After Freud lost his scientific credibility, psychology became very dismissive of dreams. The leading scientific theory held that dreams consisted of mental detritus, the scraps and fragments of memories that your brain didn’t want to remember. While Freud mined our nighttime thoughts for hidden meanings – they were symbol laden narratives of wish-fulfillment – this modern theory held that our dreams were entirely meaningless, a montage of hallucinations. They were the result of our hippocampus taking out the trash.
But dreams have been slowly making a comeback. There’s now a large body of evidence demonstrating that certain parts of the sleep cycle are an essential part of the learning process. While you are sleeping, your brain is carefully consolidating your knowledge, figuring out what you should remember and what you should forget.
That’s the larger theoretical context for this latest study of dreaming, which demonstrated that, even while we are sleeping, sensory stimuli (especially smelly stimuli) can affect what our brain chooses to remember.
Scientists studying how sleep affects memory have found that the whiff of a familiar scent can help a slumbering brain better remember things that it learned the evening before. The smell of roses — delivered to people’s nostrils as they studied and, later, as they slept — improved their performance on a memory test by about 13 percent.
In the study, neuroscientists from two German institutions, the University of Lübeck and the University Medical Center Hamburg-Eppendorf, had groups of medical students play a version of concentration, memorizing the location of card pairs on a computer screen. Upon learning the location of each pair, the students received a burst of rose scent in their noses through masks they wore. The researchers delivered the fragrance in bursts because the brain quickly adjusts to strong smells in the air and begins to ignore them.
The students went to sleep about a half-hour later, with electrodes on their heads tracking the depth of their slumber. Neuroscientists divide sleep into stages, including deep (or slow wave) sleep and the shallow, dream-rich state called rapid eye movement (or REM) sleep.
The brain is thought to process newly acquired facts, figures and locations most efficiently in deep sleep. This restful state usually descends within the first 20 minutes or so after head meets pillow and may last an hour or longer, then recur once or more later in the night. The researchers delivered pulses of rose bouquet during this slow-wave state; the odor did not interrupt sleep, and the students said they had no memory of it.
But their brains noticed, and retained an almost perfect memory of card locations. The students scored an average of 97 percent on the card game, compared with 86 percent when they played the game and slept without being perfumed by nighttime neuroscience fairies.
The students did not get the same boost when they received bursts of the fragrance just before sleep or in REM sleep rather than in deep slumber, and their improvements were not due to practice, the study found.
Just think about how much more Proust would have remembered if he’d smelled the madeleine in his sleep.