Our brains pay less attention to aspects of the world that appear to remain the same. We need this mental relief to experience life without being overwhelmed, but it can keep us from noticing when patterns change.
The world is a very big, very complicated place, and your brain will take any chance it can get to simplify it. If the brain picks up on a regular pattern, it will stop looking at it as a sound in its own right and more like a natural part of the background. University of Wisconsin-Madison psychologist Keith Kluender explains:
"In perception, whether visual or auditory, sensory input has a lot of structure to it. Your brain takes advantage of the fact that the world is predictable, and pays less attention to parts it can predict."
In general, this is a good thing, but it can sometimes make us deaf to sounds our brains don't expect. To demonstrate just how powerful this effect can be, Kluender and his colleagues created an orderly set of new sounds that were somewhere between a tenor saxophone and a French horn. The noises were also on a spectrum from abrupt like a plucked string to gradual like a bowed string. They then played these noises in the background while the test subjects played with Etch-Sketches for about seven minutes.
Here's where it gets a bit weird. The subjects were asked to identify which of three sounds was different from the others. If the odd sound was different in the same way the background noises had been - say, it varied in its instrument and the length of the noise - then they had no problem. But if the different sound didn't fit the pattern - like, say, it didn't have enough saxophone in the noise - the test subjects were lost, incapable of telling the difference between the sounds.
If the subjects hadn't spent the last several minutes establishing a pattern of sounds, they likely wouldn't have had as much trouble. Without the newly built-in expectations, they would have just evaluated the sounds in isolation and, in all likelihood, figured out the odd one out. But their brains had taken those noises for granted, and so they couldn't do it. It's mental efficiency gone overboard.
This finding fits well with previous theoretical descriptions of how an efficient brain should work, although this is the first time it's been demonstrated this clearly in people. Most of the time, our ability to predict noises is very useful, as fellow researcher Christian Stilp explains:
"The world around us isn't random. If you have an efficient system, you should take advantage of that in the way you perceive the world around you. That's never been demonstrated this clearly with people."
Indeed, despite the subjects' inability to identify the sound, this effect is far more helpful than not, as Kluender explains with a rather nefarious example:
"That's part of why people can understand speech even in really terrible conditions. You can press your ear to the wall in a cheap apartment and make out a conversation going on next door even though the wall removes two-thirds of the acoustic information. From just small pieces of sounds, your brain can predict the rest."