Some very interesting new research from Weizmann Institute was recently published in Nature Neuroscience. Scientists there have discovered that people can actually learn during sleep, which can unconsciously modify their behavior while awake.
Researchers from the Institute’s Neurobiology Department, and experts from Loewenstein Hospital and the Academic College of Tel Aviv- Jaffa conducted 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.
Sleep-learning studies are extremely difficult to conduct, so the experts had to make sure the participants were really asleep during the “lessons.” 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 reasoning behind using pairing tones and odors was neither wake the subject, yet the brain processes them and even reacts to them during sleep. Certain odors can even help the participants to have a sound sleep.
The sense of smell, or sniffing, holds a unique non-verbal measure that can be examined. Results showed that the brain acts just as it does when it is awake when dealing with smells. When we smell a pleasant aroma we inhale deeply, and when we smell something bad we cut our inhalation short.
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).
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 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.
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, but “dream amnesia” (which makes people forget their dreams) may operate on any conditioning during that stage. Non-REM sleep, is important for consolidating memory, so it could also be playing a role in this form of sleep-learning.
Researchers hope to further investigate brain processing in altered states of consciousness such as sleep and coma. The thinking is that now that there is proof that a certain level of sleep learning is possible, scientists want to discover all the varying types of information that can be learned and how much of each type can we remember.