Some college students facing exams used to try to study by sleeping with earphones on while playing a recording of material they had not managed to cover. It was regarded as a waste of time.

Now, however, researchers at the Weizmann Institute of Science in Rehovot believe sleeplearning of new things may be possible – although it is not yet clear whether this would include complex information.

Prof. Noam Sobel, research student Anat Arzi and colleagues published an article in Sunday’s latest issue of the prestigious Nature Neuroscience on learning new information while sleeping.

They found that if certain odors are presented after sounding tones during sleep, people will start sniffing when they hear the tones alone – even when no odor is present – both during sleep and, later, when awake. In other words, people can learn new information while they sleep, and this can unconsciously modify their waking behavior.

Sleep-learning experiments are notoriously difficult to conduct.

First, researchers must be sure that the subjects are actually asleep and stay that way during the “lessons.”

The most rigorous trials of verbal sleep learning have failed to show any new knowledge taking root. While more and more research has shown the importance of sleep for learning and consolidating previous information in one’s memory, none had managed to show actual learning of new information in an adult brain during sleep.

Sobel and Arzi – who is 31 – worked together with Sobel’s group at the Rehovot institute’s neurobiology department with researchers from Ra’anana’s Beit Loewenstein Rehabilitation Hospital and the Academic College of Tel Aviv-Jaffa.

They decided to experiment with a type of conditioning that involves exposing subjects to a tone followed by an odor, so that they soon exhibit a similar response to the tone as they would to the odor.

The pairing of tones and odors presented several advantages – neither wakes the sleeper (in fact, certain odors can promote sound sleep), yet the brain processes them and even reacts during slumber.

Russian physiologist Ivan Pavlov studied conditioning of dogs to salivate when they heard a tone instead of being exposed to food, but this experiment involved delayed conditioning.

In addition, the sense of smell holds a unique non-verbal measure that can be observed – breathing in deeply to sense an odor. The researchers found that, in the case of smelling, the sleeping brain acts much as it does when awake. When there is a pleasant odor, we inhale deeply, but we cut our inhalation short when we encounter an unpleasant smell.

This variation in sniffing could be recorded whether the subjects were asleep or not, Finally, this type of conditioning, while it may appear quite simple, is associated with some higher brain areas – including the hippocampus, which is involved in memory formation.

In the experiments, the subjects slept in a special lab whose walls were covered with stainless steel to prevent irrelevant odors from being absorbed.

Their sleep state was continuously monitored. As they slept, a tone was played, followed by an odor – either pleasant like shampoo or unpleasant like rotten fish. Then another tone was played, followed by an odor at the opposite end of the pleasantness scale. Over the course of the night, the associations were partially reinforced, so that the subject was exposed to just the tones as well. The sleeping volunteers reacted to the tones alone as if the associated odor were still present – by either sniffing deeply or taking shallow breaths.

The next day, the now-awake subjects again heard the tones alone – with no accompanying odor. Although they were not aware of having listened to them while sleeping, their breathing patterns belied this.

When exposed to tones that had been paired with pleasant odors, they sniffed deeply, while the second tones – those associated with bad smells – provoked short, shallow sniffs.The team then asked whether this type of learning is tied to a particular phase of sleep.

In a second experiment, they divided the sleep cycles into rapid-eye movement (REM) and non-REM sleep, and then induced the conditioning during only one phase or the other. Surprisingly, they found that the learned response was more pronounced during the REM phase, but the transfer of the association from sleep to waking was evident only when learning took place during the non-REM phase.

Sobel and Arzi suggest that during REM sleep we may be more open to influence from the stimuli in our surroundings, but so-called “dream amnesia” – which makes us forget most of our dreams – may operate on any conditioning occurring in that stage of sleep. In contrast, non-REM sleep is the phase that is important for memory consolidation, so it might also play a role in this form of sleep-learning.

Although Sobel’s lab studies the sense of smell, Arzi told The Jerusalem Post she intends to continue investigating brain processing in altered states of consciousness such as sleep and coma.

“The plasticity of the brain fascinates me. Once, sleep was considered to be ‘reversible death,’ but our brains can do things when we sleep. Now that we know that some kind of sleep learning is possible,” said Arzi, “we want to find where the limits lie – what information can be learned during sleep and what

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