By Christian Jarrett

The famous studies of London’s taxi drivers – showing they have larger hippocampi (the comma-shaped brain structure in the temporal lobes) than controls – have become a staple of undergrad psychology courses and a classic example of how your brain changes according to what you do with it. Many other studies have also implied an association between hippocampal size and navigational ability – for instance, people with Alzheimer’s, who have lost neurons in this brain structure, tend to experience problems finding their way around. For some time, then, an obvious, though tentative, inference has been that better navigators have bigger hippcampi, with London taxi drivers (and their mastery of “the knowledge” of the city’s convoluted streets) and people with Alzheimer’s representing opposite extremes of the spectrum. However, a new study, released as a preprint at bioRxiv, raises questions about how far we can safely generalise from the taxi driver and Alzheimer’s-based research.

Steven Weisberg and his colleagues tested young adults’ navigation skills and assessed the size of their hippocampi and found the two were not significantly correlated. “The hippocampus plays a crucial role in spatial navigation in humans, but the volume of the hippocampus may not be a biological marker for navigation ability among typical populations,” the researchers concluded.

The ninety participants (average age 23; 54 women) had their brains scanned and they completed a range of psychological questionnaires and tests, including a series of computer-based navigation tasks. These navigation tasks required them to learn the routes between buildings in a virtual environment, to point to the location of various buildings, and to drag and drop buildings in the correct location from memory as a way to test their ability to form a cognitive map of the environment.

Critically, the researchers did not find a significant correlation between hippocampi volume and navigation performance, which varied widely across the participants. This was true whether looking at overall hippcampal volume, right hippocampus only or right posterior hippocampus, and remained the case when controlling for differences between participants in other cognitive and demographic factors, such as their mental rotation ability, verbal IQ and gender.

Weisberg and his team believe there are at least three explanations for their null results. First, perhaps the association between hippocampus size and navigation ability only pertains to extreme groups, like taxi drivers and people with Alzheimer’s, and not to the spread of ability in the general population. Second, perhaps hippocampi volume is too crude a measure, with navigational ability depending more on connectivity within the hippocampus and between the hippocampus and other relevant brain areas. Last, maybe hippocampi volume is associated not so much with navigational skill, but the use of particular navigational strategies (raising questions about what kind of cognitive processes exactly the London taxi drivers’ “knowledge” depends on – for instance, the researchers suggested, perhaps it relies not a cognitive map in the usual sense, but rather “an enormous catalogue of associational data”).

Although the new findings represent much needed research on the neural basis of navigational skills among the healthy general population, they are limited by the study’s use of computer-based navigation tasks – it’s possible the results would have been different using a task in the real world. Also, there was no assessment of brain activity and no measure of the kind of navigational strategies the participants used, so we can’t know what part they may have played in the results.

The researchers said future research could address these shortcomings but that as things stand their paper has “implications for the role of the hippocampus in general navigation, and for the extrapolation of findings in expert and impaired groups to healthy, young adults.”

—Everyday taxi drivers: Do better navigators have larger hippocampi? [this study is a preprint meaning that it has not yet been peer reviewed and the final published version may differ from the version that this report was based on]

Christian Jarrett (@Psych_Writer) is Editor of BPS Research Digest