Brain circuitry that makes people 'zone out' identified

MIT scientists have identified a brain circuit that can trigger small regions of the brain to fall asleep even while the rest of the brain remains awake, which explains why we sometimes 'zone out'.
The discovery may help design new sleep and anesthetic drugs, researchers said.
This circuit originates in a brain structure known as the thalamic reticular nucleus (TRN), which relays signals to the thalamus and then the brain's cortex, inducing pockets of the slow, oscillating brain waves characteristic of deep sleep.
The researchers believe the TRN may help the brain consolidate new memories by coordinating slow waves between different parts of the brain, allowing them to share information more easily.

"During sleep, maybe specific brain regions have slow waves at the same time because they need to exchange information with each other, whereas other ones don't," said one of the lead authors Laura Lewis, from Massachusetts Institute of Technology (MIT).
The TRN may also be responsible for what happens in the brain when sleep-deprived people experience brief sensations of "zoning out" while struggling to stay awake, researchers said.

The team began its study of local control of alertness or drowsiness with the TRN because its physical location makes it perfectly positioned to play a role in sleep, Lewis said.
The TRN surrounds the thalamus like a shell and can act as a gatekeeper for sensory information entering the thalamus, which then sends information to the cortex for further processing.

Using optogenetics, a technique that allows scientists to stimulate or silence neurons with light, the researchers found that if they weakly stimulated the TRN in awake mice, slow waves appeared in a small part of the cortex. With more stimulation, the entire cortex showed slow waves.
"We also found that when you induce these slow waves across the cortex, animals start to behaviourally act like they're drowsy. They'll stop moving around, their muscle tone will go down," Lewis said.
The researchers believe the TRN fine-tunes the brain's control over local brain regions, enhancing or reducing slow waves in certain regions so those areas can communicate with each other, or inducing some areas to become less alert when the brain is very drowsy.

This may explain what happens in humans when they are sleep-deprived and momentarily zone out without really falling asleep.
"I'm inclined to think that happens because the brain begins to transition into sleep, and some local brain regions become drowsy even if you force yourself to stay awake," Lewis said.
The study was published in the journal eLife.