The evidence that adult brains could grow new neurons was a game-changer, and has spawned all manner of products to try and stimulate such neurogenesis, to help fight back against age-related cognitive decline and even dementia. An important study in the evidence for the role of experience and training in growing new neurons was Maguire’s celebrated study of London taxi drivers, back in 2000.
The small study, involving 16 male, right-handed taxi drivers with an average experience of 14.3 years (range 1.5 to 42 years), found that the taxi drivers had significantly more grey matter (neurons) in the posterior hippocampus than matched controls, while the controls showed relatively more grey matter in the anterior hippocampus. Overall, these balanced out, so that the volume of the hippocampus as a whole wasn’t different for the two groups. The volume in the right posterior hippocampus correlated with the amount of experience the driver had (the correlation remained after age was accounted for).
The posterior hippocampus is preferentially involved in spatial navigation. The fact that only the right posterior hippocampus showed an experience-linked increase suggests that the right and left posterior hippocampi are involved in spatial navigation in different ways. The decrease in anterior volume suggests that the need to store increasingly detailed spatial maps brings about a reorganization of the hippocampus.
But (although the experience-related correlation is certainly indicative) it could be that those who manage to become licensed taxi drivers in London are those who have some innate advantage, evidenced in a more developed posterior hippocampus. Only around half of those who go through the strenuous training program succeed in qualifying — London taxi drivers are unique in the world for being required to pass through a lengthy training period and pass stringent exams, demonstrating their knowledge of London’s 25,000 streets and their idiosyncratic layout, plus 20,000 landmarks.
In this new study, Maguire and her colleague made a more direct test of this question. 79 trainee taxi drivers and 31 controls took cognitive tests and had their brains scanned at two time points: at the beginning of training, and 3-4 years later. Of the 79 would-be taxi drivers, only 39 qualified, giving the researchers three groups to compare.
There were no differences in cognitive performance or brain scans between the three groups at time 1 (before training). At time 2 however, when the trainees had either passed the test or failed to acquire the Knowledge, those trainees that qualified had significantly more gray matter in the posterior hippocampus than they had had previously. There was no change in those who failed to qualify or in the controls.
Unsurprisingly, both qualified and non-qualified trainees were significantly better at judging the spatial relations between London landmarks than the control group. However, qualified trainees – but not the trainees who failed to qualify – were worse than the other groups at recalling a complex visual figure after 30 minutes (see here for an example of such a figure). Such a finding replicates previous findings of London taxi drivers. In other words, their improvement in spatial memory as it pertains to London seems to have come at a cost.
Interestingly, there was no detectable difference in the structure of the anterior hippocampus, suggesting that these changes develop later, in response to changes in the posterior hippocampus. However, the poorer performance on the complex figure test may be an early sign of changes in the anterior hippocampus that are not yet measurable in a MRI.
The ‘Knowledge’, as it is known, provides a lovely real-world example of expertise. Unlike most other examples of expertise development (e.g. music, chess), it is largely unaffected by childhood experience (there may be some London taxi drivers who began deliberately working on their knowledge of London streets in childhood, but it is surely not common!); it is developed through a training program over a limited time period common to all participants; and its participants are of average IQ and education (average school-leaving age was around 16.7 years for all groups; average verbal IQ was around or just below 100).
So what underlies this development of the posterior hippocampus? If the qualified and non-qualified trainees were comparable in education and IQ, what determined whether a trainee would ‘build up’ his hippocampus and pass the exams? The obvious answer is hard work / dedication, and this is borne out by the fact that, although the two groups were similar in the length of their training period, those who qualified spent significantly more time training every week (an average of 34.5 hours a week vs 16.7 hours). Those who qualified also attended far more tests (an average of 15.6 vs 2.6).
While neurogenesis is probably involved in this growth within the posterior hippocampus, it is also possible that growth reflects increases in the number of connections, or in the number of glia. Most probably (I think), all are involved.
There are two important points to take away from this study. One is its clear demonstration that training can produce measurable changes in a brain region. The other is the indication that this development may come at the expense of other regions (and functions).