Circular waves of electrical impulses spiral round the sides of our heads each night as we sleep, helping the brain file our memories. Named after the iconic coiffure of the Rebel Alliance’s leading lady, these “Princess Leia brainwaves” are thought to strengthen the connections between the brain regions that store the various components of individual memories, ensuring they remain in sync with one another.
A new study appearing in the journal eLife reveals how scientists used electrodes to study the neural activity of a group of patients while they slept. In particular, they were interested in observing short signals known as spindles, which arise in a brain region called the thalamus and spread throughout the cortex, typically during the period between deep sleep and dream sleep.
Previous studies have shown that these spindles play a major role in consolidating the day’s memories, by transferring them from the hippocampus – which holds short-term memories – to the neocortex, where longer-term memories are stored.
In the past, scientists have only been able to observe spindles at one point in the brain at a time, and had assumed that they arise simultaneously at multiple points throughout the cortex. Yet by using electrodes implanted into the brains of epileptic patients as part of separate research into the nature of seizures, the study authors were able to observe neural activity throughout subjects’ brains as they slept.
This revealed that spindles do not arise simultaneously throughout the brain, but in fact spread like “a wave that begins in a portion of the cortex near the ear, spirals through the cortex toward the top of the back of the head and then on to the forehead area before circling back”.
Each circuit takes around 70 milliseconds to complete, and is repeated many times over the course of the night.
While the researchers can only hypothesize as to the purpose of these Princess Leia brainwaves, they believe that they strengthen the connections between distant parts of the brain that store different elements of the same memories. For example, the audible, visual, tactile and emotional components of our memories are all held in different brain regions, so it is important to create strong links between these areas – kind of like syncing an audio track to a video channel.
"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," said study co-author Terrence Sejnowski in a statement. "There are also disorders including schizophrenia that affect sleep spindles, so this is really an interesting topic to keep studying."