LONDON - A curious sight peculiar to London is that of men zooming around on mopeds, apparently doing nothing other than slamming the brakes on to crane their necks at ill-placed street signs before ticking off an item on a list attached to a piece of hardboard on the handlebars. These men (they are almost all men) are engrossed in the task of what is known colloquially as "doing the knowledge" - learning the map of London and its suburbs in sufficient detail to pass the difficult examination that will allow them to register as London taxi drivers.

Anyone who has taken a registered "Hackney carriage" in London, as the cabs are quaintly known, will have been impressed by the skill with which their drivers negotiate their way through the highways and byways of the city, always knowing an alternate route to avoid jams and never getting lost. The reason is the test that they have to pass, which involves giving the best route between two places - which may not even be well-known landmarks.

Taking in and maintaining such a large and detailed mental map apparently causes part of the brain to grow. A team of neurologists at the Institute of Neurology at University College London has reported that the posterior hippocampi of licensed London taxi drivers are significantly larger than those of controls who do not drive taxis.

Eleanor Maguire, of the Wellcome Department of Cognitive Neurology at the Institute of Neurology in London, said it was a widely held view that if the brain of an adult was damaged, the amount it could grow to compensate for the damage - its plasticity - was limited. She said, "Now it appears that that there is a capacity for the healthy adult human brain to change in size in response to environmental demands. It is possible that, in the future, rehabilitation programs could use this information." No one knows, she said, whether other regions of the brain may be able to experience similar plasticity in response to environmental stimuli.

The hippocampus, which is an evolutionarily ancient part of the brain, is a swelling in the floor of the lateral ventricle. It is known to be involved in the ability to form new memories, and is the target for many experimental drugs aimed at enhancing learning and memory, particularly in Alzheimer's disease.

The study by Maguire, together with colleagues from the same department and from the Institute of Child Health at University College London, provides further clues to its function. They report their data in a paper in the April 11, 2000, issue of Proceedings of the National Academy of Sciences, titled "Navigation-related structural change in the hippocampi of taxi drivers."

Maguire and her colleagues decided to find out whether it was possible to detect morphological changes in the healthy human brain associated with extensive experience of navigation. They studied a group of 16 right-handed male licensed London taxi drivers with a mean age of 44 years, all of whom had been in the job for more than 18 months. The average time that this group had been "on the Knowledge" was two years, with some training full-time and some part-time. The researchers also assembled a control group of 50 healthy, right-handed males who did not drive taxis.

When the researchers measured the brains of the two groups, they found that the right and the left hippocampi of the taxi drivers were significantly larger than those of the controls. There were no differences throughout the rest of the brain. In controls, however, there was relatively more gray matter on both sides in the anterior hippocampi than in the taxi driver group.

Maguire and her colleagues conclude in their PNAS paper that their results suggest "professional dependence on navigational skills in licensed London taxi drivers is associated with a relative redistribution of gray matter in the hippocampus." As this begged the question of whether people with this particular arrangement of hippocampal gray matter might be predisposed to take up careers involving navigational skills, they decided to examine the correlation between volume and amount of time spent as a taxi driver. The team found that right hippocampal volume correlated with the amount of time spent as a taxi driver, positively in the posterior and negatively in the anterior hippocampus. They write: "We believe that these data suggest that the changes in hippocampal gray matter - at least on the right - are acquired. As such, this finding indicates the possibility of local plasticity in the structure of the healthy adult human brain as a function of increasing exposure to an environmental stimulus."

Left hippocampal volume did not correlate with subjects' number of years of taxi-driving, however. This suggests, the researchers write, "that the left hippocampus participates in spatial navigation and memory in a different way from the right hippocampus. We may speculate that the left hippocampus complements its partner by storing memories of people and events that occur in the rich context of taxi driving in the real world, where an over-arching framework - such as integrating information into an existing map - is not required."

So it may be the left hippocampus that the taxi driver kicks into action when leaning back and bending his fare's ear with his unwanted opinion about everything from the problems posed by refugees to the inadequacies of the English cricket team. Such conversations are renowned for starting with "I had that General Pinochet/James Watson/Hillary Clinton [insert the name of any famous person to get the idea] in the back of my cab once." Hence the headline on the news story reporting this work which appeared in The Independent newspaper in mid-March: "Taxi drivers' secret: 'I had all them brain cells in the back of my head.'"

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