Brain structure snapshots reveal science behind why only half of London taxi trainees pass the Knowledge test:

The scientists followed a group of 79 trainee drivers and a “control group” of 31 non-taxi drivers. Magnetic Imaging Resonance (MRI) scans were used to take brain structure “snapshots” of the volunteers, who were also given certain memory tasks.

http://www.mirror.co.uk/news/top-stories/2011/12/08/brain-structure-snapshots-reveal-science-behind-why-only-half-of-london-taxi-trainees-pass-the-knowledge-test-115875-23620070/

The 39 trainees who qualified had greater volumes of grey matter in their posterior hippocampus than the 40 who failed.