In praise of the London Cabbie

On a recent trip to London, I travelled by Black cab from Kings Cross Station to a small place in Brixton. The journey took just over 30 minutes but crossed a network of large roads, roundabouts, one way streets and finally to my destination, a small street off a street off a street, so to speak. All of this was done in jig time with pinpoint accuracy and not an A to Z street map in sight. I marvelled at the cabbie’s ability not just to navigate the endless warren of roads but also to know the one-way system, exactly where road works were being conducted and yet still find the fastest and most direct route from start to finish. This task is seemingly repeated with ease for each passenger who steps into a Black Cab over the course of each day. It was then I recalled some talk about the test the London taxi driver has to pass to obtain the Black Cab license.

 Acquiring ‘The Knowledge’

The test is such an intellectual ordeal it is fondly known as The Knowledge. Incredibly, this test has been around since the mid-19th Century when Hansom Cabs were London’s vehicle of choice. Even since the first motor car was introduced in 1903 the test has not changed much at all. To this day it still requires that the driver memorise every street within a 6 mile radius of Charing Cross Station (all 25,000 of them) plus all 20,000 landmarks – museums, embassies, theatres, clubs, police stations and all 320 routes which connect them. 

This equates to every inch of the 113 square miles within a 6-mile radius of this ancient city.

Compare this to Manhattan with its 23 square miles and San Francisco at 47 square miles and the London Taxi drivers’ task seems mammoth. Indeed, passing “The Knowledge” involves successfully completing a series of on the spot tests to describe the “shortest run” between two random locations within this 6 mile radius. Many of the 320 routes detailed in the so called Blue Book Runs are unremarkable but all need to be memorised.  Not surprisingly only about 40 percent out of the 7000 annual applicants pass the test, taking an average of 3 to 4 years to do so. While they say no one can fail the Knowledge – they can just give up – perhaps we can now see why!

Natural talent or a well adapted Hippocampus?

All of this points to spectacular talents in spatial awareness and memory on behalf of the London Taxi driver or at least a well-adapted hippocampus; for this is the region most involved in spatial orientation and navigation. Shaped like a seahorse, it is named after the Greek word hippos or “horse” and kampos or “sea monster” and is nestled in the brain’s temporal lobes. This is one of the regions involved in the forming, organising and storing of memories, in particular the act of memory consolidation: where short term memories are converted to long term ones. While the link between damage to the hippocampus and amnesia has long been accepted, recent research has pointed to the regions involvement in episodic memory, imagination and even predicting the future. But what of the hippocampal involvement in navigation and spatial awareness?

The spectacular skills of the nutcracker 

Scientists intrigued by the concept of spatial memory looked at the caching behaviour of several animals including a species of bird charmingly called ‘Clarks Nutcracker’. Apparently these birds are marvellously adapted to open pinecones with their bills  and gather the protein-rich seeds in a pouch under their tongue. Flying often miles away from the gathering site, they stash the seeds an inch below the surface on south facing slopes where the snows melt early. This process they repeat over and over in the autumn burying up to 33,000 seeds in 2,500 separate locations.

In early spring, even with snow covering the ground they still manage to relocate their cache, retrieve their seeds and to feed their young. Scientists discovered that these food-storing birds have a much larger hippocampus than non-caching birds and moreover that this enlargement is seasonal, relating to when they cache and retrieve their food. Researchers believe that the neuronal plasticity of their brain has helped these birds to use landmarks and the relationships between them to hide their seeds, encoding the spatial location of each or creating a neuronal architecture involved in recollection of cache sites. It seems that the demands of survival have driven these birds to adapt and their hippocampus may have enlarged accordingly.

But what of the navigational abilities of humans?

Early literature hinted that hippocampal damage in humans may give rise to disturbances in navigation and spatial awareness. Neurological disease or damage in these cases,  however, often involved more than just the Hippocampal region and so confirming its role in the symptoms was difficult. It was the advent of functional brain imaging that provided the opportunity to explore cognitive processes in vivo in humans.

Taxi drivers compared to non drivers

Licensed London taxi drivers’ made a perfect cohort to examine due to their remarkable capacity to navigate. Researchers in University College London looked at a group of these taxi drivers and a group of matched controls who did not drive taxis. The structural MRIs showed that the posterior hippocampi of the taxi drivers were significantly larger than those of the controls. Indeed, the longer the taxi driver had been driving, the larger the volume of their right posterior hippocampus. Interestingly, their anterior hippocampus had correspondingly shrunk, perhaps as an accommodation to the posterior enlargement. One side-effect of this was that the ability to acquire new visuospatial information was compromised in the taxi driver group. The authors speculated that a greater posterior hippocampal volume might come at a cost to new spatial memories and gray matter volume in the anterior hippocampus. They concluded that:

“… although the hippocampus does not support navigation in isolation from other brain regions, it seems to be crucial to the storage and use of mental maps of our environments”.

Taxi drivers compared to Bus Drivers

Following on from this, the same researchers decided to compare taxi drivers and bus drivers. You may think that both taxi and bus drivers have similar experience in getting around London: they both drive full time, along similar routes and for a comparable number of hours each day. However, when researchers compared bus and taxi drivers brains, it was only the taxi drivers’ hippocampus which had enlarged, while the bus drivers’ remained unchanged.  Interestingly, they also looked at the brains of taxi drivers who had retired and observed a shrinkage of the region back to normal after they quit driving, which suggests a sort of use it or lose it phenomenon. While the lack of enlargement in bus drivers may seem surprising, the demands of their job differ from that of the taxi driver in one fundamental way. Bus drivers have to navigate along a predetermined route while the taxi driver must generate the new route in their brain each and every time.

Longitudinal study on Taxi Drivers

While the initial cross-sectional studies in the field were informative, it was impossible to establish a causal relationship with the observed hippocampal changes. For this, a longitudinal investigation of the same individuals before and after the intervention was required. So in order to establish whether taxi drivers’ skills were predetermined rather than acquired, a group of trainee taxi drivers were studied using structural MRIs before commencing the training and they were followed over four years. Researchers in University College London examined 79 male would-be taxi drivers prior to commencing the training for The Knowledge, and an equal number of matched controls. Prior to their training, no differences were discernible between the groups in the structure of their brain, IQ or memory. Over several years, those who passed the test, those who trained but did not pass and a control group were followed. On repeat testing the hippocampus of those who qualified had significantly increased in size, while there was no visible change in brain structure in either the control group or those who trained but did not pass The Knowledge.

The latter group are worth considering for a moment. None of those who dropped out (or otherwise didn’t pass the Knowledge) stated that the test was too onerous. Most blamed time commitments, financial demands and family obligations. It is interesting to consider how these individuals differed from the drivers who succeeded. Could the hippocampal plasticity express itself only in certain individuals? Are there intrinsic reasons why some trainees dropped out? What of the Hippocampal adaptation in the ubiquitous Uber drivers? With the use of Sat-Nav its easy to imagine their Hippocampus being less well developed!  As yet these questions remain unanswered but the researchers suggested that genetics may play a role.  They queried whether the trainees that qualified “may have had a genetic predisposition toward plasticity that the nonqualified individuals lacked“. Either way it seems that there is a capacity for local neuroplastic changes to occur in the brain structure of healthy adult humans in response to environmental demands.

Brain plasticity and the treatment of pain

So where does this all fit into our world of treating people with persistent pain? Could it be a useful way to illustrate the inherent plasticity of the brain and its ability to change all through adulthood? Can we use this tale to reassure patients that they can adapt in a good way to stresses or demands? It certainly is a lovely way to herald the adaptability of the brain.

So perhaps the next time you are in a London Black Cab, pause for a moment to think about the majesty of the brain and its neuronal plasticity. Consider that your driver has shown an amazing capacity to acquire and use, for navigation, a highly complex spatial representation of London to get you from A to B. Even marvel at the ability of the brain to endlessly adapt to the demands of the environment, but never forget that the brain does not navigate you: the person does! So doff your cap to the person behind the wheel and hail the London Cabbie!

– Blanaid Coveney

Blanaid is a practicing physiotherapist in Dublin, Ireland. Her professional interests include Pain, Epidemiology and all things brain related.

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