In Montreal, we have two seasons: winter and construction. And winter is nearly over. As road closures make our winding one-way streets even more difficult to navigate, it’s sometimes hard to imagine the days before apps like Google Maps helped recalculate your route. But if your iPhone is dead, it’s useful to remember: you’ve got a built-in backup for getting where you need to go. It’s called a cognitive map, and it is encoded deep inside your brain.
Long before we carried computers in our pockets, scientists were curious about how our brains process our surroundings in order to tell us where we are, and how to get to where we want to go. The concept of the cognitive map, a mental representation of our physical environment, was introduced by Edward Tolman, a professor at UC Berkeley, in 1948. He suggested that this allows us to make flexible decisions about navigating our environment, which is why it’s sometimes referred to as our “internal GPS.” When we encounter an obstacle, like streets closed for construction, our brains refer to our cognitive map to quickly adapt an alternate route on the fly. But new research published in Science suggests that how the brain represents distance within our cognitive map is more malleable than anyone previously realized.
In the early 1970s, John O’Keefe, a neuroscientist at the University College London, began recording the electrical activity of neurons in a region of the brain called the hippocampus. These recordings consist of a single neuron’s action potentials, the electrical signals that neurons use to communicate. (They sound like a series of popsof varying frequency — imagine making popcorn, or the finale of a fireworks display.) By using this technique to essentially eavesdrop on the hippocampus, O’Keefe discovered that some of the cells were pretty much silent most of the time.
But as rats moved through a maze, these quiet cells started to fire rapidly, each apparently linked to a specific geographical place. O’Keefe found that each of these many cells was only active in a separate location in the environment, which he called a place field. He hypothesized the place fields of these many neurons would represent every part of the animal’s environment; working together, they construct the animal’s cognitive map.
The next step was to establish how these place cells knew where and when to fire: O’Keefe still wanted to learn how the brain uses information from the world to establish and orient the cognitve map. In 1996, O’Keefe was briefly joined in his lab by May-Britt and Edvard Moser, then a pair of graduate students interested in learning how to make recordings of neurons from the hippocampus. The Mosers would soon return to Norway to begin their own laboratory. It was there, in 2005, that they discovered the medial entorhinal cortex — an area that is thought to send information to the hippocampus — also contained cells that seemed to encode spatial information.
Unlike place cells, which fire only in one specific place, each of the cells the Mosers discovered fired in multiple places. When the researchers increased the size of the rats’ arena, a pattern emerged: These new cells fired in a regular, hexagonal pattern as rats moved through the entire enclosure. The lattice-like firing pattern of these neurons led the Mosers to name them grid cells, and the Mosers thought that they might be providing the data that helped place cells determine when to fire as the rat moved into its place field in the cognitive map. (O’Keefe and the Mosers were jointly awarded the Nobel prize in physiology or medicine in 2014 for their discoveries.)
Initially, the scientists believed that grid cells’ rigid firing pattern was essentially an internal coordinate system, giving the cells the ability to assign a neural representation of any environment an animal might encounter —essentially providing the cognitive map its latitude and longitude. That’s in part because grid cells were originally only recorded in rats tested in standard, symmetrical environments like a large square or circular arena. But recently, Juilja Krupic, a researcher working with O’Keefe, found that irregular environments, like those we encounter in the real world, can actually change the way grid cells fire.
When a rat is moved from a square enclosure into a trapezoidal one, for example, the irregular shape of the environment effects how the rat’s grid cells fire, especially when the animal is close to the non-square walls. When the animal returns to squared intersections, the grid cells go back to a fixed, hexagonal firing pattern. This suggests that, in contrast to what was previously believed, the firing pattern of grid cells is actually malleable and responds to changes in the environment.
These new results also indicate that the information encoded by grid cells may be more complicated than a one-size-fits-all system used to assign distances. In turn, this raises questions about the relationship between place and grid cells. In fact, cells that respond to environmental borders like maze walls (aptly named ‘border cells’) may be playing a previously underappreciated, but important, role in controlling how grid cells fire.
These new rat studies provide insight at a cellular level about how we are able to navigate the world without Google Maps, but O’Keefe’s lab has also since made the translational jump into human research. An engineer, for example, removed all the guns and enemies from the first-person shooter video game Duke Nukem, creating a virtual environment for human test subjects to navigate while their brain activity was being monitored using magnetic resonance imaging (MRI). The results show the hippocampus — the same region of the brain that generates place signals in rats — is also active while human subjects navigate this virtual environment.
To move these findings out of video games and into the real world, Eleanor Maguire, a colleague of O’Keefe’s, studied the hippocampi of London taxi drivers, who need to pass a rigorous exam memorizing more than 25,000 streets in order to get their license. Maguire found that the prospective drivers’ hippocampi actually grow as they learn to navigate the streets of London. This supports the idea that, like rodents, our hippocampi are involved in constructing and maintaining our cognitive map, both of which can change as we learn about our environment.
The hippocampus is much more than just a part of our “internal GPS” — it is also intricately linked with how we form and store memories. (The hippocampus consolidates declarative memories, like facts or personal experiences, for long term memory storage.) Importantly, the hippocampus is also one of the first regions to be affected by neurodegenerative diseases. So researchers hope that continuing to learn how neurons — including grid cells, place cells, and border cells — interact to process spatial information may aid in the early detection of neurodegenerative disease.
The more we learn about this part of the brain, the more we’ll understand about how our brains process and respond to the ever-changing environment around us.