To drive a licensed black cab in London, an applicant must pass a test known simply as the Knowledge. It is one of the most demanding rote-memorization examinations in the world. Candidates must memorize roughly 25,000 streets within a six-mile radius of Charing Cross station, along with thousands of landmarks, hotels, restaurants, theatres, hospitals, and government buildings. They must then be able to plan the optimal route between any two named points in real time, without consulting a map.

Most candidates spend three to four years studying. They ride mopeds around the city, working through standard "runs." Roughly half of those who attempt the exam eventually pass.

In 2000, the neuroscientist Eleanor Maguire at University College London published a study using magnetic resonance imaging to compare the brains of qualified London cab drivers to a control group of comparable adults who had not done the training. The cabbies had a measurably larger posterior hippocampus. The hippocampus is the brain region most associated with spatial memory and navigation.

The result, published in the Proceedings of the National Academy of Sciences, was significant for two reasons. It suggested that intensive spatial training could literally reshape adult brain structure. And it directly challenged a long-standing assumption that the gross anatomy of the adult human brain was essentially fixed.

Follow-up studies tracked candidates through training. The hippocampal change appeared in those who passed and was absent in those who dropped out. Other work found that the change partially reversed after cab drivers retired and stopped using the mental map daily.

The London cabbies became one of the clearest pieces of evidence for what neuroscientists now call structural neuroplasticity in the adult brain.