Israeli scientists have found the strongest evidence yet that people can learn new information while they are sleeping, rather than simply strengthening memories already made.
Anat Arzi from the Weizmann Institute of Science in Rehovot, Israel played tones to sleeping volunteers before wafting smells of deodorant, shampoo, rotten fish, or dead animals past their noses. The smells triggered a sniffing reflex and the pleasant ones drew stronger sniffs. Then, when Arzi played the tones alone, the volunteers still sniffed, and more strongly to tones that had been paired with nice odors.
This conditioned response lasted through the night and into the next morning when the volunteers woke up. Although they still sniffed when they heard the tones, none of them were aware of what they had learned.
“This work is transformative in that it shows that humans can acquire information not only without awareness but also in a non-conscious state,” says Kimberly Fenn, a psychologist from Michigan State University, who was not involved in the study.
“There’s a kind of dogma that says the brain encodes new information when it’s awake, and consolidates memory while asleep,” explained Jan Born, a sleep researcher from the University of Lübeck. “This paper shows that the contrast between these two modes of activity isn’t that sharp.”
Decades of research has already shown that the brain actively processes information while we sleep, and we can strengthen existing information with the right triggers, such as odors present at the time of initial learning. But its ability to learn while snoozing has been less clear.
Several groups have tested for advanced forms of learning during sleep, like picking up the links between pairs of words. All such experiments have failed. The only positive results came from studies showing that a very basic form of learning known as classical conditioning can occur in sleeping rats and infants, which begin to associate two stimuli—say, a tone and a puff of air—if they are presented together.
By contrast, Arzi’s experiments used a different technique called “trace conditioning,” where the tone and the smells are separated by more than a second. “This is considered a more advanced type of learning, and unlike classical conditioning, it depends on the hippocampus,” she said. “This is the type of learning associated with more complicated cognitive tasks, and therefore finding it in sleep is potentially important and novel.”
Arzi also took steps to ensure that her subjects were not inadvertently waking up. Throughout her study, a sleep technician monitored the volunteers’ brain activity and halted the experiment whenever they showed signs of rousing. All such trials were left out of the final analysis.
Arzi’s volunteers only learned a very simple response, and it is not clear if we can pick up more complex information while sleeping. “This does not imply that you can place your homework under the pillow and know it in the morning,” she said. “There will be clear limits on what we can learn in sleep, but I speculate that they will be beyond what we have demonstrated.”
Fenn agreed. “It is difficult to imagine that any form of declarative information, even simple vocabulary learning, could be accomplished in a sleep state,” she said. “That being said, this study may have strong implications for conditioning of undesirable behaviours.”
Addictions, for example, are sometimes treated by teaching people to associate the drugs they crave with something repulsive. Although such approaches are controversial, our hours of slumber could provide a prime opportunity to enshrine such associations, and the lack of awareness when we wake up would only be a bonus, Arzi noted. “If you have some bad habit that you want to get rid of, awareness may only hinder your efforts,” she said. “If you could learn to get rid of the habit during sleep, you may do it without awareness as to why.”
A. Arzi et al., “Humans can learn new information during sleep,” Nature Neuroscience, doi:10.1038/nn.3193, 2012.