The Brain Maps Out Ideas and Memories Like Spaces

In the parable of the blind men and the elephant, each paid attention to a different aspect of the creature. The brain may do something similar by mapping out the qualities of perceptions, experiences and abstract concepts along various dimensions, with the help of the same system that it uses to map out physical spaces. Credit: Alexandre Tamisier for Quanta Magazine. We humans have always experienced an odd — and oddly deep — connection between the mental worlds and physical worlds we inhabit, especially when it comes to memory. We’re good at remembering landmarks and settings, and if we give our memories a location for context, hanging on to them becomes easier. To remember long speeches, ancient Greek and Roman orators imagined wandering through “memory palaces” full of reminders. Modern memory contest champions still use that technique to “place” long lists of numbers, names and other pieces of information. As the philosopher Immanuel Kant put it, the concept of space serves as the organizing principle by which we perceive and interpret the world, even in abstract ways. “Our language is riddled with spatial metaphors for reasoning, and for memory in general,” said Kim Stachenfeld, a neuroscientist at the British artificial intelligence company DeepMind. In the past few decades, research has shown that for at least two of our faculties, memory and navigation, those metaphors may have a physical basis in the brain. A small seahorse-shaped structure, the hippocampus, is essential to both those functions, and evidence has started to suggest that the same coding scheme — a grid-based form of representation — may underlie them. Recent insights have prompted some researchers to propose that this same coding scheme can help us navigate other kinds of information, including sights, sounds and abstract concepts. The most ambitious suggestions even venture that these grid codes could be the key to understanding how the brain processes all details of general knowledge, perception and memory. The Amnesiac and the Hexagons On September 1, 1953, Henry Molaison, a 27-year-old man the world would come to know as “Patient H.M.,” went under the knife in a risky, experimental bid to cure a debilitating case of epilepsy. A neurosurgeon removed the hippocampus and surrounding tissues from deep within H.M.’s brain, alleviating some of his seizures but inadvertently leaving him a permanent amnesiac. Until his death more than half a century later, H.M. couldn’t encode new memories: not what he’d had for breakfast, nor the most recent news headline, nor the identity of the stranger he’d been introduced to just a few minutes earlier. H.M.’s story, though tragic, revolutionized scientists’ understanding of the role the hippocampus plays in how the brain organizes memory. Years later, another hippocampus-centered revolution transpired and earned its pioneers a Nobel Prize: the discoveries, decades apart, of two types of cells, which made it clear that the hippocampal region’s fundamental functions included not just memory but also navigation and the representation of two-dimensional spaces. The first of these came in 1971, when researchers uncovered “place cells,” which essentially fire to indicate one’s current location. John O’Keefe, a neuroscientist at University College London, and his colleagues monitored the brain activity of freely roaming rats and observed that some of their neurons fired only when they were in specific parts of their cages. Some became active as a rat sniffed around, say, its enclosure’s northeast corner, but otherwise remained quiet; others fired in the cage’s center. That is, the cells encoded a sense of place (“you are here”) — and together, they created a map of the entire space. (When the rat was put in a different cage or room, these place cells “remapped,” encoding different local positions.) These findings inspired the proposal that the hippocampus might be creating and storing “cognitive maps” (an idea first put forth by psychologist Edward Tolman in the 1940s to explain how rats could suss out new shortcuts to rewards in mazes) beyond spatial ones. At the very least, the hippocampus seemed like a promising place to start looking for hints of such maps. That work eventually led a then-married pair of scientists at the Norwegian University of Science and Technology, May-Britt Moser and Edvard Moser, to direct their attention to the entorhinal cortex, located just next door to the hippocampus. The region provides major inputs to the hippocampus — and is also one of the first areas of the brain to deteriorate in Alzheimer’s disease, which affects both navigation and memory. There, the researchers found what they called grid cells, which experts now think may be the most compelling candidate for cognitive mapmaker. Unlike the place cells, grid cells do not represent particular locations. Instead, they form a coordinate system that’s independent of location. (As a result, they’re popularly known as the brain’s GPS.) Each grid cell fires at regularly spaced positions, which form a hexagonal pattern. Imagine the floor of your bedroom is tiled with regular hexagons, all the same size, and each hexagon is divided into six equilateral triangles. As you walk across the room, one of your grid cells fires every time you reach a vertex of any of those triangles. Different sets of grid cells form different grids: grids with larger or smaller hexagons, grids oriented in other directions, grids offset from one another. Together, the grid cells map every spatial position in an environment, and any particular location is represented by a unique combination of grid cells’ firing patterns. The single point where various grids overlap tells the brain where the body must be. This kind of grid network, or code, constructs a more intrinsic sense of space than the place cells do. While place cells provide a good means of navigating where there are landmarks and other meaningful locations to provide spatial information, grid cells provide a good means of navigating in the absence of such external cues. In fact, researchers think that grid cells are responsible for what’s known as path integration, the process by which a person can keep track of where she is in space — how far she has traveled from some starting point, and in which direction — while, say, blindfolded.