How Sleep Helps the Brain Navigate Space
Scientists have known for decades that certain neurons in the hippocampus are dedicated to remembering specific locations where an animal has been. More useful, however, is the ability to remember where places are relative to each other, and it hasn’t been clear how those mental maps are formed. A study by MIT neuroscientist Matthew Wilson and his colleagues sheds light on that question.
The researchers allowed mice to explore mazes freely for about 30 minutes a day over several days. While the animals wandered and during their sleep, the team monitored hundreds of neurons they had engineered to flash when electrically active. Wilson’s lab has previously demonstrated that animals refine their memories by dreaming about their experiences.
The recordings showed that the “place cells” remained equally active for days. However, activity in another group of cells, which were only weakly attuned to individual places, gradually changed. This change correlated not with locations but with activity patterns among other neurons in the network, leading to the formation of a more accurate cognitive map of the maze. Sleep played a crucial role in this process: When mice explored a new maze twice with a siesta in between, those allowed to sleep during the break showed significant refinement in their mental maps, while those that stayed awake did not.
“On day 1, the brain doesn’t represent the space very well,” says research scientist Wei Guo, the study’s lead author. “Neurons represent individual locations, but together they don’t form a map. But on day 5, they form a map. If you want a map, you need all these neurons to work together.”
