Taxi drivers given brain scans by scientists at University College London had a larger hippocampus compared with other people. This is a part of the brain associated with navigation in birds and animals.

The scientists also found part of the hippocampus grew larger as the taxi drivers spent more time in the job.

I never noticed part of my brain growing - it makes you wonder what happened to the rest of it. David Cohen, taxi driver

"There seems to be a definite relationship between the navigating they do as a taxi driver and the brain changes," said Dr Eleanor Maguire, who led the research team.

She said: "The hippocampus has changed its structure to accommodate their huge amount of navigating experience."

The research confirms something which London's black-cab drivers have suspected for some time - learning their way around the capital is a brain-straining feat.

In order to drive a traditional black cab in London drivers have to gain "the knowledge" - an intimate acquaintance with the myriad of streets in a six-mile radius of Charing Cross.

Taxi driver: No ordinary brain

It can take around three years of hard training, and three-quarters of those who embark on the course drop out, according to Malcolm Linskey, manager of London taxi school Knowledge Point.

"There are 400 prescribed runs which you can be examined on but in reality, you can be asked to join any two points," he told BBC News Online.

Click here for your experiences of 'brainy' cabbies.

"Most people learn by visualisation but we do have a few tricks which we teach them, for example 'little apples grow quickly' gives you the order of the theatres on the north side of Shaftesbury Avenue: Lyric, Apollo, Gielgud, Queen's."

But 'the knowledge' is definitely worth learning - black cab drivers are self-employed can earn significantly more than minicab drivers. A black cab fare from Shepherd's Bush to Heathrow might cost £50, compared with £28 for a minicab.

Brain changes

The hippocampus is at the front of the brain and was examined in Magnetic Resonance Imaging (MRI) scans on 16 London cabbies.

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.

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Dr Maguire said: "One particular region of the hippocampus, the posterior or back, was bigger in the taxi drivers.

"The front of the hippocampus was smaller in the taxi drivers compared to the controls.

"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.

David Cohen from the London Cab Drivers' Club said he was surprised by the findings: "I never noticed part of my brain growing - it makes you wonder what happened to the rest of it.

"You do have to have a retentive memory but you also need a placid temperament to drive in London traffic," he added.

Parkinson's disease

The UCL researchers think 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 diseases such as Parkinson's.

Dr Maguire said: "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.

"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."

Dr Maguire's research is published in the US scientific journal, Proceedings of the National Academy of Sciences.

Click here to read about your experiences of 'brainy' cabbies.