Sunday, September 27, 2020
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Is sleep-learning via sensory input possible?

People can learn new information while they sleep, and this can unconsciously modify their waking behavior, according to new research from the Weizmann Institute of Science in Israel, published in Nature Neuroscience.


Experiments that deal with sleep-learning are difficult to conduct for a few reasons:

  • You have to make sure the subject isn’t faking it.
  • You have to make sure whatever you’re doing in the “lesson” doesn’t wake them up.
  • You have to make sure they didn’t already know what you’re “teaching” them.

Assuming these things can be properly controlled, the idea is that before a person went to sleep, he didn’t know something. Then you presented that information (sensory input, by any sense except sight, obviously, because his eyes had better be closed). And when he woke up, the information had been retained.

If that doesn’t happen, you haven’t demonstrated sleep-learning. Aural sleep-learning has failed in many, many experiments. This is where the sleeper-learner would listen to a tape of someone giving a lecture or reading a book—or something much less involved, such as a poem.

Upon waking, if the person could recite the poem, sleep-learning would have been shown, provided all three of those bullet points were working in the experiment. But scientists have had great difficulty showing that aural sleep-learning works reliably.

That’s one of the reasons Prof. Noam Sobel, research student Anat Arzi, and Dr Sobel’s group in the institute’s Neurobiology Department decided to experiment with a type of conditioning that involves exposing subjects to a tone followed by an odor. If this experiment is performed when the subject is awake, subjects would eventually learn that the tone was always paired with the odor and exhibit a similar response to the tone as they would to the odor.

Working in collaboration with researchers from Loewenstein Hospital and the Academic College of Tel Aviv, Jaffa, they wanted to see if subjects would “learn” this same association if the tone-odor combination were “taught” to them while they were sleeping.

The good news is that neither tones nor odors tend to wake people up. In fact, some odors promote a deeper sleep. Furthermore, the sense of smell produces what is almost an unconscious response: sniffing. This can be observed reliably without forcing the subject to report. If the sniffing response occurs after sleep-learning in response to the tone, after the subject has been conditioned to expect the odor, they’ll know the experiment worked.

As far as smelling goes, your brain works pretty much the same while you’re awake as it does when you’re asleep. If there’s a pleasant odor in your vicinity, you’ll inhale deeply whether you’re awake or asleep; conversely, you’ll stop inhaling if you smell a nasty odor.

Scientists also knew this type of conditioning is associated with some higher brain areas, like the hippocampus, which is involved in memory formation. The ultimate question is, Can we form memories from sensory input while we’re sleeping?

As they slept, a tone was played, followed by an odor—either pleasant or unpleasant. 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 had no conscious recollection of listening to them during the night, their breathing patterns told a different story. 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.

Aha! If somebody knows a way to encode trig identities in the odor of a range of flowers so students would inhale deeply, we might be on to something.

But seriously, this is some interesting brain-based research using biology, psychology, and fun with friends and strange odors.

Paul Katula is the executive editor of the Voxitatis Research Foundation, which publishes this blog. For more information, see the About page.

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