Scientists have known for decades that certain neurons in the hippocampus are dedicated to remembering specific locations where an animal has been. More useful, though, is remembering 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 colleagues sheds light on that question.
The researchers let mice explore mazes freely for about 30 minutes a day for several days. While the animals were wandering and while they were sleeping, the team monitored hundreds of neurons that they had engineered to flash when electrically active. Wilson’s lab has shown that animals essentially refine their memories by dreaming about their experiences.
The recordings showed that the “place cells” were equally active for days. But activity in another group of cells, which were only weakly attuned to individual places, gradually changed so that it correlated not with locations, but with activity patterns among other neurons in the network. As this happened, an increasingly accurate cognitive map of the maze took shape. Sleep played a crucial role in this process: When mice explored a new maze twice with a siesta in between, the mental maps of those allowed to sleep during the break showed significant refinement, while those of mice 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.”
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