Cab drivers' grey matter enlarges and adapts to help them store a detailed mental map of the city, according to research.
Taxi drivers given brain scans by scientists at University College London had a larger hippocampus compared with other people. This part of the brain is associated with navigation in birds and animals.
The scientists also found that part of the hippocampus grew larger as the taxi drivers spent more time in the job.
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"There seems to be a definite relationship between the navigating they do as a taxi driver and the brain changes," said team leader Dr Eleanor Maguire.
"The hippocampus has changed its structure to accommodate their huge amount of navigating experience."
Evidence that the brain is able to change physically according to the way it is used could have important implications for people with brain damage or brain diseases such as Parkinson's, she added.
"It has long been thought that if there's damage to the brain there's only a limited amount of plasticity in an adult that can help them recover," Dr Maguire said.
"Now direct things in the environment, like navigation, appear to show changes in the brain. So we could in the future see some rehabilitation programmes that use that kind of knowledge."
Intensive training
The hippocampus is at the front of the brain and was examined in Magnetic Resonance Imaging (MRI) scans on 16 drivers of London cabs. The drivers had spent an average of two years in intensive training for "the Knowledge" - the vast number of street names and routes in the capital which they must learn to get their licence.
The tests found the only area of the taxi drivers' brains that was different from the 50 other "control" subjects was the left and right hippocampus.
"One particular region of the hippocampus, the posterior or back, was bigger in the taxi drivers," said Dr Maguire. "The front of the hippocampus was smaller in the taxi drivers compared to the controls.
Grey matter
"This is very interesting because we now see there can be structural changes in healthy human brains."
The posterior hippocampus was also more developed in taxi drivers who had been in the career for 40 years than in those who had been driving for a shorter period.
Dr Maguire's research is published in the US scientific journal, Proceedings of the National Academy of Sciences.
