In the sleeping brain, the hippocampus strengthens memory by going over the day’s events.

Brain Areas Team Up During Sleep In the sleeping brain, the hippocampus strengthens memory by going over the day’s events. When a rat runs a maze, for example, hippocampal neurons involved in the activity will fire in the same pattern after the animal goes to sleep. New research suggests that other brain areas take part in these “rehearsals” and that they act in concert. Daoyun Ji and Matthew Wilson of the Massachusetts Institute of Technology trained rats to navigate mazes with distinctive visual patterns, such as black and white stripes or geometric shapes. The investigators took recordings from electrodes surgically implanted in the rats’ hippocampal and visual cortices, both during the maze test and while the rats slept. Their results, published in the January Nature Neuroscience, were consistent with previous research: during running and sleeping, neurons fired in similar bursts of activity, or “frames,” in both parts of the brain. “It’s likely that the two brain areas were replaying the same event,” Wilson says. The finding suggests that separate brain areas work together to build a memory. The hippocampus, though responsible for navigation and memory, does not directly contribute to visual recognition. Reactivation in the visual cortex during sleep—when the eyes are shut and there’s nothing to see—suggests that this area does more than just perceive. “The visual cortex may be part of a memory-forming network in which sensory areas provide individual components of the memory,” Wilson concludes.