It’s Monday morning and your about to head out to catch your bus for work. You’re in a bit of a rush because, when you were getting dressed, you couldn’t find a two matching socks and you had to dig through the clean laundry still sitting in your dryer to find a matching pair. As you approach the door, you reflexively pat your pockets, checking for everything you need to bring with you. Keys? Check. Wallet? Check. Cell phone? Your phone is not there. Your heart starts racing and a drop of sweat starts forming below your hairline. “Where did I put it last night??” You only have a few minutes to catch the bus, or else you’re going to be late for your morning meeting. You rustle through your things. It’s not in your nightstand, on the bathroom counter, or on your coffee table. A feeling of hopelessness starts creeping in, but then you suddenly remember you can locate your phone using an app on your computer.
You race to your computer and open the app. The dot on the map sits squarely over your apartment, so you know your phone is there somewhere. You’re half way there. But the GPS tracker can’t tell you where in your apartment your phone is. So you send a signal to your phone to make it beep.
You hear a faint ringing sound coming from somewhere in your apartment. You stand up. It rings again. You turn your head towards where the sound is coming from — your bedroom. Another beep. You scurry into your bedroom and follow the beeps until you find your cell phone sitting on the floor, under a pile of clothes. Victory!
Your memory failed you that day, and so did most of your sensory systems that help you find things in your surroundings. What helped you find your phone? Your brain’s uncanny ability to localize the source of sounds.
We’ve discussed this phenomenon briefly before, but its just an important part of our daily lives that we had to revisit it.
How Did Your Brain Locate your Cell Phone?
Having an ear on both sides of your head does more than just balance out your face; their positions relative to each other helped you locate the source of your cell phone ringer. Since your ears are separated from each other, sounds will reach your ears at slightly different times and at slightly different volumes, and this helps your brain calculate where sounds are coming from.
For instance, if someone is talking into your left year, that ear will pick up the sound of their voice before your right ear does, and your left ear will also find the sound a tiny bit louder than your right ear will. Your brain can detect these minute differences in timing and volume and use this information to calculate that the sound of the person’s voice is coming from the left.
Your brain calculates this information in your brain stem, an area that sits right above your neck. Specifically, the calculator consists of a group of cells a group of cells called the superior olive (because it looks like an olive). This region contains a two-dimensional map of the world that radiates from your head, as if the map were drawn on the brim of a hat.
When a sound hits your hears, the signals from each one travel directly to the superior olive and meet up. Let’s pretend that instead of sounds, they were cars for a second. If the two cars are going the speed limit and leave at the same time, they’ll meet in the middle. But if one car gets a head start, it will pass the midpoint before it sees the other car. If you really wanted to, you could measure the spot between where the cars were supposed to meet and where they actually did and calculate how much earlier one car started driving before the other. Your superior olive does this with sound.
If a sound comes from your left side, the signal from your left ear will get a head start and meet up with the signal from your right ear closer to the right side of your superior olive. Conversely, if the sound comes from the right, the signals from your two ears will meet towards the left side of your superior olive. If a sound comes from straight ahead, the two signals will meet up in the middle. Based on where the signals meet, they will excite a particular set of neurons in your superior olive that tell the conscious part of your brain where the sound originated. And voilà, you found your phone!
The Best Sound Locators in the World
Owls are nocturnal, so they hunt at night, typically in complete darkness. As a result, unlike most animals, they cannot rely on their eyes to find prey. Furthermore, they don’t use heat or smell to detect prey either – they rely entirely on their sense of hearing. With living prey that flies and jumps all over the place, barn owls’ sound localization abilities need to be spot on, or else they’ll miss their dinner (literally).
Barn owls are better at localizing sounds than humans for several reasons. First, their ears are asymmetrical. While our ears sit in the same spot on either side of our heads, a barn owl’s left ear is slightly higher than their right ear and it points downwards, while the right ear points upwards. This makes it easier for barn owls to not only localize sounds in a horizontal direction (called azimuth) like our superior olive does, but also in a vertical direction (called elevation).
How Humans Detect Elevation
Just because we humans aren’t as proficient in detecting differences in elevation than barn owls, we can still do it. We just use a different method. After all, that’s why you were able to tell your phone was on the floor and not on the floor in the room above you.
We humans detect elevation using our outer ears – the part you can see, which is called the pinna. When high-pitch sounds reach your outer ear, they bounce around the curves in your pinnae for a while, like a basketball bouncing on a rim, and eventually make it into your middle ear (the “net,” where your ear drum sits). This by itself doesn’t give your brain enough information to calculate the elevation of a sound, however.
That’s because the real world is a bit more complicated than a single shooter with a basketball. There are more players on the court. If several people shot a basketball at a net at the same time, they’ll all bounce on the rim in different ways and might bump into, or interfere, with each other, and if they make it to the net, they’ll reach it at different times. Like basketballs, sound waves are bouncing all over the place, and they interfere with each other, too. In this case, certain high-pitch sound waves interfere with lower-pitch ones, causing the high-pitch waves to drop in volume.
Sounds approaching your ears will hit your ears at different angles depending on their elevation, which will affect the amount of interference. And this, in turn, will affect the volume your ear drum senses. Your brain picks up these volume changes and works backwards to determine what elevation these sounds are coming from.
We’re approaching the edge of knowledge on this, and no one knows exactly how this process works yet. Indeed, its a complicated, but that just goes to show how incredibly powerful our brains are. As our brains our healthy, we don’t have to think about localizing sounds – it just happens!
The Challenge of Sound Localization
Compared to depending on our sense of hearing to locate objects, it seems easy to do that using our senses of vision and touch. But don’t take your sense of hearing for granted. The next time you hear someone say “WATCH OUT!!” and you instinctively turn to them, see them pointing to a baseball approaching your head, and you duck out of the way, don’t thank your reflexes – thank your brain, which helped you find the source of that voice that ultimately helped you avoid a clonk to the head.