Visitors to Japan often comment on the way taxi doors open as you approach — at the touch of a button by the driver; and that those drivers generally wear smart white gloves. I apologize for the competitive tone, but there is something far more remarkable about London taxis: their drivers.

To qualify as a London taxi-driver and be able to cruise the streets plying for hire in an iconic “black cab” (though other colors are now also allowed), trainees must embark on an epic quest, known in near-mystic terms as “the Knowledge,” in order to be awarded a coveted license.

This requires that trainees wishing to join the ranks of around 25,000 others currently authorized to helm a black cab in the capital’s 1,578-sq.-km Metropolitan Police District — many of whom own their vehicles and are self-employed — learn 25,000 streets by name, and their intensely tangled layout. As well, they have to commit to memory some 20,000 landmarks such as hospitals, hotels and theaters — and then remember how to get between any and all of those streets and landmarks using the shortest possible route.

The learning process generally takes three to four years, during which wannabe cabbies typically brave all weathers astride step-through scooters that they tack and jibe all over the place.

As well, there are classroom tests to be passed every month before the whole process culminates in a series of written and practical driving exams that only about half the trainees who embark on the Knowledge ever ultimately pass.

This changes their brains forever.

Scientists already knew that in London cabbies, a part of the brain involved in memory is bigger than in non-taxi-drivers. Now, a study conducted by Eleanor Maguire of University College London shows that the experience of “doing the Knowledge” actually changes the very structure of their brains, too.

Specifically, the study found there is more gray matter in the back part of a London cabby’s hippocampus compared to non-taxi-drivers — and less in the front compared to non-taxi-drivers.

For years it was thought that the adult brain was unmalleable, that it didn’t change after about the age of 20 or so. But Maguire’s work, along with other findings, provides added evidence that learning changes the adult brain — so offering hope after all to those trying to learn a language later in life, and also to those undergoing rehabilitation after a brain injury.

“The human brain remains ‘plastic’ even in adult life, allowing it to adapt when we learn new tasks,” says Maguire, whose report is published in the journal Current Biology (DOI reference 10.1016/j.cub.2011.11.018).

She and colleague Katherine Woollett wanted to know if the brain of a London taxi-driver might have changed in order to accommodate an internal “map” of London.

To find out, they followed a group of trainees — and a group of non-taxi-drivers as control subjects — capturing images of their brain structure over time and testing their memory.

At the start, study participants showed no differences in either brain structure or memory. But after three to four years, after the cabbies had completed the Knowledge, it was another story.

Then, Maguire and Woollett found an increase in gray matter in the back part of the hippocampus of those trainees who qualified as black-cab drivers. Changes were not observed in those trainees who failed to qualify, or in the non-taxi-driver controls.

Maguire says: “By following the trainee taxi-drivers over time as they acquired — or failed to acquire — the Knowledge, we have seen directly and within individuals how the structure of the hippocampus can change with external stimulation.”

Woollett and Maguire speculate that the findings may reflect a rise in the rate at which new neurons are generated and survive when faced with a significant cognitive challenge, noting that the hippocampus is one of the few brain areas where the birth of new neurons is known to occur. Successful training might also strengthen the connections between existing neurons.

As all that shows, our understanding of how this key part of the brain works is getting clearer all the time.

In Japan, for instance, Kaoru Inokuchi’s team at the Mitsubishi Kagaku Institute of Life Sciences, University of Toyama, have found that when rats learn new things, the new neurons that grow in the brain as a result cause other memories to decay. So for them, it’s a trade-off.

In the Japanese Nutcracker, by the way — not that ballet beloved at Christmas, but the bird (Hoshigarasu) — the hippocampus is larger than in other birds. That’s likely because a Nutcracker has to remember where it has buried hundreds of seeds and nuts.

London taxi-drivers have told experts a lot in other ways, too — and not just when a scientist sitting peacefully in the back has been shot their immortal line inviting conversation: “Oi, you’d never believe who I had in my cab last week . . .”

By scanning the brains of taxi-drivers while they played a video game simulating driving in London streets, researchers at University College London previously found that the hippocampus is active when the driver plans the route — but not during the driving of the route.

Then it only becomes active again if the destination is changed during the course of the journey.

But there is still a lot we don’t know — and likely a lot more those cabbies’ brains have yet to tell us.

We don’t know, for instance, whether those who succeed in becoming taxi-drivers have some inherent advantage over those who do not, Maguire adds.

“Could it be that those who qualified are genetically predisposed toward having a more adaptable, ‘plastic’ hippocampus? This leaves the perennial question of ‘nature versus nurture’ still open?” she notes for one.

Some new taxis in Tokyo have vibrating massage seats. Well, I can counter that too: Some of the older taxis still in service in London judder along like they’ve got no suspension and are driving across cobbles. (For any enthusiasts out there, the prime culprit is the Austin FX4 cab). Well, it’s a similar vibration to those Tokyo ones — if not nearly as relaxing.

And the white gloves? It’s a Shinto thing, and indicates that the cab is spotlessly clean and pure. For the duration of the drive the taxi is your personal, mobile shrine.

London taxis offer no such serene haven. But the drivers: even their brains are different.

Rowan Hooper (@rowhoop on Twitter) is the News Editor of New Scientist magazine. The second volume of Natural Selections columns translated into Japanese is published by Shinchosha at ¥1,500. The title is “Hito wa Ima mo Shinka Shiteru (The Evolving Human).”

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