Nestled in a part of the brain called the medial entorhinal cortex, grid cells fire off regularly spaced signals as a rat moves through the world, marking a rat’s various locations. Individual grid cells work together to create a mental map of the environment. But scientists didn’t know what happens to this map when an animal no longer needs it, such as during sleep.
Grid cells, it turns out, maintain their mapmaking relationships even in sleeping rats, report two teams of researchers, one from the University of Texas at Austin and one from the Norwegian University of Science and Technology in Trondheim. (The Norway group includes the researchers who won a Nobel Prize in 2014 for discovering grid cells (SN Online: 10/6/14).) By eavesdropping on pairs of grid cells, researchers found that the cells maintain similar relationships to each other during sleep as they do during active exploration. For instance, two grid cells that fired off signals nearly in tandem while the rat was awake kept that same pattern during sleep, a sign that the map is intact. The results provide insights into how grid cells work together to create durable mental maps.