Thinking hard weighs heavy on the brain

When the mind is at work, the brain literally gets heavier.

That fact may be surprising, but it isn’t new: In the 1880s, Italian scientist Angelo Mosso built an intricate full-body balance and reported that mental activity tips the scales. Now, a modern-day version of Mosso’s “human circulation balance” backs him up. Compared with a brain at rest, a brain listening to music and watching a video is indeed heavier, David Field and Laura Inman of the University of Reading in England report January 9 in Brain.

While teaching a course about brain-imaging techniques, Field grew curious whether Mosso’s general approach would work. So he and some students decided to find out. “It was a bit of a mad idea, to be honest,” Field says.

At the heart of both balances lies a simple seesaw lever. As weight shifts in a body, presumably from blood moving, the lever tilts the head or the feet downward, Mosso observed. Field and Inman’s contraption doesn’t actually tip. The researchers put a sensitive scale under the head end, which would register changes in force.

After lots of troubleshooting, which involved eliminating signals created by bodily processes that move blood such as breathing and heart beats, Field and Inman were able to test mental tasks. Fourteen participants were asked to lie still on the lever, and listen to music or listen to music and simultaneously watch a video of colorful geometric shapes. The part of the brain that detects sound is relatively small, Field says, so the audio plus video test was used to activate a wider swath of the brain and increase the chances of a measurable blood shift.

Right after a two-second blip of either audio or audio and video, blood leaves the brain, as measured by a drop in force, Field and Inman found. This quick dip in blood volume, a phenomenon that’s also seen in functional MRI, may represent the brain preparing for work by shunting waste-ridden blood out via the jugular vein. Seconds after that, a surge of new blood enters the brain, increasing the force measured by the scale.

These changes in force were very small — about 0.005 newtons — and most prominent in the people who both listened to music and watched a video, Field says. It’s hard to calculate how much blood rushes into the brain with each mental task. To know that value, scientists would need to know the distance of the head from the lever’s fulcrum, which could be easily measured, and exactly where the blood came from, which is nearly impossible to know.  

In his original experiments, Mosso found that tasks that required more mental energy made the brain heavier. Reading a page from a mathematics manual seemed to tip the balance more than reading a page from a newspaper. Strong emotions also tipped the scales: When a subject read a letter from an angry creditor, Mosso wrote, “the balance fell all at once.”

Until recently, Mosso’s scientific manuscripts had not been described in detail. But Stefano Sandrone of King’s College London unearthed Mosso’s papers in archives and published a description in Brain in 2013.

“We have been neglecting Mosso and his work for so many years. It’s good that someone has begun to find interest in the papers that he wrote,” Sandrone says of the modern-day experiment. He and his colleagues are working on an exhibition of Mosso’s original balance.

Many neuroscientists use functional MRI to detect changes in blood flow in the brain. Usually, fMRI spots regional differences, as when a little blood moves from one part of the brain to another. In contrast, the balance describes overall changes in brain workload, Field says.

The balance is not going to replace modern neuroimaging as a way to see what happens inside the brain. But with refinements, it might ultimately prove to be useful, Sandrone says. “The more measures we have, the more we can approximate the complexity of the brain.” 

Editor’s note: This story was updated on January 28, 2014, to correct the date of Angelo Mosso’s work and the wording of the quote from his experiment: “The balance fell all at once.”