For instance, if certain odours are presented after specific tones during sleep, people will start sniffing when they hear the tones alone - even when odour is absent, both during sleep and later, when awake, say researchers.
Sleep-learning experiments are notoriously difficult to conduct. For one thing, one must be sure that the subjects are actually asleep and stay that way during the "lessons," the journal Nature Neuroscience reported.
Noam Sobel, professor of neurobiology at the Weizmann Institute's and research student Anat Arzi, along with colleagues from Loewenstein Hospital and the Academic College of Tel Aviv - Jaffa, chose to experiment with a type of conditioning that involves exposing subjects to a tone followed by an odour, so that they soon exhibit a similar response to the tone as they would to the odour.
"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," said Arzi, according to a Wizemann statement.
The pairing of tones and odours presented several advantages: Neither wakes the sleeper (in fact, certain odours can promote sound sleep), yet the brain processes them and even reacts during slumber.
Moreover, the sense of smell holds a unique non-verbal measure that can be observed - namely sniffing.
Researchers found that, in the case of smelling, the sleeping brain acts much as it does when awake: We inhale deeply when we smell a pleasant aroma but stop when assaulted by a bad smell.
This variation in sniffing could be recorded whether the subjects were asleep or awake. Finally, this type of conditioning, while it may appear to be quite simple, is associated with some higher brain areas - including the hippocampus, which is involved in memory formation.
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