As London taxi drivers in training are busy learning how to navigate the city's thousands of streets and places of interest over a period of years, the experience actually changes the very structure of their brains, according to a report published online in Current Biology, a Cell Press publication.

The findings add to evidence that learning changes the adult brain and should come as encouraging news for life-long learning and also rehabilitation after brain injury, says Eleanor Maguire of University College London.

Becoming a licensed London taxi driver is no easy feat. Trainees must acquire what's known as "the Knowledge," learning 25,000 streets and their very complicated layout as well as 20,000 landmarks. The learning process generally takes three to four years, culminating in a series of exams that only about half of trainees ultimately pass.

Maguire's earlier studies of London taxi drivers showed that they have more gray matter in the back part of a brain structure called the hippocampus compared to non-taxi drivers, and less in the front.

The hippocampus plays important roles in memory and spatial navigation. Those studies suggested that the brain might have changed in order to accommodate an internal "map" of London.

In the new study, Maguire and colleague Katherine Woollett directly examined this idea by following a group of trainee taxi drivers and non-taxi driver controls, 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. Three to four years later, it was another story: the researchers found an increase in gray matter in the back part of the hippocampus of those trainees who qualified as taxi drivers. Changes were not observed in those trainees who failed to qualify, or in the non-taxi driver controls.

"The human brain remains 'plastic' even in adult life, allowing it to adapt when we learn new tasks," says Maguire. "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 an increase 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.

It remains less clear whether those who succeeded at becoming taxi drivers had some inherent advantage over those who did not, Maguire adds. "Could it be that those who qualified are genetically predisposed towards having a more adaptable, 'plastic' hippocampus? This leaves the perennial question of 'nature versus nurture' still open."