Brain waves during REM sleep solidify memories in mice, scientists report in the May 13 Science.
Scientists suspected that the eye-twitchy, dream-packed slumber known as rapid eye movement sleep was important for memory. But REM sleep’s influence on memory has been hard to study, in part because scientists often resorted to waking people or animals up — a stressful experience that might influence memory in different ways.
Richard Boyce of McGill University in Montreal and colleagues interrupted REM sleep in mice in a more delicate way. Using a technique called optogenetics, the researchers blocked a brain oscillation called theta waves in the hippocampus, a brain structure involved in memory, during REM sleep. This light touch meant that the mice stayed asleep but had fewer REM-related theta waves in their hippocampi.
Usually, post-learning sleep helps strengthen memories. But mice with disturbed REM sleep had memory trouble, the researchers found. Curious mice will spend more time checking out an object that’s been moved to a new spot than an unmoved object. But after the sleep treatment, the mice seemed to not remember objects’ earlier positions, spending equal time exploring an unmoved object as one in a new place. These mice also showed fewer signs of fear in a place where they had previously suffered shocks.
Interfering with theta waves during other stages of sleep didn’t seem to cause memory trouble, suggesting that something special happens during REM sleep.