The structure of a London taxi driver's brain changes during the gruelling process of learning the quickest way around the capital, scans reveal.
Dozens of trainee drivers had MRI scans before and after they acquired "The Knowledge", memorising hundreds of journeys and street names.
The University College London team, writing in Current Biology, found brain parts linked to memory grew bigger.
They said it proved the brain could adapt to new tasks, even in adulthood.
Earlier studies of the brain of the cabbie had already noted the increase in "grey matter" in the hippocampus, an area found at the base of the brain.
However this research tried to work out if the change had happened during the intensive learning period prior to starting work, or on the job itself.
They scanned a total of 79 trainees, just before they started to learn the "All-London" Knowledge, which can take between two and four years to complete.
Would-be taxi drivers have to learn 320 routes within a six mile radius of Charing Cross, which covers a mind-boggling 25,000 streets and 20,000 landmarks and places of interest.
Throughout the process, any changes to their brains were mapped by regular MRI scans.
Compared with similar scans from non-taxi drivers, those who had attempted the Knowledge had increased the size of the posterior hippocampus - the rear section of the hippocampus which lies at the front of the brain.
As would be expected, they were better at memory tasks involving London landmarks than the non-cabbies, but this advantage appeared to come at a price, as the non-cabbies outperformed them in other memory tasks, such as recalling complex visual information.
Prof Eleanor Maguire, who led the study, said: "The human brain remains 'plastic', even in adult life, allowing it to adapt when we learn new tasks.
"By following the trainee taxi drivers over time as they acquired - or failed to acquire - the Knowledge, a uniquely challenging spatial memory task, we have seen directly and within individuals how the structure of the hippocampus can change with external stimulation.
"This offers encouragement for adults who want to learn new skills later in life."
The reasons why the brain was able to adapt remain unclear, although the hippocampus is one of the few areas of the brain in which new cells can grow.
Dr John Williams, head of neuroscience and mental health at the Wellcome Trust, which helped fund the research, said: "Only a few studies have shown direct evidence for plasticity in the adult human brain related to vital functions such as memory, so this new work makes an important contribution."