You probably could walk around your home with your eyes closed and still manage to navigate from, say, the kitchen to the bedroom with relative ease. But how? How do our brains know how to navigate from one place to another? That’s the subject of decades’ worth of research pioneered by the newest Nobel Prize winners: John O’Keefe, May-Britt Moser, and Edvard I. Moser.
In the early 1970s, O’Keefe discovered what he called “place cells,” neurons in the hippocampus that represent a single spot in our surroundings — sort of like a drop pin for your brain. Decades passed without significant additional findings, until 2005, when the Mosers (a husband-wife team in Norway) identified “grid cells,” which work with the place cells to act kind of like a navigational system in our brains.
Place cells, grid cells, the brain’s “navigational system” — fascinating? Yes. Completely confusing? Also yes. To understand more about this area of research, Science of Us spoke with Joshua Jacobs, who teaches biomedical engineering at Drexel University. As a researcher, Jacobs has explored this neural network in humans, and he helped explain some of the neuro-gobbldygook.
What exactly are place cells?
First, let’s talk a little about the way most of these studies are done. So researchers insert a microelectrode into the hippocampus of a rat, which is done through surgery. Once the animal has recovered from the surgery, then you let it run around in a room, and you’re keeping track of its location in an environment. So you’re recording exactly where the rat is, and meanwhile you’re measuring the activity of different brain cells.
And so a place cell is a neuron that is active only when the rat is at one particular spot in the environment. For all other spots in the environment, it’s quiet. But at that one spot in the environment, it’s very active. So it’s sending a signal for that particular place in the environment — hence the name, “place cells.”
Wait, so — how many of these do we have? Do we need one for every place where we’ve ever been or will ever go?
That’s actually a thing that theoretical neuroscientists think a lot about. And that’s an open question: How many place cells do we have, and does every place cell really represent a new, unique location? Or do some cells represent multiple locations, and we just haven’t looked at that in that detail yet, because that’s very hard to do? These are all very important questions. But right now, the best understanding is that a place cell represents one location in a given environment.
So if you look around your office, you have one cell that represents where you are sitting at your desk, another cell representing the area near the door, another representing the chair in the corner, that sort of thing.
What about grid cells? How are they different?
Place cells only fire at one location in your environment. Grid cells fire periodically as you move about your environment, and when we record that brain activity, a triangular pattern forms.
In relation to place cells, grid cells kind of act like the coordinates. In the same way you can specify where you are with a GPS, with longitude and latitude, this triangular grid makes a different kind of coordinate system that signals your location to your brain.
I’ve been reading a lot of analogies that call this network the brain’s “inner GPS,” and you’ve just alluded to it, too. So is that a pretty accurate metaphor?
I’ve been thinking about that. I mean, on one level, the metaphor is dead-on — these are cells that tell you where you are in an environment. And each place cell tells you about one location — they say, yes, you are at this location, or, no, you’re not at that location. And grid cells tell you about where you are in an environment in a different way — like the coordinate system I mentioned.
But the metaphor is not perfect. With GPS, there’s one coordinate used for the entire world — you know, latitude and longitude. That tells you where you are in the entire world. But a place or grid cell only tells you where you are in one particular environment — usually in a room, is the way they usually study it.
It’s possible that there are worldwide coordinate systems in the brain, somewhere, but we don’t really have any real evidence of that yet.
So building on all of this, I’m curious about what all of this might say about our sense of direction. If you have a really bad sense of direction, for example, could that mean that your place and grid cells are somehow out of whack?
When navigation goes wrong, there is reason to think that grid and place cells have been disrupted. So that’s exactly right. I don’t have any direct evidence that a good navigator, versus a bad one, might have especially good grid and place cells, but that’s not an unreasonable jump to make.
And that’s a reason why this research is so important — these cells are important for understanding why navigation goes wrong sometimes. Because if you disrupt people’s grid and place cells, it does screw up their navigation. For example, Alzheimer’s patients have damaged hippocampi — which is where these cells are found — and they also have navigational difficulties. So this is very good, but indirect, evidence of why these networks are important.
Are there other major reasons why this research is important?
Yes, there’s one other major reason. Not only are these cells important for navigation, they’re also important for memory. Specifically, place cells are involved in representing memory for spatial locations. Now, this is all a newer line of research, but there’s reason to think that when you remember a location, that those particular place cells activate.
Let’s take the example of, Where did I leave my car in the parking lot? It may be that the place cells that represent the location of the car in my environment, that those are the cells that “turn on” when I’m trying to think of where I left my car.
What are the next steps for this research?
I mean, what would be great, right, is to have some kind of brain recording system that seeks out which cell tells you the particular spot where I left my car, and to stimulate that cell so you can remember where you parked. We’re not at that point yet.
But we are at the point where we’re understanding the network in a more general way. Eventually, this could translate to treatments for Alzheimer’s patients, or others with spatial memory problems, to enhance their navigational networks, and make sure those cells are operating properly.
That’s the direction this is going in, but that’s far off. But overall, the grid and place cells are part of the work that could one day lead to those findings.