If you’ve ever had the misfortune to step in a puddle and get your shoes soaking wet, you probably spent the next few hours keenly aware of the “squish, squish, squish” sound you made with every step. But the truth is, your footsteps make noise whether or not your feet are wet; you just don’t notice unless there’s something different about it. As a recent neuroscience study discovered, that’s by design. Forget noise-canceling headphones — you’ve got a noise-canceling brain.
Can’t You Hear Me Knocking?
The idea that the brain can ignore some sounds in its environment is nothing new. In fact, it’s essential for normal hearing. Hearing aids, for instance, have to use filters to dampen background noise so that wearers are better able to understand speech and distinguish other sounds — without it, they’d just hear an unintelligible cacophony. The brain has more information than a hearing aid, however, since it’s also in charge of the body’s own noise-making movements. That means it knows when to ignore footsteps, breathing sounds, and chewing noises, to name a few. (As for the sounds your dinner companions make, you’re not so lucky.) Even more important, it’s able to predict when those sounds might happen so it can ignore them at the right moment.
But scientists weren’t sure how exactly the brain knows to ignore these sounds. To find out, researchers from Duke University and NYU turned laboratory mice into fuzzy little gamers: The mice were put on treadmills and hooked up to an augmented reality system. That way, the researchers could control the sounds the mice heard. The team monitored how fast the mice ran on the treadmill and played specific tones to match the tempo of their steps.
“We decided to make the sound as artificial as possible to push the mouse’s brain beyond what it was evolved to do,” said NYU Assistant Professor and study co-author David Schneider in a statement. All the while, the team recorded the brain activity of their rodent subjects.
After a few days of this, brain cells in each mouse’s auditory cortex (the brain’s hearing center) stopped responding to the artificial tones — but only while they were running. They could even watch this noise filter build up in the neurons over several days:
Same Thing We Hear Every Night, Pinky
The researchers already knew that the brain’s motor center turns on noise-canceling neurons in the auditory cortex, but this experiment showed them just how precise the brain is about it: Once the mice had learned the artificial footstep tone, their motor cortex only turned on the noise-canceling neurons that were tuned to the sound — that is, their hearing center could still listen for other noises that weren’t related to running.
Even cooler, they found that once the mice had learned this running sound, they actually became better at detecting other sounds. “For mice, who are prey animals, this ability to ignore the sounds they make probably allows them to better detect other sounds — like a cat who might be stalking them while they’re running over dried leaves,” Schneider wrote on Twitter.
This better explains what might be happening in the human brain when you stop hearing your footsteps in gravel or your keys jangling in your pocket. But it also opens up a new avenue for exploring learning and memory. As Schneider wrote, “… we think that studying these systems might help us learn a bit about how memories are formed and how they’re used to predict the future.”