PNAS: Looks inside the brain while sleeping, deciphering how memories are stored

A new study looked deep in the brain where, during sleep, previous learning was reactivated, improving memory
.

Northwestern's neuroscientists, in collaboration with clinicians at the University of Chicago Epilepsy Center, studied the electrical activity of the brains of 5 patients at the center when they heard sounds made by the research team as part of a
learning exercise.

Five patients who volunteered to participate in the study had electrode probes implanted in the brain with the aim of investigating potential treatments for
epilepsy.

Previous studies have used EEG recordings captured by head electrodes to measure memory processing during sleep, and this is the first study to
record this electrical activity from inside the brain.

The study found that participants performed significantly on
the next morning's recall test.
By providing visual data to identify brain regions involved in the nighttime memory storage process, the brain activity map allows researchers to take a big step
forward in understanding how memory storage works.

Although the number of patients studied was necessarily small, it was possible to draw strong conclusions because all 5 patients showed the same pattern
of memory improvement and electrical activity.

Ken Paler, director of Northwestern's cognitive neuroscience program and senior investigator of the study, said: "We're investigating how people manage to remember what they've learned instead of forgetting them
.
" "Our view is that sleep contributes to this ability
.
"

Paller is a professor of psychology and the James Padilla Chair Professor
of Arts and Sciences in Northwestern University's Weinberg College of Arts and Sciences.

Study collaborators include neurology and neurosurgery researchers at the University of Chicago, and psychology researchers
at Northwestern University, the University of Michigan, and Middlebury College (Vermont).

How the study was conducted

One night, while each patient slept in a hospital room, the team recorded electrophysiological responses
to 10 to 20 recurring sounds.
All sounds are played very quietly to avoid being woken up
.
Half of the sounds are related to objects and their precise spatial location, which patients learn with their laptops before going to bed, such as the clanging of car keys, to help recall their location
.

After sleep, the researchers found systematic improvements in spatial memory, replicating previous findings
recorded using scalp EEG.
The patient more accurately indicates the remembered location
on the laptop screen.

New data from implanted brain electrodes show that the sounds of objects that appear during sleep trigger an increase in oscillatory activity, including an increase in theta, σ, and gamma EEG bands
.

When sounds are heard during sleep, electrophysiological activity occurs in the medial temporal region of the hippocampus and adjacent cerebral cortex, reflecting the reactivation and strengthening
of corresponding spatial memory.

The gamma response is consistent
with the degree of improvement in spatial memory after sleep.
This electrophysiological evidence led the researchers to conclude that sleep-based memory storage enhancement occurs in these brain regions
.

"The traditional assumption is that when people sleep, these sounds are muffled
," Paler said.
Instead, these sounds allow us to demonstrate that when memories are reactivated, brain structures such as the hippocampus respond to help us retain the knowledge
gained while awake.

Sometimes, memory and forgetting seem random
.
We can remember irrelevant details and forget what
we most want to remember.
The new answer to this long-standing puzzle, highlighted in this study, is that when we sleep, memories are revisited, even when we wake up unaware that it happened," Paller said
.