Grid Cells Encode Both Meaning and Position

Locations linked to reward get special representations in the brain, new research suggests.

New research shows that grid cells don’t simply encode an animal’s location; they also represent how important that location is to the animal. For example, grid cells that represent spots that are reliably tied to food tend to fire more often. The findings, published in Science in March, demonstrate that non-spatial properties, such as reward, can influence how the brain represents space.

Viewing the environment from above, gray traces show how the animal moved through the space. Colored lines show individual cases of the animal going to the rewarded location (red box; only the second environment has a reliably rewarded location). Credit: Butler et al. Science 2019

Neuroscientists have been exploring the representation of space in the brain for decades, discovering a widening array of cell types that represent different navigational factors, including place cells, head direction cells and running speed cells. Grid cells, found in the medial entorhinal cortex, fire in a periodic pattern as an animal moves through space. Scientists knew that place cells change their firing in response to rewards and tasks, but they believed grid-cell responses remained stable.

In the new study, William Butler and Kiah Hardcastle in the lab of Lisa Giocomo, a neuroscientist at Stanford and an investigator with the Simons Collaboration on the Global Brain, used two different navigational tasks: rats either had to forage for food randomly placed in an environment, or they were trained to receive food in a specific location that they would go to in response to a sound. These two tasks took place in different environments, enabling the researchers to map out how grid cells represent space in the context of each task.

Viewing the animal's environment from above, yellow spots indicate the locations in which this grid cell fires. Red lines mark the grid's orientation, which is aligned with the walls of the environment in the foraging task (left) but slightly angled in the rewarded environment (right). Credit: Butler et al. Science 2019

Researchers found that grid cells in the two environments responded differently. For example, grid-cell responses have a particular orientation: the specific direction the animal needs to move in order for the cell to fire periodically. The exact locations at which grid cells fire can change in different environments, but this direction usually remains the same. The new study found that reward can modify a cell’s orientation, however. In their experiments, grid cells shifted their orientation between the random foraging environment and the rewarded environment.

Both Giocomo’s study and a second study, from Jozsef Csicsvari‘s group at the Institute of Science and Technology Austria, published in the same issue of Science showed that rewards influences grid-cell firing patterns — particularly near the reward location. Giocomo’s team found that increased firing in the environment with the rewarded location peaked near the reward and faded with distance from it. This boost in firing may help the brain more accurately encode significant locations.

The Giocomo lab also built a location decoder — an algorithm that predicts the animal’s location based on neural activity — for each environment. They found that neural activity better predicted the animal’s location in the rewarded environment. Specifically, the prediction was more accurate for locations near the reward. This suggests that the changes in grid-cell representation may serve to give the animal a more precise estimate of its location, particularly as it approaches the rewarded area.