This is a guest post in our #NPGsfn11 blog series and posted on behalf of Moheb Costandi.
The brain encodes two distinct maps of the route from one location to another and switches between the two at different phases of the journey, according to new research presented earlier this week at the annual meeting of the Society for Neuroscience in Washington, D.C.
We know that a brain structure called the hippocampus, in the medial temporal lobe, is essential for spatial navigation and for encoding spatial memories. It contains at least four different cell types that encode maps of the environment, but exactly how this occurs is unknown.
According to one model, the hippocampus encodes a Euclidean path, or straight line, between point A and point B. Another suggests that it encodes the true path between the two locations, incorporating diversions around obstacles.
To do so, they recruited 24 participants and gave them maps of London’s Soho district to study; all were “Soho virgins” who had no previous experience of the area. The participants were then taken on a two-hour guided tour of the area.
The next day, they had their brains scanned while watching a high definition first-person film of the areas they had explored. The researchers paused the film at junctions and asked the participants to indicate which direction they should take to reach a particular location.
“We know that the hippocampus is the hub of the navigation system,” says Spiers. “Some researchers think it encodes the distance to the goal as the crow flies, others say it maps distance along paths you can take to the goal.”
But when they analysed their data, Spiers and Howard found that the hippocampus actually does both. When the participants were en route to their destination, the anterior, or front end, of the hippocampus was activated and the level of activity was closely related to the distance from the destination. The closer the participants were to the desired location, the less active was the anterior hippocampus.
At turning points, however, a different pattern of activity was observed. When the participants had to decide whether to turn left or right, the posterior, or back end, of the hippocampus, was engaged and became more active the closer they were to their destination: “It’s a linear scale,” Spiers explains. “The posterior hippocampus becomes more active the closer you are along the path.”
The results provide a new understand of how the brain’s navigational system operates, showing what kind of spatial information is encoded by which brain regions and also when the information is used.
The hippocampus appears to be encoding two different maps of the path to a journey, with the anterior region tracking the Euclidean distance to the final destination and the posterior hippocampus tracking the “true path” and homing in on the destination.
“According to previous models of spatial navigation, the hippocampus does one or the other,” says Spiers. “We found it actually does both, and that it flips between the two in an elegant way. There’s no model in which it does both and certainly not one proposing that it flips between the two.”
Mo trained as a molecular/ developmental neurobiologist and now works as a freelance science writer. His work has appeared in Nature, New Scientist, Scientific American and Technology Review, among others, and his blog, Neurophilosophy, is hosted by the Guardian.