Our brains don’t rest when we sleep. Electrical waves ripple through our noggins as our neurons talk to each other. Now, researchers have shown that when these waves don’t interact properly, we can lose our long-term memory. The work may help explain why older adults are so forgetful, and it could lead to new therapies to treat memory loss.
To find out how sleep contributes to memory loss in old age, Randolph Helfrich, a neuroscientist at the University of California (UC), Berkeley, and his team gave healthy 70- and 20-year-olds a memory test. Participants were trained to match 120 common, short words—for example, “bird”—with nonsense words made of combinations of random syllables, like “jubu.” Once they learned the word-nonsense word combos, the volunteers played a version of the game “memory.” They had to match the word pairs twice: once about 10 minutes after they’d mastered the task, and again a few hours after waking from a full night’s rest. While they slept, researchers recorded the electrical activity in their brains.
As expected, the older adults’ ability to remember the word pairs in the morning was worse than their young counterparts’. The electrical recordings revealed one reason. Two kinds of brain waves—slow oscillations, large undulations that promote restorative sleep, and sleep spindles, transient bursts of short waves—are tell-tale marks of deep, typically dreamless, non–rapid eye movement sleep. But these waves are out of sync in older people, the researchers report today in Neuron. This out-of-step activity, they say, interrupts communication between the parts of our brains that store short- and long-term memories. In effect, Helfrich says, the prefrontal cortex where long-term memories are stored needs to tell the hippocampus—the part of the brain where all memories go first—that it’s ready to receive information; if brain waves aren’t in sync, this communication gets lost. So do the memories.
To figure out what causes this asynchrony, the researchers looked at participant’s brains using structural magnetic resonance imaging, which uses radio waves to view internal organs. They found the part of the brain that makes slow oscillations—and long-term memories—was smaller in older participants, suggesting that older adults are more forgetful because this region atrophies over time. “That atrophy is enough to impair the mechanism to bring the brain waves together in time to really store memories overnight,” Helfrich says.
Emery Brown, a neuroscientist at the Massachusetts Institute of Technology in Cambridge, is skeptical. He calls it “a bit of a stretch” to draw such conclusions based on the brain scans. He’d like to see the results reproduced in other settings first.
Still, Phyllis Zee, a neurologist and neuroscientist at Northwestern University in Chicago, Illinois, says the research connects a lot of dots between sleep and memory. She’s curious whether the results will hold up in adults who are at risk for neurological conditions such as Alzheimer’s disease.
Elissaios Karageorgiou, a neurologist and neuroscientist at UC San Francisco, would have added another layer of analysis. For example, if researchers didn’t know whose memory results they were looking at, could they have predicted the unsynchronized brain waves? But he agrees that the findings could potentially lead to memory therapies. Optimizing the timing of these brain waves with electrical or magnetic stimulation, for example, could be one way to improve peoples’ memories.