The brain is wired to recognize when a noise we’re expecting to hear doesn’t sound quite right. You go to close your car door, but don’t push it hard enough – you know you’ve made a mistake when you don’t hear the characteristic thump and click. Until now, scientists weren’t sure exactly how the brain managed this, but new research has found a population of brain cells that only have one job – to tell us when something sounds wrong.
Predicting the sound that should arise from a particular action is an ability that’s not unique to humans. Lots of other animals do it too, and previous research has shown that their brains can signal when a sound is not as it should be.
A team from New York University previously investigated this in a mouse model, and now sought to take their research to the next level by determining whether there are specific neurons whose job it is to respond to erroneous sounds.
First, they trained the mice to expect a particular sound when they pressed a lever. Once the association between the sound and the lever-press had become ingrained, the researchers started to vary the sounds the mice heard, mimicking situations humans face like closing the car door incorrectly.
Electrophysiological recordings identified a population of neurons in the auditory cortex that were inactive when all the sounds were as they should be. But, as soon as the sound did not match expectations, these “prediction-error neurons”, as the scientists termed them, fired up to signal that something was wrong.
Even more amazingly, different subgroups of these neurons worked to alert the brain not only to the fact that something was wrong, but also how it was wrong. For example, one set of neurons became active when the sound heard by the mice was quieter than it should be, while another only fired when the sound was late.
“When a movement makes an unexpected sound, it can violate our expectations in a lot of different ways,” said senior author David Schneider in a statement. “Different neurons are active when a movement makes too quiet a sound, and other neurons when the movement makes the wrong sound.”
Although this work is in mice for now, the discovery could have implications for the study of auditory learning in humans.
“Neurons like these might be vital in learning how to speak or how to play a musical instrument. Both of those behaviors involve lots of trial and error, lots of mistakes, and lots of learning from mistakes,” explained lead author Nicholas Audette.
Schneider added, “Do expert musicians have better prediction error neurons than novices? And in diseases in which speech is underdeveloped, are prediction error neurons malfunctioning?”
The brain relies a lot on expectation. Some have even referred to it as a “prediction machine”. It’s why, for example, we don’t notice the blind spot in our visual field – the brain simply fills in the blanks with what it expects to be there. Research has suggested that expectation can have a powerful effect on how we perceive the world and our interactions with it.
That being the case, it makes sense that the brain would have sophisticated mechanisms for detecting the unexpected.
“Brains are remarkable at detecting what’s happening in the world, but they are even better at telling you whether what happened was expected or not,” said Schneider. “We found that there are specific neurons in the brain that don't tell you what happened, but instead tell you what went wrong.”
The study is published in the Journal of Neuroscience.