A cognitive map featuring that level of detail, as you might imagine, requires a fair amount of storage space, and, sure enough, University College London neuroscientist Eleanor Maguire found that the back part of the hippocampus in London taxi drivers is enlarged compared with that of the general population. The longer they’ve been driving, the bigger the gap. Maguire also found, though, that the front part of the hippocampus gets correspondingly smaller. “So there is a price to pay for their expertise,” she says. This difference showed up in tests of visuo-spatial memory, including one in which the drivers were asked to memorize the position of sixteen objects on a table, then put them back in place after they’d been removed. “They were incredibly poor at doing that,” she says. While it isn’t yet clear whether this happens because the requirements of storing a map of London take over other parts of the brain or because of some other process, what these studies do make clear is the brain’s plasticity: its very structure is shaped by the demands we place on it.