By Elizabeth Mitchell
Science reporter, BBC News
The knowledge: London cabbies are famous for knowing their way around
Scientists have uncovered evidence for an inbuilt "sat-nav" system in the brains of London taxi drivers.
They used magnetic scanners to explore the brain activity of taxi drivers as they navigated their way through a virtual simulation of London's streets.
Different brain regions were activated as they considered route options, spotted familiar landmarks or thought about their customers.
The research was presented at this week's BA Science Festival.
Earlier studies had shown that taxi drivers have a larger hippocampus - a region of the brain that plays an important role in navigation.
Their brains even "grow on the job" as they build up detailed information needed to find their way around London's labyrinth of streets - information famously referred to as "The Knowledge".
"We were keen to go beyond brain structure - and see what activity is going on inside the brains of taxi drivers while they are doing their job," said Dr Hugo Spiers from University College London.
The scientists used functional magnetic resonance imaging (fMRI) to obtain "minute by minute" brain images from 20 taxi drivers as they delivered customers to destinations on "virtual jobs".
The scientists adapted the Playstation2 game "Getaway" to bring the streets of London into the scanner.
After the scan - and without prior warning - the drivers watched a replay of their performance and reported what they had been thinking at each stage.
"We tried to peel out the common thoughts that taxi drivers tend to have as they drive through the city, and then tie them down to a particular time and place," said Dr Spiers.
The series of scans revealed a complex choreography of brain activity as the taxi drivers responded to different scenarios.
The hippocampus was only active when the taxi drivers initially planned their route, or if they had to completely change their destination during the course of the journey.
The scientists saw activity in a different brain region when the drivers came across an unexpected situation - for example, a blocked-off junction.
Another part of the brain helped taxi drivers to track how close they were to the endpoint of their journey; like a metal detector, its activity increased when they were closer to their goal.
Changes also occurred in brain regions that are important in social behaviour.
Taxi driving is not just about navigation: "Drivers do obsess occasionally about what their customers are thinking," said Dr Spiers.
Animals use a number of different mechanisms to navigate - the Sun's polarized light rays, the Earth's magnetic fields and the position of the stars.
This research provides new information about the specific roles of areas within the brains of expert human navigators.