Mastering London Topography Changes Hippocampal Structure in Taxi Drivers

Can spatial training alter the anatomy of the hippocampus in humans as it does in animals? To test this idea, a group of investigators studied the brains of licensed London taxi drivers (Maguire et al., 2000). These drivers are very well suited for such a study because, in order to obtain a license, they must undergo intensive training about London streets and locations (called colloquially “being on The Knowledge”). This training requires 2 years, on average, and it is followed by a stringent set of police examinations.

In an earlier study using positron emission tomography (PET), the same investigators had found that having London taxi drivers recall complex routes around the city caused activation of a network of brain regions that included the right hippocampus; recall of landmarks for which the subjects had no knowledge of their location within a spatial framework activated similar brain regions, except for the right hippocampus (Maguire et al., 1997).

In the later study the experimental subjects were 16 right-handed men whose careers as licensed drivers ranged from 1.5 to 42 years; all had healthy general medical, neurological, and psychiatric profiles. Structural MRI scans of their brains were obtained and measured. For comparison, the investigators studied MRI scans of 50 healthy right-handed males who were in a similar age range and did not drive taxis. The only brain regions that showed structural differences between the taxi drivers and control subjects were the right and left hippocampi (Figure 1). The posterior part of the hippocampus was significantly larger in taxi drivers than in controls in both the right and the left hemispheres (Figure 2). In contrast, the anterior part of the hippocampus was significantly smaller in taxi drivers than in controls. There was no significant difference between taxi drivers and controls in either the midportion (body) or the overall size of the hippocampi.

Figure 1 Region including the hippocampus

Figure 2 Area that is larger in experienced drivers than in new drivers

Could these differences reflect an innate predisposition to learn to navigate the streets of London? To test this possibility, the investigators plotted the volume of the anterior and the posterior hippocampus against the number of months each person had spent as a taxi driver. The volume of the anterior right hippocampus correlated significantly negatively with the duration of taxi experience (r = –0.6, p < 0.05), and the volume of the posterior right hippocampus correlated significantly positively with the duration of taxi experience (r = 0.6, p < 0.05). Thus, the greater the duration of taxi experience, the greater the anatomical effect in the right hippocampus. These correlations strongly indicate that experience alters the hippocampus, rather than initial hippocampal differences reflecting an innate disposition to acquire spatial knowledge. The left hippocampus did not show significant changes with duration of experience, suggesting that the left and right hippocampi participate differently in spatial navigation and memory.

Although this report offers surprising new information, like much innovative research it also raises many new questions. For example, is there a behavioral correlate to the reduction of size of the anterior hippocampus in taxi drivers? A report of this research in The Economist (“Neuroscience,” 2000) comments ironically, “Whether the loss of frontal [hippocampal] tissue has any relationship with the robust political opinions for which London cabbies are renowned is an area that remains mercifully uninvestigated” (p. 83).

In a commentary on this study, Terrazas and McNaughton (2000) pointed out some surprises and some paths for future work. For example, they expressed surprise that changes in the brain with experience appear to accumulate for as long as 20 years; rather, they expected the greatest changes to occur earlier, when knowledge acquisition was greatest. MRI measures can be done repeatedly in the same subjects, so these investigators would like to see longitudinal studies of some taxi drivers. To test whether it is only acquisition of spatial knowledge that produces localized effects in the hippocampus, they suggested comparable studies with groups that have similarly high demands of nonspatial learning, such as years of legal training and practice or medical studies and practice. Clearly the research by Maguire et al. has raised as many questions as it has solved. (Photos from Maguire et al. 1997; courtesy Eleanor Maguire.)