The first such evidence involved London cabdrivers, who turned out to have larger hippocampi than their non-cabdriver contemporaries. British neuroscientists reported a finding that, even a few years earlier, would have been dismissed as a neurological impossibility. They scanned the brains of 16 London taxi drivers and compared them with the brains of 50 control subjects. The taxi drivers, who in the course of their work develop an intricate mental map of their huge metropolis, had larger posterior hippocampi. Furthermore, the greater the number of years spent on the job, the larger the hippocampi were in individual drivers.
The hippocampi, of course, are presumed to be critical for learning and memory, including spatial memory, and anyone in a position to memorize multiple routes in the huge metropolis that London is must be straining his or her hippocampi to the utmost.
A lifelong denizen of large cities, I am nonetheless stymied by the sheer size and irregularity of the British capital. Whether this finding could be replicated in New York is very much in doubt. In London, a cabdriver cannot receive his license until he can pass a test identifying nearly every street in the city. This is certainly not the case in New York, where even a basic command of the English language doesn’t seem to be a prerequisite. The London cabdriver findings may very well have been the first direct demonstration of a relationship between the size of a macroscopic brain structure and the environmental factors contributing to its use.
As in any correlational observation, the direction of causation between hippocampal size and cab driving as a vocation is ambiguous. Unlike their New York counterparts, the London cabdrivers are supposed to undergo a very rigorous test and are true professionals. One might say that only those with the oversized hippocampi make the cut.
But this argument is undermined by another correlation — between the hippocampal size and the number of years on the job. In other words, the longer a person was employed as a cabdriver, the larger the hippocampi were. This seems to clarify the direction of causation: The hippocampi appear to increase in size as a function of time spent driving a cab on the streets of London.
This finding is particularly striking if one considers the fact that more years on the job usually implies a more advanced age, which in turn would predict a certain decrease in size as a result of age-related brain shrinkage. But it appears that the effect of hippocampal stimulation by job demands overrides the effects of aging.
Additional findings of a similar kind were not long in coming. One of them concerns the angular gyrus, a cortical region on the juncture of the parietal, temporal and occipital lobes. In right-handed people, the left angular gyrus is among the most important substrates of language, particularly of the ability to process complex relations.
Scientists from the Wellcome Department of Imaging Neuroscience at the Institute of Neurology in London have shown that in bilingual individuals the left angual gyrus contains more gray matter than in people in command of only one language, and the underlying white matter is denser. This means that there are more neurons and/or more connections in the language area of the people who mastered more than one language.
This was true both in early bilinguals (people who acquired the second language early in life) and in late bilinguals (people who acquired the second language later in life). Again, one might argue that the direction of causation is in question, but bilingualism is usually a function of personal circumstances (such as resettlement from one country to another) or an accident of birth (being born in a multilingual environment), rather than a function of aptitude-based selection. To the extent that the angular gyrus findings are indeed a reflection of experience-driven plasticity, it appears that these affects are not limited to very young people but are present also at later stages of life.
Yet another study involves Heschl’s gyrus (also known as the traverse gyrus), a region of the temporal lobe important for processing acoustic stimuli. It turns out that Heschl’s gyrus is larger in professional musicians than in musically untrained people — in fact, twice as large. As with the London cabdrivers, there is a positive relationship between the size of Heschl’s gyrus and the amount of time spent practicing an instrument. This latter correlation argues strongly in favor of practice being the cause and Heschl’s gyrus size the consequence.
So it appears that cognitive activity promotes neural tissue growth, and that such neuroplastic effects can be quite targeted and specific: Certain kinds of mental activity are related to morphological changes in those parts of the brain most directly responsible for supporting them. In the three examples just discussed — London cabdrivers, bilinguals, and professional musicians — the morphometric findings reflected the effects of very long-term cognitive activities of particular kinds, measured in years and perhaps even in decades.
But the effects of neuroplasticity can manifest themselves within a much more compressed time frame, as a juggler study has shown. A group of volunteers without any prior juggling experience were trained in a three-ball juggling routine for three months, until they reached a certain level of proficiency. MRI scans taken before and after the training period were compared, and a considerable increase in the amount of gray matter was found in the temporal lobes in both hemispheres and in the left parietal lobe. Then the training was discontinued for three months, and a third MRI scan was performed. It revealed reduction of the amount of gray matter in all the regions that had benefited from training earlier.
This truly spectacular finding shows that even relatively brief but sustained cognitive activity is capable of affecting brain morphology and is detectable in neuroanatomically specific ways. As the findings of this kind continue to accumulate, the existence of lifelong neuroplasticity and its dependence on cognitive activity will be no longer in doubt.
This article is excerpted from Elkhonon Goldberg’s book, “The New Executive Brain: Frontal Lobes in a Complex World.”
