Are learning and unlearning bedfellows?

The human brain's capacity for learning is astonishing, capable of even memorizing meaningless sounds if repeated. Hence, a pattern of white noise, like that produced by an untuned radio, can be learned if heard only a few times, even if the listener isn't paying attention.

The researchers chose this kind of passive auditory stimulation—particularly well suited for subjects who are asleep—to explore the connection between learning and sleep. They exposed sleeping volunteers to noise that incorporated repeated sounds and then tracked their brain activity using electroencephalography. Since the brain reacts differently to new, unfamiliar sounds as opposed to learned sounds, electroencephalographic analysis can show researchers whether a sound has been memorized, even when the subject is not awake.

Analysis of brain activity in subjects while asleep, and of their behavioral responses after waking, demonstrated they recognized the noises heard during both REM sleep and stage N2 sleep. These findings reveal that our brains are able to learn while in these two sleep phases. And indeed, during both phases—even though they are very different in terms of brain activity—the brain can process complex information, whether endogenous or exogenous. However, while previous research on humans and other animals has shown that certain types of learning, like conditioning, can occur during sleep, this study reveals that we can also memorize new representations and objects (here, auditory 'objects') while asleep.

This study also yielded new information about deep sleep (stage N3). The researchers discovered just the opposite to hold true during this sleep stage—sounds previously learned during N2 sleep are forgotten, or unlearned, as if erased from memory. Furthermore, after waking, subjects find these sounds even harder to learn than completely new ones.

These findings support the idea that N2 and REM sleep favor cerebral plasticity and active memory consolidation, while N3 sleep seems to fulfill the necessary function of unloading memories that would otherwise accumulate day after day. These novel results are important because they could reconcile two theories on sleep's function in memory that are often deemed contradictory: one theory regards sleep as a time for consolidating knowledge acquired during the day; the other posits it to be a sieve for discarding information no longer needed. Having made their discovery, the researchers are now considering the precise neural mechanisms underlying this double role sleep plays in memory. "Formation and suppression of acoustic memories during human sleep" is published in Nature Communications .

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