Navigating a Taxi Driver's Brain

Qualified London taxi drivers know their way around over 25,000 streets in the capital. And, if you scan their brains, you find that the structure called the hippocampus, which contains a mental map of the world around us, is much bigger than it is in the average non-taxi driver. But was it bigger to begin with, or did learning London like the backs of their hands trigger the cabbies brains to change? Now, UCL's Eleanor Maguire thinks she knows...

Eleanor -   Animals who do a lot of navigating often have a bigger hippocampus than animals of the same species who don't engage in much navigation.  So we wondered if the same would be true of the human brain and whether those who navigated a lot would also have a bigger hippocampus than those who didn't navigate so much.  And so, about 11 years ago, I studied this using Magnetic Resonance Imaging on some London taxi drivers and we indeed found that they had greater grey matter volume in part of their hippocampus than people who didn't navigate so much.

Chris -   I guess one problem though or one criticism of that is that it's purely observational in the sense that you look at this group, they're taxi drivers.  Do they have a very big hippocampus because they're taxi drivers or do they have a job as a taxi driver because they have a very big hippocampus which means they're endowed with the very good map in their head.

Eleanor -   Yes and that's a very important point and in fact, that's one of the prime observations of the current study, was to try to see if we could document within specific individuals the change that might occur in the structure of the hippocampus, purely as a consequence of acquiring this very detailed mental map of London.

Chris -   So how did you actually do it?

Eleanor -   Well what we did was with the cooperation of the Public Carriage Office, we recruited trainees who were just starting there and training as London taxi drivers and we scanned their brains and we tested their memory.  And then off they went to try to acquire the knowledge and this takes about 4 years on average, 3 to 4 years.  And so, when people had qualified, we invited them back and we scanned their brains again and tested their memories again.  And we were able to, in the first instance look at people before they started and see if there was anything in their brain or their behaviour that could predict who would eventually qualify because the interesting thing is only 50% of the trainees actually went on to qualify.  It's an extremely tough thing trying to become a taxi driver in London.

Chris -   The odds are slightly better in medicine.  Did they give you any reason why they dropped out, the ones that did drop out because I mean, there may have been perfectly sound reasons other than cognitive ones?

Eleanor -   Absolutely and I think it's difficult to know.  It's probably quite a heterogeneous group in the sense that some people probably did find it very tough going and they just didn't have the navigational skills to pursue this, but it's also the case that embarking on this training can be time consuming, can take time away from your family, it's a big financial commitment, and in the current climate undoubtedly, some individuals had to withdraw as a consequence of those sort of issues.  So it's not easy to know exactly why people dropped out and sometimes people can say they dropped out for one reason but maybe it was another reason and so on. So it is quite a mixed group, but we did end up with a group that didn't qualify, a group that did qualify and then of course, we had control participants who didn't engage in any training at all but still, we scanned at the start and at the end of the study, just like the trainees.

Chris -   Probably, the most important question is, those people who you scanned at baseline and then they became qualified taxi drivers, did you see any differences in their brains?

Eleanor -   We did.  For those who qualified, we found that between the start and the finish of the study, the back part of their hippocampus had increased in volume and no other part of the brain had changed, just very specifically this back part of the hippocampus which is what we found in our previous sort of observational studies where we compared taxi drivers to non-taxi drivers.  So it was fully in line with our previous results.

Chris -   And the controls, they didn't show any changes?

Eleanor -   No, the controls and those who didn't qualify, their brains remained exactly the same from start to the finish of the study.

Chris -   Now what about other measures of cognition because you said you also tested their memories in other ways? So rather than just looking at the structure of the brain, you also looked at function. What were the differences then before and after?

Eleanor -   Well obviously, the first thing we did was we tested people's general intellectual ability just to make sure that there were no differences in that regard.  So the IQs for example of the individuals were all very similar.  We then tested their basic knowledge of London in terms of understanding spatial relationships between landmarks in London.  And then we did a whole range of other memory tests that looked at their ability to remember verbal material, words or pictures or other types of spatial information.  So we did those tests at the start and then we did parallel versions of those tests at the end of the study.

Chris -   And how did the results of the before and after compare amongst all the groups?

Eleanor -   Well we found that the controls didn't change and we found that the trainees, particularly the qualified trainees became much better in terms of their knowledge of London and the proximity of landmarks to each other which of course you'd expect because they were trying to actually learn that information.  But what was most interesting was that on other tests of spatial memory, those who qualified actually performed worse at the end of the study than they did at the start.  And this is something we found previously in our studies of taxi drivers that although they are experts in terms of navigating around London, perhaps there's a little bit of a price to pay for that expertise in that they become a little bit worse at dealing with information of other kinds.  And that kind of makes sense you know, somethings got to give when you're taking in a lot of information.

Chris -   And of course, you're left with another problem which perhaps you'll answer in another 10 years which is, those people that didn't drop out and did show this change, is there something special about them and that their brain is more adaptable, it can incorporate new cells, make more grey matter when they need to do a task like this, compared with people who find it less easy?

Eleanor -   Yes, I think that is another important question and so, we must consider the reasons for why people fail to qualify.  It may be that there are genetic predispositions to hippocampal plasticity in the individuals who qualified, allowing them to expand their knowledge and so, expand the volume of their posterior, the back part of their hippocampus.  And there may be other individual differences that come into play as well.  So I think this is a very important issue because what we all want to know is given any individual, what can they hope to achieve?  How much can they learn?  What capacity does their memory and their hippocampus have?  So I think it's going to be very important to understand these individual differences in future studies.

Chris -   That's was Eleanor Maguire from the University College London and she published that work this week with her colleague Katherine Woollett in this month's Current Biology.