How the brain makes maps of its world
Evidence suggests that the hippocampus creates ‘maps’ that share more features with subway maps than street maps.
The hippocampus is one of the most easily recognizable structures in the brain owing to its characteristic seahorse-like shape. Brain imaging studies in the 1990s famously showed the hippocampus to be larger in London taxi drivers than in other people, suggesting that it plays a role in spatial navigation. This was consistent with previous findings in rodents, which had shown that the hippocampus is active when animals find their way through mazes.
Electrode recordings have revealed that whenever an animal is in a specific location of a particular environment (for example, in the back left-hand corner of a small room with white walls) one or a small number of cells within the hippocampus will fire to encode that location. When the animal moves to a new location within the same environment, other cells will fire to encode the new location. In this way, the population of cells — which are known as place cells — can together construct a virtual ‘map’ of the environment.
It is generally assumed that this hippocampal map represents space in terms of the absolute distances and angles between locations, rather like a street map. However, this type of geometric map appears inconsistent with the results of certain experiments. Yuri Dabaghian and co-workers proposed instead that the hippocampal map is based on topology, or the relative order of locations and the connections between them, rather like a subway map. Subsequently, computer models demonstrated that virtual simulations of place cells could effectively ‘learn’ the topological features of different environments.
Now, Dabaghian and co-workers provide their own empirical data to support the existence of a hippocampal ‘subway-style’ map by recording the electrical impulses from place cells in the rat hippocampus as the animals ran through a U-shaped maze. The maze was constructed so that its arms could either be straight or folded into zigzags. Changing the maze in this way does not alter its topology because the relative order of its various components — such as the positions of food wells in the arms — is unchanged, but it does alter the maze’s geometry. Notably, as rats ran through different conformations of the maze, the activity of the place cells in their hippocampi remained largely unchanged, consistent with a map based on topology rather than geometry.
By providing evidence that hippocampal maps have more in common with subway maps than street maps, the work of Dabaghian and co-workers offers an explanation for previously challenging results and provides a framework for further experiments into hippocampal memory function.
To find out more
Read the eLife research paper on which this story is based: “Reconceiving the hippocampal map as a topological template” (August 20, 2014).
eLife is an open-access journal that publishes outstanding research in the life sciences and biomedicine.
