Short-term memory of events 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.
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. The Salk Institute scientists who discovered these
circular "Princess Leia" oscillations, which are described in the
think the waves are responsible each night for forming associations between different aspects of a day's memories.
‘The 'Princess Leia' waves in the cortex are responsible each night for forming associations between different aspects of a day's memories.’
Sleep spindles - a type of brain wave pattern known to occur in the
earliest stages of non-REM sleep - are associated with memory
consolidation. Previous studies showed that the more sleep spindles a
human brain exhibits overnight, the more numbers one would remember the
next day. But exactly how these sleep spindles related to memory was
unclear, and scientists were limited by the fact that electrodes could
only detect these spindles at one place in the brain at a time.
"The scale and speed of Princess Leia waves in the cortex is
unprecedented, a discovery that advances the Brain Research through
Advancing Innovative Neurotechnologies (BRAIN) Initiative," says
Terrence Sejnowski, head of Salk's Computational Neurobiology
"For a long time, neuroscience researchers had to record activity at
one point in the brain at a time and put many data points together
without seeing the whole picture simultaneously," says Lyle Muller, a
Salk research associate and first author of the new work. Scientists had
long believed that each sleep spindle oscillation peaked at the same
time everywhere in the neocortex of the brain.
Sejnowski and Muller wanted to see the broader picture, however, and
turned to large-scale recordings, called intracranial
electrocorticograms (ECoGs), that can measure activity in many areas of
the brain at once. Patients with epilepsy often have ECoG arrays
temporarily implanted in their brains to determine the location in the
brain of epileptic seizures, so the scientists were able to study all
the data collected from five such patients on healthy, seizure-free
When they crunched the ECoG data from each night, the researchers
were in for a surprise: the sleep spindles weren't peaking
simultaneously everywhere in the cortex. Instead, the 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 at is the same speed it takes for neurons to communicate
with each other."
Throughout the night, the researchers observed the same rotating
patterns, each lasting about 70 milliseconds but repeating hundreds and
hundreds of times over a matter of hours.
Why would different areas of the neocortex need to communicate to
store memories? One single memory is composed of different components
(smell, sound, visuals) that are stored in different areas of the
cortex. As a memory is being consolidated, Muller and Sejnowski
hypothesize, circular sleep spindle waves help form the links between
these different aspects of a single memory.
"If we understand how memories are being linked up like this in the
brain, we could potentially come up with methods for disrupting memories
after trauma," says Sejnowski. "There are also disorders including
schizophrenia that affect sleep spindles, so this is really an
interesting topic to keep studying."