Amazingly, our brain tells us when something sounds right or wrong –such as if the refrigerator door is closed properly, a soccer ball is hit in an unintended direction or a piece of music is off-key. But how does it accomplish this?
A team of neuroscientists at New York University has recently uncovered how the brain works to make distinctions between “right” and “wrong” sounds, research that provides a deeper understanding of how we learn complex audio-motor tasks like speaking or playing music – and also what goes wrong in neural disorders such as schizophrenia.
“We listen to the sounds our movements produce to determine whether or not we made a mistake,” said Prof. David Schneider at NYU’s Center for Neural Science and senior author of the paper, which appears in Current Biology under the title “Precise movement-based predictions in the mouse auditory cortex.”
“This is most obvious for a musician or when speaking, but our brains are actually doing this all the time, such as when a golfer listens for the sound of her club making contact with the ball,” he said.
“Our brains are always registering whether a sound matches or deviates from expectations. In our study, we discovered that the brain is able to make precise predictions about when a sound is supposed to happen and what it should sound like.”
The researchers focused their work on better understanding everyday phenomena. We know what a car door closing should sound like because we have shut them countless times, but when we leave the seatbelt in the door jamb of the car and try to shut it, we hear something different – a “clank” rather than a “thump.”
It’s the same when a musician hears a note that fits the melody rather than one that disrupts it.
Schneider and his colleagues studied the neurological activity of mice when they performed tasks that are like humans closing a car door. The scientists trained mice to push a lever with their paws and played a tone every time the lever reached a particular position. Eventually, the mice learned exactly how the lever was supposed to sound.
If the researchers removed the sound, played the wrong sound, or played the correct sound at the wrong time, the mice adjusted their behavior, just as humans would do if a car door did something unexpected.
The scientists recorded the mice’s brain activity during these behaviors: specifically, how the neurons responded in the auditory cortex, one of the brain’s “hearing centers.” These neurons were only minimally active when a mouse pushed a lever and heard the expected sound, but if the researchers changed the sound to the wrong one or even shifted the timing of the sound slightly, these neurons responded vigorously.
“The auditory cortex seems to signal not what was heard, but whether what was heard matched or violated its expectations,” observed Nicholas Audette, the lead author on the study and a postdoctoral fellow in the Schneider lab.
Does every sound break through barriers?
The researchers also found that if they omitted the sound altogether –similar to not shutting a door hard enough – they observed a select group of neurons become active at the time the sound should have happened. “Because these were some of the same neurons that would have been active if the sound had actually been played, it was as if the brain was recalling a memory of the sound that it thought it was going to hear,” Schneider explained.
In addition to its role in predicting self-generated sounds during everyday behaviors, the same brain circuitry that the team members are studying is thought to malfunction in diseases such as schizophrenia, leading to the perception of “phantom voices” that aren’t actually there.
They hope that by understanding these circuits in the healthy brain, they can begin to understand what might go wrong in diseased ones.