Previous studies have highlighted the important role sleep plays in learning and memory formation. New research suggests, during sleep, a person's brain replays memories that go un-recalled when awake.

For their study, neuroscientists in Germany recruited epilepsy patients electrodes implanted in their brains for surgical planning. The electrodes allowed scientists to precisely record brain activity patterns.

Researchers had participants memorize a series of images. Each image was associated with a unique pattern of brain activity. Later, scientists measured the participants' neural activity while they napped. Researchers were able to recognize the gamma band activity signatures of each images.

Their analysis showed, during sleep, the participants' brains reimagined each of the images. After napping, scientists quizzed participants on the pictures they were asked to memorize. The tests proved the brain, during sleep, considers images that later go unremembered.

"The forgotten images do not simply disappear from the brain," Hui Zhang, a neuropsychologist at the University of Bonn, said in a news release.

As the new research proved, reactivating an image in the brain during sleep doesn't guarantee memorization. However, scientists observed that when gamma band activity was accompanied by ripples inside the brain's hippocampus, a region important to memory formation, the image was likely to be later recalled.

Scientists also found the gamma band activity happening during the first viewing of an image could be divided into two stages, a superficial stage and a deep processing stage. Only images triggering the deep processing stage of gamma band activity -- both during first viewing and while sleeping -- were later recalled by participants.

The research -- published this week in the journal Nature Communications -- provided scientists new insights into the ways the brain recalls and organizes memories during sleep and while awake.

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