Using MRI brain scans, researchers in Britain discovered that cab drivers had increased the grey matter in their posterior hippocampus after over three years of intensive training to memorize London's numerous street routes.

First implemented in 1865, the "Knowledge of London" Examination System (also known as "The Knowledge") requires cab drivers to memorize 320 standard routes and points of interest covering around 25,000 streets. This is done in order to show that London cab drivers know the city like the back of their hand, without needing to refer to any map or satellite navigation system.

Cab drivers need to pass the qualification exams based on The Knowledge in order to get their professional license. It's been reported that twelve attempts are usually made before passing the test.

"By following the trainee taxi drivers over time as they acquired... the Knowledge, a uniquely challenging spatial memory task, we have seen directly and within individuals how the structure of the hippocampus can change with external stimulation," said Professor Eleanor Maguire of University College London, one of the study's authors, as quoted by LiveScience. "This offers encouragement for adults who want to learn new skills later in life," she added.

The hippocampus, which plays a vital role in memory formation and retention, is the region responsible for spatial navigation.

Although the cab drivers did develop more grey matter in their posterior hippocampus, their anterior hippocampus was significantly reduced as a result. This affected their ability to process visual information, revealed the study published in Current Biology.

"What is not clear is whether those trainees who became fully-fledged taxi drivers had some biological advantage over those who failed. Could it be, for example, that they have a genetic predisposition towards having a more adaptable, 'plastic' brain? In other words, the perennial question of 'nature versus nurture' is still open," further explained Maguire.

Participants in the study included seventy-nine cab driver trainees and a control group composed of thirty-one non-cab drivers. The MRI scans were taken before the trainees began studying for The Knowledge and right before they took their qualification exams.