Cabbies who acquire "the knowledge", a stern test involving in-depth mastery of London's streets, suffer changes to their brains that could make them blinkered, research has shown.
Those who pass the test develop more grey matter in the posterior hippocampus, the part of the brain they use to recall a mental map and calculate the shortest route from A to B.
But this comes at a price because the tens of thousands of streets and landmarks they need to remember "fill up" their brain to its maximum capacity, researchers said.
This only applies to the hippocampus, which is concerned with memory and spatial navigation, but it backs up previous research which shows taxi drivers struggle more than most drivers to adapt to changes in the road network or to driving in unfamiliar cities.
Dr Katherine Woollett, one of the co-authors of the study from the Wellcome Trust Centre for Neuroimaging at University College London, said: "The posterior hippocampus is at its full capacity, it cannot incorporate any more of this same type of knowledge because it is full."