The brain may flush out its waste products after a mental workout

The brain’s “waste disposal system” may kick in after intense neural activity – and it might be possible to turn on the process intentionally.

Until recently, this system was thought to activate only during sleep, but now researchers have seen it ramping up in people after they watch flickering chequerboard patterns on a screen.

The finding provides a tantalising hint that people may be able to deliberately flush out waste products from their brain by staring at intense visual stimuli, says Laura Lewis at Boston University in Massachusetts.

“The real surprise was that they found it in awake people,” says Edoardo Rosario de Natale at the University of Exeter in the UK, who wasn’t involved in the work.

The brain’s waste disposal system involves cerebrospinal fluid (CSF) being pumped into the brain and leaving through a network of fine tubes called the glymphatic system, which was only discovered in 2012.

Animal research suggests the fluid flushes out waste products made by brain cells, including harmful compounds that may be involved in Alzheimer’s disease and Parkinson’s disease, such as beta-amyloid and alpha-synuclein.

Since the glymphatic system’s discovery, there has been a surge of research aiming to understand how boosting fluid flow could help improve brain health, but much of how the system functions in people is still unclear.

Lewis’s team took advantage of a new brain-scanning technique, using existing magnetic resonance imaging machines, that highlights any CSF that has newly entered into the fourth ventricle of the brain, a cavity at the base of the head. Fluid that enters this chamber drains out through the glymphatic system.

They asked 20 volunteers to watch a screen inside the scanner that displayed a pattern known to cause high brain activity: a flickering black- and-white spiral chequerboard. The display was turned on and off at 16-second intervals for about an hour, apart from during short breaks.

When the pattern was shown, this caused a rise in blood flow to the brain’s visual centres, as expected. When the screen went dark, blood flow reduced and CSF flow into the brain increased.

The brain-scanning technique couldn’t reveal if the fluid left through the glymphatic vessels, nor if there was a reduction of waste products within the brain. These are questions that need to be tackled next, says Rosario de Natale. “This is opening a new door.”

“It’s still an open question whether the fluid goes directly into the brain tissue or if it sloshes around in the ventricle. But we definitely think that it has an effect on fluid in the rest of the brain,” says team member Stephanie Williams, also at Boston University.

“We’re very interested now to understand the effect of these changes in fluid flow and how it intersects with brain health,” says Lewis.