How your brain knows where you are

Most animals, including humans, need to be able to navigate around their environment to find food and water, to return to shelter and learn where there is danger. This means that our brains are astonishingly adept at working out where we are. Kate Jeffrey is a neuroscientist at UCL and she spoke to Georgia Mills about how our brains can pull this off... 

Kate - Well a very important thing when you're walking round a familiar environment is memory. So you've been there before, for example you come out of the underground tube station and there's the street. The first thing you brain has to do is recall, based on what you can see now, its map of the last time you were there and that map has an orientation to it, so a compass direction which you've learned about as well and then you have to decide where you want to go. So somewhere in that map is a goal so, so for example, if you want to go to the pub or the post office or something. So you have to try and recall in your memory where that goal is and then plan a path from one to the other.

Georgia - And what form does this map take in the brain?

Kate - Well, like all forms of knowledge, it's fundamentally activity of neurons. And we think that some of the neurons that are particularly important for space are these things called space cells, which are found in the hippocampus which is a little part of the brain, very deep in the part of the brain behind the ears - the temporal lobe. These neurons become very active when you go to a particular place. So each neuron has it's own place where it becomes active and, we think, what these neurons are doing is signalling that you're at a particular place. And often it's a place that you've been to before, in which case it will be the same neuron that was active the last time you were there.

Georgia - Okay. So, for example, when I go into my kitchen, would my kitchen neuron light up?

Kate - That's right, although that's not just one neuron - its many. So as you're walking around your kitchen, these neurons will become active one after the other. The map is spread among hundreds of thousands of neurons - it's not just one. But the puzzle now is a neuron being active might tell you where you are, but how do you represent where you want to be? So is there another map or is the same map and, if so, what does the activity of the place where you want to be look like? So we haven't solved that one yet - it's still a mystery.  

Georgia - As well as these place cells, which light up when you're in a familiar area, are even more mysterious neurons which can help you navigate around any space, and they're called grid cells!

Kate - Grid cells are neurons that are found in the entorhinal cortex, which is the part of the brain that sends most of the cortical projections to the hippocampus from the sense organs and so on, and these neurons were investigated because of the this projection. So it was thought that if we look at what was sending to the place cells, we might get some insights as to how the place cells know where the rat is, or the human.

And it was discovered that these neurons are also active when the rat goes into specific places. But it's not just one place for these cells, it's multiple places, and these multiple places are evenly spaced and regularly ranged. The neurons themselves are randomly scattered throughout the entorhinal cortex, but the patches of activity are equally spaced. It's hard to imagine quite what that means if you haven't seen a neuron but, when the rat goes into a particular place, a grid cell will be active. If the rat walks 30 cm north, let's say, that cell will become active again. The rat walks another 30cm north and that cells becomes active again. And if you plot out all of the places this neuron likes to fire, you find it makes this really regular pattern. So it's a very extraordinary discovery.

It's a type of thing that we would never have predicted in a million years but, once it was discovered, it immediately all fell into place - aha, this how the place cells know how far the rat has walked. It's using these grid cell, and these grid cells are switching on and off, and on and off, and on and off as the rat walks and that's how the place cells know how far the rats gone. So now we're trying to understand exactly all of that works.

Georgia - Yeah, that's incredible. I'm assuming it doesn't matter how fast or slow the rat is going, they seem to be able to know the distance. Do we have any idea how these neurons are lighting up so consistently?

Kate - Not yet. There are lots of theories and you're quite right, it's independent of how fast or slowly the rats walking, so the cells are able to interpret the sensory information that's coming in based on how fast they know the rat's going. So, somewhere in the brain is some neurons that are calculating how fast the rat's going, so the hunt is on now to find out where those are. They may be in the entorhinal cortex; there's some evidence for that, but then the question becomes well, how do they know how fast the rat's walking? And there's a whole lot of ways that might be: from the visual world and the speed with which it moves past the eyes, from the movements the rat's making, how fast the legs are moving, and so on, and so on. So a lot of information is being integrated to make these calculations. 

Georgia - Our brains, it turns out, can layer grids over our environment while we move - pretty impressive. But, with all our reliance on our smartphones and satnavs to get us around, will our navigation centres, our hippocampuses, shrivel away and die.

Kate - That's a very good question. Is it really use it or lose it? So I think it's probably too soon to tell the answer to that. My hunch is probably not, because the hippocampus is very busy. You're using it all of the time just to walk around your house, even though you don't need your phone and GPS or anything. You still needing your hippocampus to know where you are in your house; how do I get from my bedroom to my bathroom and so on, so it's always busy?

Maybe it's not as busy these days as it would have been in olden times when we were navigating through the forests. That's quite possibly true but, on the other hand, we stimulate our brains in a lot of other ways with our watching television, and our virtually reality games, and so on. So I think we're not going to become a species of decrepit, brain damaged people unable to function. I think we'll adapt; we always adapt and technology always ends up enriching our lives, I think.