Earlier studies on sleep focused on how it stabilizes and consolidates new memories that are acquired while awake. Very few scientific papers touched upon the topic of sleep-learning.
Existing research showed that the sleeping brain replays information acquired during wakefulness. By replaying the experience during sleep, the brain reinforces the strength of the memory trace. It also embeds the newly obtained information in the same area where the organ stored earlier memories. (Related: Sleeplessness can make you angrier, more frustrated, new study shows.)
Given the beneficial effects of replay during sleep on the storage of memories from periods of wakefulness, researchers from the University of Bern theorized the possibility of first-play of memories while asleep. The preliminary evaluation of new data, first-play usually takes place when a person is awake. If it takes place during sleep, first-play can embed a memory trace in the brain that will reach the next period of wakefulness.
In their experiment, Bern researchers got their participants to take naps during the middle of the day. The participants were exposed to foreign words and their translations while asleep.
The results showed that it is possible to link the newly acquired words of another language to earlier associations stored during previous periods of wakefulness. Once the participants woke up, they could take advantage of the links formed during sleep to get the meaning of foreign words as if they learned them while awake.
The process of recovering sleep-formed connections received support from the hippocampus. The region of the brain that manages associative learning during wakefulness, the hippocampus played a significant role in the retrieval process.
The Bern researchers also found that the state of the brain cells affects the formation of semantic associations between foreign words and local translations. To successfully form a semantic link, the neurons needed to be in an active state.
Once a person enters deep sleep, neurons will start working together more closely. Brain cells will remain active for a short period. Then they will simultaneously enter a state of inactivity. The active state is referred to as "Up-state" while the opposite state of inactivity is the "Down-state." The brain cells alternate between the different states every 0.5 seconds.
The semantic associations could only be encoded and stored in the brain if the second member of a word pair played several times during an Up-state period of brain cell activity. The researchers used the example of the German-English word pairs "tofer (small) = key" and "guga (big) = elephant." If the second word of those pairs repeated themselves two to four times while the neurons were in an active state, the participants displayed a higher chance of remembering if the foreign word indicated a small item or a large object.
"It was interesting that language areas of the brain and the hippocampus -- the brain's essential memory hub -- were activated during the wake retrieval of sleep-learned vocabulary because these brain structures normally mediate wake learning of new vocabulary," explained Bern researcher Marc Züst, the co-first-author of the paper. "These brain structures appear to mediate memory formation independently of the prevailing state of consciousness -- unconscious during deep sleep, conscious during wakefulness."
Züst and his colleagues believed that their findings challenge the existing theories on sleep and memory. In particular, they suggested that a person does not need to be awake to form a strong memory.