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We can tell which direction a sound is coming from using just two ears, but how do we do that? What would happen if our ears switched sides?

Hosted by: Brit Garner
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Sources:
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Sounds:
https://freesound.org/people/eguaus/sounds/331882/
https://freesound.org/people/Adam_N/sounds/349260/
https://freesound.org/people/DanielsonIII/sounds/68396/
https://freesound.org/people/mobaudio/sounds/384487/
[INTRO ♪].

If you’ve ever played Marco Polo, you know that it can be pretty tricky to locate sounds with your eyes closed. [crowded swimming pool sounds] But assuming you don’t swim violently into a wall trying to tag someone, you can do it. Your brain can figure out the difference between what each of your ears is hearing and use that to tell where a sound is coming from.

But how do we do it, and what would happen if our right ear suddenly switched places with the left? This might seem like a silly question, but it’s actually one that people study with a device called a pseudophone. The pseudophone was invented in the 1920s, and it makes it seem like a sound is coming from somewhere that it’s not.

It’s basically a couple of old-school ear trumpets with sound-proof tubing attached to them. The user puts an ear trumpet on either side of their head, and then runs the tubing from each trumpet to their other ear. More modern pseudophones electronically flip-flop the sound going to each ear in a pair of headphones, but… just savor that ear trumpet image for a second. [rumbling].

Generally, humans can adapt to most things— like when you suddenly step into bright sunlight, or when you get used to loud music at a concert. But one thing we can’t seem to adapt to is switching around our ears. This one researcher in the ‘20s, I kid you not, wandered the streets of Berlin for more than 80 hours while wearing a pseudophone.

He was trying to see if he could become habituated to it and train his brain to flip-flop the sounds back to normal. Turns out he couldn’t. With his eyes closed, he still pointed in the wrong direction when he heard sounds. [wooden board hitting floor] If anything, all that practice made him better at pointing to exactly 180 degrees opposite from where the sound was actually coming from.

All of this suggested to researchers that there really is something important about how and when each of our ears encounters a sound that allows us to tell where it’s coming from. See, under normal circumstances, we’re actually really good at localizing sound. We can determine the angle that a sound is coming from within one to two degrees.

We do it through a process called binaural sound localization, which means exactly what it sounds like:. We use both ears to pinpoint a sound’s location. We locate sound horizontally using the difference in intensity between what each ear is hearing and the tiny time delay between when one ear hears it and when the other one does.

So, for example, if headphones played a sound in both your ears at the same time, your brain would do the math and conclude that the sound was happening inside your head. To do the same thing vertically, we use the flappy external parts of our ears, which are called pinnae or auricles. The different shapes and contours of our auricles at different angles make sounds sound different.

Auricles also help us sort out the location of stuff that’s in the cone of confusion— and, yes, that is totally a technical term. It’s the region in front of and behind you where sound generally reaches each ear at the same time. With hearing, it’s generally accepted that we don’t really build out a map of 3D space on the eardrum or anything like that.

Instead, it’s really all about your brain constantly doing these calculations, most of which are done in a region called the superior olivary complex in the brain stem. Except… that’s probably not the whole story, either. For one, there’s a lot of evidence that moving the head and body changes our ability to figure out where sounds are coming from, too.

A 1980 pseudophone study and a 2018 blindfold study found that people were better at localizing sounds when they could move their head and walk around— although moving their head made them worse at it when they were wearing the pseudophone. This kind of hearing—where we think about a sound’s location in relation to ourselves— is called egocentric hearing. And a 2017 study from PLOS Biology suggested that there might be a difference between it and so-called allocentric hearing, where we think about a sound’s location more objectively.

It’s kind of the difference between thinking about your buzzing phone as being “on your left” or “over there on the table.” When the researchers recorded ferret brains, they found that some cells fired differently when the ferrets turned their heads to reflect the fact that the sound was now coming from somewhere else in relation to them. But other neurons didn’t change how they fired. This suggests that, if we’re anything like ferrets, we maybe do build out some sort of audio map of the world after all.

Studies have also shown that our sense of sight can also play a role in how we process where sound is coming from. And for people who are blind, it’s possible to use sound to build out a kind of map the way most seeing people do with sight. Other weird things can affect our sense of sound, too.

There are a couple of studies that suggest that for evolutionary reasons, “looming” sounds—you know, like when something [rumbling increasing in volume] big and loud is approaching you— are perceived differently. We’re likely to overestimate the intensity of sounds that are getting closer to us. And there’s actually a different pattern of neural activity for sounds that are getting closer as opposed to those that are moving away.

From a survival standpoint, that makes sense. But regardless, as always when it comes to our brains, things are more complicated than they appear. Except how silly people must’ve looked with an old-fashioned pseudophone on their head.

That seems pretty straightforward. Thanks for watching this episode of SciShow Psych! If you’d like to keep learning about the brain with us, you can go to youtube.com/scishowpsych and subscribe. [OUTRO ♪].