The study suggests that while people sleep, if certain odors are presented after hearing tones, people start sniffing even if there is no odor presented when they hear the same tones. This happens during sleep and even when people wake up.
There have been several past studies explaining the importance of sleep for learning and memory consolidation. However, none of them have been able to show the human brain actually learning new information during sleep.
Professor Noam Sobel, research student Anat Arzi, Sobel's team from the Institute's Neurobiology Department, and experts from Loewenstein Hospital and the Academic College of Tel Aviv- Jaffa, decided to try an experiment with a type of conditioning that exposes participants to a tone followed by an odor, so that they soon experience a similar response to the tone as they would to the odor.
The researchers found many advantages from pairing tones and odors, for example, neither wakes the subject, yet the brain processes them and even reacts during sleep. Certain odors actually even help the participants to have a sound sleep. On the other hand, sleep-learning studies are extremely difficult to conduct, so the experts had to make sure the participants were really asleep during the "lessons."
Associations made whilst sleeping appear to be retained after waking up.
It didn't matter if participants were asleep or awake, this variation in sniffing could be recorded either way. This type of conditioning, while appearing so simple, is also associated with some higher brain areas, like the hippocampus (involved in memory formation).
In order to continuously monitor the subjects' sleep state, the subjects slept in a special lab during the experiments. Even if a participant woke up for a second, the results had to be disqualified.
During sleep time, the subjects heard a tone that was followed by either a pleasant or an unpleasant odor. Then another tone was heard, followed by an odor (at the opposite end of the pleasantness scale from what they previously smelt).
The associations were partially reinforced throughout the night, in order to expose the subjects to the tones alone. The volunteers sniffed deeply or took shallow breaths when they heard the tones without the odor, reacting the same way as if the associated odors were still present.
After volunteers awoke the next day, they heard the tones again with no odor following. Since they were asleep the night before, they had no conscious memory of ever listening to them, but their breathing patterns were showing something different. When the tones that were paired with bad smells were played for the subjects, they produced short, shallow sniffs; and when they heard the tones that were associated with nice odors, they sniffed deeply.
The team conducted a second experiment to find out if this type of learning was tied to a particular phase of sleep. In order to do so, they divided the sleep cycles into rapid eye movement (REM) and non-REM sleep where they induced the conditioning in only one phase or the other.
Researchers were surprised to see that the REM phase showed a more pronounced learning response. However, being able to transfer the learned association from sleep to waking was only found when the learning happened in the non-REM phase.
REM sleep may make us more open to stimuli in the environment, according to Sobel and Arzi, but "dream amnesia" (which makes people forget their dreams) may operate on any conditioning during that stage. Non-REM sleep, they explained, is important for consolidating memory, so it could also be playing a role in this form of sleep-learning.
Since Sobel's lab focuses on the sense of smell, Arzi hopes to further investigate brain processing in altered states of consciousness such as sleep and coma.
"Now that we know that some kind of sleep learning is possible, we want to find where the limits lie- what information can be learned during sleep and what information cannot."