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Review of: Distances between real-world locations are represented in the human hippocampus.

Link to original F1000 review: http://f1000.com//8492957

This exciting new article advances our understanding of the information encoded by the human hippocampus. We have long known that the hippocampus contains a spatial representation of the environment {1}, but now Morgan et al. reveal that the human hippocampus may scale its activity depending on the Euclidean distance between recently observed landmarks.

The authors used a functional magnetic resonance imaging (fMRI)-adaption paradigm. During scanning, subjects were sequentially presented with a large set of photographs of familiar landmarks. For each image, the researchers calculated the Euclidean distance between it and the preceding landmark. While the authors report a number of results, the important finding is that activity in the left anterior hippocampus was significantly correlated with Euclidean distance between landmarks. Thus, the further apart the landmarks were, the greater the fMRI signal in this hippocampal region. This implies that the human hippocampus encodes real-world distances in its activity.

This finding extends my previous fMRI research, which found that the medial temporal-lobe activity of London taxi drivers was positively correlated with the Euclidean distance to the goal, during the navigation of a virtual simulation of London (UK) {2}. In the study by Morgan et al., the subjects were not required to consider the distance between landmarks. Therefore, the new result implies that distance-scaled activity may be automatic in the human hippocampus.

As Morgan et al. point out, understanding the underlying mechanism will require more research and, crucially, cellular recording studies.

References

1. O'Keefe and Nadel, “The hippocampus as a cognitive map.” Oxford: Clarendon Press, 1978 [ISBN: 0-19-857206-9]. (Accessed 24 February 2011).
2. Spiers and Maguire, Hippocampus 2007, 17:618-26 [PMID:17492693].