Brain cells that plan where to go

Researchers at the RIKEN Center for Brain Sciences (CBS) in Japan have discovered a region of the brain that encodes where an animal plans to be in the near future. Linked to internal maps of spatial location and past movements, activity in the newly discovered grid cells accurately predicts an animal’s future locations in its environment.

Published in Science On August 15, the study helps explain how planned space navigation is possible.

It may sound easy, but moving through the world requires a fair amount of brain activity. For example, simply walking around a supermarket to do your shopping requires having internalized maps of the outside world, information about your own changes in position and speed, and memories of where you’ve been and what you’re trying to buy.

Much of this type of information is contained in cells in two connected parts of the brain, the hippocampus and the medial entorhinal cortex, or MEC, which are extremely similar in all mammals, from rats to humans. The MEC contains maps of an animal’s current position in space, a discovery that earned the animal the Nobel Prize in Physiology or Medicine in 2014.

The new study focuses on the MEC, but not on the stored information needed for spatial navigation or an animal’s current location. Instead, the experiments led by Shigeyoshi Fujisawa and Ayako Ouchi of RIKEN CBS focus on how this brain region creates maps of future positions, which are continually updated as animals move.

As the rats roamed an open square field in search of freely available water and moved it to different locations, the researchers recorded all movements, which included hundreds of trajectories. At the same time, they recorded the activity of individual brain cells in the MEC.

They then checked how well brain activity over time corresponded to changes in the rats’ position.

They found that the activity of certain brain cells in the MEC created an internal grid that mapped future positions in the field. For example, an MEC cell might encode a certain location in the field, but only when a rat reached a spot 30 to 40 cm earlier on a route that eventually passed through that location, regardless of the direction the rat came from.

This is very different from the grid cells that won the Nobel Prize, which only fire when an animal is in a specific location. The authors called the newly discovered neurons “feedback grid cells” and conducted several follow-up experiments to get a better idea of ​​what exactly they encode.

First, they tested whether the newly discovered grid cells predicted future location in terms of distance or time relative to the present. They found that both were encoded, although the “grid-ness” of the cells was higher when distance was taken into account.

They also found that future positions were faithfully encoded across different situations, whether the rats were trying to reach specific targets or were randomly searching for food. This means that the function of predictive grid cells is not limited to goal-directed behavior.

“This study provides important insights into the mechanisms of spatial navigation and episodic memory formation in hippocampal and entorhinal cortical circuits,” Fujisawa says. “In the future, we would like to clarify the mechanism by which these predictive grid cells are organized.”