Last week my friend and Map Lab co-conspirator Betsy Mason wrote about her love of geologic maps. These maps have crazy colors and patterns that indicate different kinds of rocks, and you can learn a lot about the geological forces that created spectacular places like the Grand Canyon by studying them. Betsy got hooked on these maps when she was a grad student in geology.

I studied maps in grad school too. The ones I studied aren't nearly as nice to look at – I'll just concede that point right up front. But I am going to try to convince you they're even more amazing than Betsy's rock maps. You know why? Because they're made from living cells and they exist inside your brain.

Your brain actually has lots of maps. The brains of virtually all animals do. They use these maps to locate threats and opportunities and find their way through the world. Without brain maps, they wouldn't just be lost, they'd be somebody else's lunch.

>You need to pinpoint the sound and calculate a flight path that will put your talons in touch with your dinner. And you need to do it fast.

The brain maps you're most likely to have heard of before reside in the hippocampus, a part of the brain that's important for memory and navigation. Neuroscientists have found "place cells" and "grid cells" that encode locations in the hippocampus of rats. In humans, they’ve found that part of the hippocampus of London taxi drivers gets bigger as they learn the Knowledge required to navigate the city's labyrinth of streets. This is all fascinating stuff, but I'm going to save it for another post.

In this post I'm going to tell you about another type of brain map that gets less attention, but is every bit as important. While the hippocampus maps help animals (including cabbies and the rest of humanity) remember locations for minutes to years, the maps I worked on help animals pinpoint things that are happening RIGHT NOW.

I studied these maps in the brains of barn owls. Here is the thing about owls: they hunt at night, and they have to use their hearing as well as their vision to catch their prey. And they have maps in their brains that help them do this.

Pretend for a minute that you're a hungry barn owl up in a tree waiting for a tasty mouse to scurry by on the forest floor. You hear a rustle in the leaves. You need to pinpoint the sound and calculate a flight path that will put your talons in touch with your dinner. And you need to do it fast.

The owl's brain locates the source of the dinner mouse the same way your brain would locate a sudden noise in a dark alley as you're walking alone at night. The details are a little different, but the principles are the same.

First off, let's say the dinner mouse is off to the left a bit. That means the sound it makes is going to reach your left year a tiny fraction of a second before it reaches your right ear. No problem. Your brain is all over that: 25 microseconds delay between the ears; OK… dinner mouse must be 10 degrees to the left. You take off.

Sheribeari /Flickr

But of course the mouse is moving around, and now you are in midair and you have to keep adjusting your course.

How far up ahead is dinner mouse? Unfortunately, the time difference between your ears doesn't give you much help with that. But you have another trick. Because your ears are a little bit asymmetrical – your right ear canal is angled slightly upward and your left ear canal points slightly downward – sounds that come from above you (or from up ahead if you’re flying parallel to the ground looking down for dinner mouse) will be a few decibels bit louder in your right ear.

See how it works? The difference in timing between the ears gives you the left-right position, and the difference in loudness gives you the up-down position (or up and back if you're flying). The sound level and timing differences between the two ears are analogous to latitude and longitude in the owl's brain map of sound. Without this brain map, the owl would never catch its dinner mouse.

So where does the owl keep this map exactly? It's in a place called the optic tectum, which is a fleshy peanut-shaped nubbin protruding from the midbrain. It's not much to look at, but what it does is really cool.

Now imagine that you are a neuroscience grad student. It's a rough life, but it's better than being the owl in the scenario I'm about to describe.

You've just inserted a very thin electrode, about as thin as a human hair, into the optic tectum of a barn owl. The owl is anesthetized so he doesn't feel any pain. Your electrode is hooked up to a whole bunch of electronics, and when the neurons at the tip of your electrode fire, you can see a whole bunch of blips on a computer screen and hear a burst of noise on an audio monitor. If you don't do anything else, you'll hear a steady chatter of neurons firing. No big deal.

Let's say you have a small speaker you can move around and play bursts of sound at different locations in front of the owl. You move it all around, covering the entire hemisphere in front of the owl. For the most part, the neurons at the tip of your electrode don't respond. They just keep chattering along.

But then! When you play a sound from one particular location, let's say it's 8 degrees left, and 12 degrees up from the center of the owl's head, the neurons go crazy. You see a barrage of spikes on the computer screen and the audio monitor sounds like you've tuned into a firefight between two gangs with automatic weapons. You've found the auditory receptive field for these neurons. When a sound originates from this area of space, the neurons go nuts. When a sound comes from somewhere else, they don't care.

Congratulations! Another 12 hours of collecting data like this and you'll have a point to put on a graph, and you'll be that much closer to getting your Ph.D. Never mind that everyone else your age with half a brain is just finishing med school or making a fortune in tech while you're stuck in the lab again on a Saturday night. It's totally going to be worth it.

Imagine you're an owl standing in front of a translucent hemisphere. Flashes of light or sounds at the numbered locations map onto corresponding positions in your brain's optic tectum. (Non-artist's rendering: Greg Miller)

Now you move your electrode down, a bit deeper into the tectum. The receptive field of these neurons is a little different. It's still 8 degrees left, but now it's only 6 degrees up. A little farther down, and it's at 0 degrees. Then minus 6. And so on. Elevation is mapped from top to bottom in the owl's tectum, much as it is in actual space.

The horizontal dimension (or azimuth, more precisely) is mapped along the long axis of the tectum. Neurons at the end closest to the beak have receptive fields directly in front of the owl. As you move gradually towards the back, you find neurons with receptive fields increasingly off to the side.

You've just taken a whirlwind tour of the owl's map of auditory space. For any given point in space, there's a corresponding spot in the owl's tectum that's monitoring it, just waiting for something to happen there.

But that's just the start of the cool stuff that the tectum does.

Instead of moving a speaker around, let's say you darken the room and move a tiny light around. You'll find that neurons in the tectum have visual receptive fields as well as auditory ones. What's more, their visual and auditory receptive fields match up: A neuron that responds to a sound at 8 degrees left, 12 degrees up will also respond to a light there. The auditory and visual maps are aligned.

And that's not all. Let's say you flip a switch and deliver a tiny electric zap to this neuron. What happens next is both creepy and amazing. The owl's head moves 8 degrees left, and 12 degrees up.

So the tectum has not two but three maps that are overlaid and aligned with one another: auditory, visual, and motor, which is what neuroscientists say when they're talking about the parts of the brain that plan and execute movements. The optic tectum is the place where the neurons that do all the computations needed to locate a sound – comparing the timing and level difference between the two ears, for example – hand off the results of those calculations to the neurons that figure out which muscles have to contract, and by how much, to move the head to that exact location. Because this arrangement works so well, when the owl in the tree hears a sound or catches a glimpse of a mouse, it can respond in time to catch its dinner.

Other animals have similar brain maps, but the details differ in interesting ways. Pit vipers, for example, can detect infrared light, and their optic tectum contains an infrared map. In humans and other mammals, the analogous part of the brain moves the eyes, not the head (barn owls can't move their eyes independently of their head, so moving their head is the only option for shifting their gaze).

I should make it clear, just in case it's not, that I'm not the one who discovered all this. Or any of it. That happened long before I got to grad school, and everything I've described so far is just the background for the research I did.

I studied how other parts of the owl’s brain represent space and what happens during development. The map in an owl's optic tectum, for example, isn’t very accurate when the owl is born. It gets better as the owl grows up and interacts with the world around it. Those interactions refine the map, making tiny adjustments to account for things like individual differences in the size of the head and orientation of the ears. The result is a more accurate map that's customized for the individual.

I won't bore you with the details of all that. In fact, the details even bore me a little, which probably tells you something about why I left science to become a journalist.

Even so, the owl's auditory map still strikes me as an amazingly cool example of how circuits of neurons solve a real-world problem. It’s such an elegant mechanism. And it underlies a survival skill that’s essential for any animal: orienting to the world around it.