How do London cabbies learn the Knowledge, and how does it change the brain?

The Knowledge is the training that prospective London taxi drivers undertake to learn the city's streets, landmarks and routes. National Geographic reports that it takes two to four years. A longitudinal brain imaging study followed trainees over about four years and found that those who qualified gained gray matter volume in the posterior hippocampus, a region linked to spatial memory. Trainees who failed and a control group showed no such change. [1] [2]

A review of neurocognitive studies describes two kinds of learning: map-based study and practical visits on the street. The core material is the Blue Book, a set of 320 origin-destination pairs. Students learn each run, and the surrounding quarter-miles, by visiting them many times, often on a scooter with a map of the run attached to the windscreen. These trips create memories that help later recall of street sequences, places of interest and traffic rules. [3]

On maps, students first use the so-called dumbbell method. They identify the quarter-miles at the start and end of a run and trace the connecting route, then add nearby points to build a network. They also pick subgoals called 50% markers, such as bridges where the river must be crossed. To memorize street sequences they use acronyms, mnemonics, short stories, mental walks through the rooms of an imaginary house, and historical or personal associations, often in combination. Trainees are not paid, so the process is costly as well as slow. During training the correct route is judged by shortest distance, not time, though qualified drivers say they later take time into account. [3]

The study scanned trainees at the start and again about four years later, alongside control participants. Among trainees who qualified, gray matter volume rose in the posterior hippocampi on both sides, with no significant change elsewhere in the brain. Trainees who did not qualify showed no significant change anywhere, including the hippocampi, and neither did the controls. The authors note that the unsuccessful trainees went through the same training environment, so the change cannot be put down to the training procedures or to general attempts to learn. [2] [4]

Earlier studies had compared experienced taxi drivers with other people and found a larger posterior hippocampus. Those studies could not show cause and effect. The longitudinal design, following the same people before and after, is what supports the conclusion that learning London's layout most likely produced the change. [2]

After training, qualified trainees were better than controls at judging spatial relations between London landmarks. They were worse at delayed recall, after 30 minutes, of a complex visual figure. BBC News also reported that non-taxi drivers outperformed the cabbies on some tasks. The researchers did not observe a decrease in anterior hippocampal volume, which earlier studies had suggested. They speculate that the posterior increase may come first and that an anterior decrease may follow, possibly already underway but not yet visible on MRI. This is a hypothesis, not a finding. [2] [5]