People can acquire completely new memories while they are asleep, reports a new study in Nature Neuroscience. While it is already known that sleep can strengthen existing memories, this work demonstrates that people can also learn a new association during sleep.
Anat Arzi and colleagues required participants not to remember new facts, but simply to form a link between a particular tone and a pleasant or unpleasant smell. While participants slept, they smelt various smells, while simultaneously listening to sounds of different tones. Some of these smells were very unpleasant, such as rotting fish, while others were pleasant, such as shampoo. None of these smells or sounds were strong enough to wake the subjects but the authors noted that even while asleep, they inhaled more deeply only when they smelt a pleasant smell. After learning, they also inhaled more deeply when they heard a tone which was previously paired with a pleasant compared to an unpleasant smell, even when no smells were present.
The next day, when participants were awake, they again heard the tones that had been paired with certain smells. The team observed that even though no actual smells were present, they inhaled more deeply when they heard tones previously paired with the pleasant smells. This happened even though the participants had no conscious recall of the pairing between the smells and tones. The authors conclude that the increase in inhalation to occurring after tones previously paired with pleasant smells suggests that the participants had formed an association between the tone and the smell.
While learning more complex things, such as memorizing facts or learning a new language, may still not be possible during sleep, this study demonstrates that it is possible to acquire a simple association while fast asleep.