| When most of us walk into a grocery store, we can find our way around what is, in fact, a fairly complex maze. We navigate the aisles swiftly, swerving a cart around other shoppers, tight corners and piles of fruit. We can collect many items, check out and proceed to the exit without much thought.
We can do this because humans have an intrinsic navigation system that helps us move around without bumping into things, remember places we've been before, and locate ourselves and others in space. Only in the last few decades have scientists started to understand how this brain GPS works, propelled by two fundamental findings about the cells that store these maps.
The first came in the 1970s, when John O'Keefe, a neuroscientist at University College London, was recording the brain activity of rats freely moving about in a space. He observed that different subsets of neurons in the hippocampus, the part of the brain central to learning and memory, fired in response to the rats' specific locations. For example, if the rats were in the northeastern part of their space, one subset would fire; another subset was active when the rats were in the southwestern part. He concluded that the mammalian brain uses these so-called place cells to create a spatial map.
Then, in the early 2000s, May-Britt Moser and Edvard Moser, a then-married couple working together at the Norwegian University of Science and Technology, discovered "grid cells" in the entorhinal cortex, a region right next to the hippocampus. These cells form an internal positioning system. Grid cells don't fire for one specific location, but rather encode a coordinate system represented as a hexagonal grid in the brain. If an animal navigates to one of the six vertices of its coded hexagon, that grid cell will fire.
Taken together, place cells and grid cells explain how the mammalian brain creates a map of the world. In 2014, O'Keefe and the Mosers jointly won a Nobel Prize for their discoveries.
Of course, our internal navigation system isn't limited to grid and place cells. And new studies are finding that the same cells involved in mapping out space might also map out other kinds of information, such as time, abstract concepts and memories.
What's New and Noteworthy
When we navigate physical worlds, we're often navigating social ones, too. In a room there might be fixed objects, such as furniture, and ever-shifting social objects, as people walk in and out. By studying social fruit bats, Michael Yartsev, who studies the neural basis of behavior, and his team at the University of California, Berkeley found that place cells encode for both social and physical environments. The cells reacted not only to a bat's location, but also to the locations of other bats nearby, as Quanta reported in 2023. The cells also seemed to carry information about individual identities: The neurons fired differently if a bat flew near a friend than they did for an acquaintance.
As studies uncover more roles for place and grid cells, some scientists are pondering whether the brain maps other knowledge the way it does space. For instance, cells might structure conceptual spaces like physical ones, according to research Quanta covered in 2019. In the study, participants watched on a screen as a bird silhouette was stretched and compressed. As the participants navigated this "bird space," their cells fired in a hexagonal grid pattern just as if they were navigating a physical space. The findings suggested that the brain might represent many kinds of information in grids.
A few years ago, researchers discovered a limitation of these studies. Grid cells have usually been studied and mapped in two dimensions, as rodents scurry around a flat area. However, work Quanta reported in 2021 shows that the brain encodes 2D space differently than 3D space. The hexagonal pattern that grid cells typically form in 2D disappears in 3D, a pair of studies showed. In three dimensions, grid cell activity becomes disorderly and complicated — a structural breakdown that suggests that grid cells may play a wider role in processes such as memory that aren't strictly spatial.
"The brain has so many surprises for us," Ila Fiete, a computational neurobiologist at the Massachusetts Institute of Technology, told Quanta. "Here's this system that you kind of understand, and it's orderly — and the brain just throws a curveball." |
