Now, with the exam over, the researchers found an increase in grey matter -- the nerve cells in the brain where processing takes place -- in the back part of the hippocampus of the trainees who passed the test. Those that failed, or never learned, had no changes to their brain structure.

A famous neuroimaging study led by Eleanor Maguire at University College London revealed that London cab drivers who successfully passed "The Knowledge"—the rigorous test requiring them to memorize 25,000 streets—showed significant structural changes in their brains. Specifically, the posterior hippocampus, a region associated with spatial memory and navigation, was significantly larger and had a higher volume of gray matter compared to those who failed the exam or members of the general public. Interestingly, the study followed trainees over several years and found that while the successful candidates grew their hippocampi through the intense mental exercise of navigating the "mental map" of London, those who dropped out or failed showed no such structural growth. This research serves as a landmark example of neuroplasticity, proving that the adult human brain can physically rewire and enlarge itself in response to high-level cognitive demands and environmental stimulation over an extended period.