What’s the difference between an Uber driver and a licensed driver of London’s venerable black taxi? (Besides price.) Surprisingly, the size of the hippocampus in their brains. To become an Uber driver, you need only a car and a smartphone GPS app. To become a London taxi driver on the other hand, you need a highly functional GPS inside your brain.
To obtain a London cabbie license, you must first pass the infamous taxi driver test. In the test, taxi drivers are given the starting and finishing points of imaginary taxi journeys across London and must describe the shortest routes on the spot in a series of oral exams. The test is known as “The Knowledge of London,” which was established in 1865 and has changed little since. The London road map is a chaotic mess of twists and turns, and looks about as orderly as a plate of boiled spaghetti. It takes wannabe cabbies 2-4 years to memorize the 25,000 streets and 20,000 landmarks within the test’s catchment area (that is, anything within a six-mile radius of Charing Cross train station). When drivers finally have the navigation skills to pass the test, the hippocampus in their brains is demonstrably bigger, according to a study from neuroscientist Eleanor Maguire’s research group at University College London.
The hippocampus is a seahorse-shaped structure near the center of the brain that is important for memory and cognition. As far as we know, it is the only region in the human brain that keeps growing into adulthood. Most neurons in the brain are generated before birth and are as old as you. In the hippocampus, on the other hand, new neurons develop throughout life.
The London cabbies’ hippocampus is an extreme example. In Maguire’s study, she and colleague Katherine Woollett followed two groups of subjects to see how hippocampal volume changes over time: Knowledge Boys – prospective London taxi drivers who were studying for The Knowledge course – and control subjects with no particular interest in the London map.
Three or four years later, the researchers checked in with both groups. Half of the Knowledge Boys had failed or dropped out (yes, the test really is that hard). Of the successful cabbies, failed-aspiring cabbies and the control, only qualified taxi drivers had noticeable hippocampal growth. The disappointed trainees, like control subjects, had the same amount of grey matter (the dark tissue consisting of neurons) in their hippocampus as before.
So why do taxi drivers have this super hippocampus? Back in the 1970s, scientists found that the hippocampus can act as the hidden GPS in our brains. American-British neuroscientist John O’Keefe studied how rats orient themselves in space. He inserted electrodes into rats’ hippocampus and recorded the neurons’ brief electric signals, indicating activity. O’Keefe put these rats in a T-shaped maze and watched them run, sit, eat, drink, groom, sniff, sleep, and relax. As the rats went about their business, he kept a close eye on neuronal activities recorded by the electrodes.
O’Keefe found that when a rat was at a specific location in the maze, no matter what they were doing there, specific cells in the hippocampus got excited. And when the rat moved to another location, other cells lit up. He called these cells “place cells,” because they signaled the places the rat was located, regardless of its behavior. These place cells tell rats where they are in a maze, and tell taxi drivers where they are on the maze-like London map. When complex navigation tasks get so complicated they take years to learn, a rat, or a taxi driver, demands more cells to support the extensive mind map. And that, Maguire speculates, is why qualified London cabbies’ hippocampus grow bigger.
More than 35 years after O’Keefe discovered place cells, a couple of Norwegian neuroscientists uncovered another player in the internal GPS system. Edvard and May-Britt Moser were studying the entorhinal cortex (entorhinal literally means “inside the nose”), a gateway that feeds information into and out of the hippocampus. The Mosers monitored neuronal activities in the entorhinal cortex while rats were running around in a big arena. They found a group of cells that became excited at certain locations.
Similar to place cells, these cells were space-sensitive. But instead of responding to just one location in the space, these cells lit up in multiple locations. When the researchers marked down all the locations one cell responded to, it created a grid overlaying the whole arena (image below). The Mosers named them “grid cells”. Scientists believe these grid cells in the entorhinal cortex lay out a coordinate system. The grid cells send that information to the place cells in the hippocampus which calculate the final location. The landmark discoveries of place cells and grid cells earned O’Keefe and the Mosers the Nobel Prize in 2014.
Pictured: Activity of a grid cell in rat (entorhinal cortex). Photo source: Torkel Hafting/Wikipedia Commons.
Navigating through physical space is not the only job of the hippocampus, though. Scientists propose the hippocampus supports a “mental map,” which not only stores maps of city routes, but also organizes other conceptual networks of information, like social networks.
To test this idea, neuroscientist Daniela Schiller and her team at New York University designed a role-play game. In the game, not unlike The Sims, subjects play a character who just moved to a new town. To find a job and a place to live, they must interact with different characters in the town and develop multiple relationships. As the game proceeds, the social structure between the player and the other characters will dynamically change. For example, upon arriving at the new town, you have no connection with anyone else. Everyone is a stranger. One day, you bump into an old friend from high school on the street, and she goes for a hug. You have two choices: “hug her for a long moment” (grow more affiliation), or “give her a brief hug” (less affiliation). Later, you find a job and your boss ask you to stay later for work. You can choose to respond “whatever you say, boss” (being more submissive) or “Can I get paid extra time?” (being more authoritative).
To see how the mind navigates these social functions, players’ brain activity was monitored by MRI during the game. When a character became more authoritative and more affiliated towards the player in the game, the player’s hippocampus became more active in real life. This suggests that the hippocampus tracks our position in a “social space,” like it does in a physical space.
In the end, it might not be a mere coincidence that we use phrases like “a close friend” or “a distant relative” to describe interpersonal relationships. These may be literal descriptions of the fundamental machinery in the brain. We need the hippocampus to navigate our way through a cocktail party, just as London taxi drivers strive to find the best rush hour route for their passengers.