Scientists have known for a while that sleep helps to strengthen previously acquired memories. But can we learn in our sleep? Although the state is characterized by loss of consciousness and a lack of response to external stimuli, the brain still may process sensory information. Studies have shown that exposure to meaningful stimuli, such as one’s own name, evokes different brain responses than does exposure to meaningless stimuli. Other studies have shown a connection between smell and memory in sleep. One in particular showed that presentation of an odor associated with a previous memory during sleep makes that memory more available for recall right after waking up.
Researchers have used differential partial reinforcement trace conditioning (DPRTC) to show that new memories might be acquired during sleep as well. Prior research has shows that certain non-trigeminal odors—that is, odors that don’t stimulate the trigeminal nerve—can be presented during sleep without risk of waking the subject. Studies show that about 30% of odors fall under this category of olfactory stimulants. Odors presented to the nose during sleep are known to cause participants to sniff, the strength of which varies depending on the pleasantness of the odors. Very pleasant odors elicit a strong sniff response while unpleasant odors elicit a very weak sniff response. Therefore, DPRTC allows for a nonverbal implicit measure of brain processing.
Prior studies have show that participants can be conditioned to a tone while awake. Researchers recently attempted the same type of conditioning while participants were asleep. Researchers first asked waking participants to sample different odors, rating the degree of pleasantness in order to establish a baseline for each participant. They then instructed participants to fall asleep while measuring their brainwave activity using electroencephalography (EEG). This allowed the researchers to ensure participants were asleep. Participants were then exposed to different odors at the same time as the researchers played different tones. Participants’ degree of sniffing, or inspiration, was measured with an electronic device. Participants were then woken up and presented with the same tones they heard during sleep, except no odors were presented at the same time. Researchers found that the strength of inspiration in response to tones in the waking state correlated with the strength of inspiration caused by exposure to the corresponding odors they were exposed to during sleep, suggesting certain sorts of associations can be learned during sleep.
The average person spends 30% of their life in a supine position. Wouldn’t it be great if we could reinforce our memory while unconscious? Just think: you could pull an all-nighter every night! But we must be careful about this study’s conclusions, for we cannot say for sure whether the pleasantness of the smell is what triggered the association. There is an important distinction between the neurological response to an odor stimulus and the actual experience of that odor as pleasant. They’re two different things. It is possible that while unconscious, “pleasantness” is a meaningless concept—associations might be made merely through the co-activation of olfactory and auditory neurons.
Why is this distinction important for learning? In order to learn something like a foreign language in our sleep, we would need to remember not only the associations between sounds of words but also the connection between words and their meaning. Like with pleasantness, it’s possible that the sounds of words could be associated with one another without any recognition that their meaning is related. So although these study show exciting results, much more research must be conducted before we can say what sort of benefits sleep may have for learning.