The London cab driver is a curiously British institution.  If you want to become one you have to apply to the Public Carriage Office in London.  The process of getting a license mixes element from a Monty Python show and Franz Kafka.  First, you have to train yourself on “the knowledge”: a term more appropriate for what you get from a spiritual retreat but applied here in a very literal sense to the geography of the 25,000 or so streets that comprise London within a six-mile radius of Charing Cross. You have to memorise this information before being called for an examination, known almost as ominously as “the appearance”.  Most aspiring taxi drivers acquire “the knowledge” by poring over maps and taking bike rides through the streets, pausing to register every landmark, for they will not just be questioned on street directions but particular prominent buildings and landmarks.  It takes 3–4 years to learn “the knowledge” and only about 50 per cent pass their “appearance”. All this for the privilege of driving one of those quaint black taxis called hackney carriages with bench seats that face each other, not very much unlike the horse-drawn carriages of the 17th century!

It is reasonable to presume that all this cramming in preparation for “the appearance” would bring about some changes in the brains of these cab drivers.  However, can this change be seen in a brain scan?  That is exactly what scientists at University College London set out to find. They took eighty prospective cabbies who were training for the examination and scanned their brains before and after they took the test.  The findings were interesting.  The region of the brain known as the hippocampus that is associated with spatial localisation was enlarged in those taxi drivers who took the test and passed.  Taxi drivers who took the test and failed, a good 50 per cent of subjects, showed no such enlargement of the hippocampus.  These results were compelling enough to make the scientists conclude that the when the cabbies who succeeded in the test prepared for the examination over a 3 to 4-year period, the brain region responsible for storing and processing the new information enlarged as an adaptive response.

Interestingly this adaptation came at a cost.  The same cabbies who had this enlargement performed poorly in a complex memory task unrelated to the map of London, compared with control subjects with no hippocampal enlargement.  So, it would appear that when huge demands are placed on the map-remembering neurons in the brain, other neurons related to the acquisition of new and unrelated memories are disadvantaged.  Adaptive brain responses may be operating in a zero-sum environment where gaining one functional skill can result in diminished skills in a different function.

The London cabbies and their hackney carriages may soon be driven into oblivion by Uber and GPS (although, if Londoners had a say in this, the Queen would have to abdicate before they gave up their black taxi cabs).  However, they have provided our first real and prospective evidence in a controlled study the that the brain is plastic well into adulthood. It would appear that you can teach an old monkey new tricks, although the monkey may, in the process, forget some of the old tricks.