over several minutes.

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
Every cure has a starting point. The Salk Institute embodies Jonas Salk's mission to dare to make dreams into reality. Its internationally renowned and award-winning scientists explore the very foundations of life, seeking new understandings in neuroscience, genetics, immunology and more. The Institute is an independent nonprofit organization and architectural landmark: small by choice, intimate by nature and fearless in the face of any challenge. Be it cancer or Alzheimer's, aging or diabetes, Salk is where cures begin. Learn more at: salk.edu.
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Nov 28, 2016   Fetal Timeline   Maternal Timeline   News   News Archive   

(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;

Scroll down to watch video at bottom of page: 
Rotating waves over five spindle oscillation cycles.


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