A medical student playing a version of concentration, memorizing the location of card pairs on a computer screen.
The New York Times
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. A rose bouquet — delivered to people’s nostrils as they studied and, later, as they slept — improved their performance on a memory test by almost 15 percent.
The new study, appearing Friday in the journal Science, is the first rigorous test of odor on human memory during sleep. The results — whether or not they can help students cram for tests — clarify the picture of what the sleeping brain does with newly studied material, and of what it takes for this process to succeed.
Researchers have long known that sleep is crucial to laying down new memories, and studies in the 1980s and ’90s showed that exposing the sleeping brain to cues associated with learning — the sound of clicking, for instance — could enhance the process. But it is only in recent years that scientists have begun to understand how this is possible.
“The idea didn’t get any traction with scientists back then, because it didn’t make sense,” said Dr. Robert Stickgold, a psychiatrist at Harvard, who was not involved in the research. He added that the new study “shows not only that sleep is important for declarative memory, but also allows us to look at exactly when and how this process might happen.”
In the study, neuroscientists from 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 a mask they wore. The researchers delivered the fragrance in bursts because the nose 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 head 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 sleep, or R.E.M.
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 it may last an hour or more, then recur 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 they 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 concentration game and slept without being perfumed by nighttime neuroscience faeries.
The students did not get the same boost when they received bursts of the fragrance before falling asleep, and their improvements were not a result of practice, the study found.
Previous research has shown that regions of the cortex, the thinking and planning part of the brain, communicate during deep sleep with a sliver of tissue deeper in the brain called the hippocampus, which records each day’s memories. What is most likely happening, the study’s authors argue, is that the cortex is reactivating the same set of neurons that fired when a particular fact was noticed or learned. The hippocampus then encodes that firing sequence back in the cortex, consolidating the memory.
“We would expect spontaneous reactivation driven by the slow-wave sleep, but by presenting the rose odor cues we intensified this activation and enhanced the transfer of these memories into the neocortex,” said Dr. Jan Born, a neuroscientist at Lübeck and an author of the study. His co-authors were Bjorn Rasch, Christian Büchel and Steffen Gais.
Olfactory sensing pathways in the brain lead more directly to the hippocampus than visual and auditory ones. That may be why smells can be linked so closely to memory, and may revive forgotten joys, humiliations and other remembrances of things past.
To check their reasoning, the researchers took M.R.I. images of some of the students’ brains during their rose-scented slumber. As predicted, regions of the cortex became noticeably more active, as did the hippocampus.
The findings suggest that distinct sleep states may be specialized to integrate different kinds of information. The researchers found, for example, that the rose scent did not enhance memories of a learned, finger-tapping sequence — a rhythmic memory that does not appear to be consolidated by the hippocampus.
Likewise, rose fragrance during R.E.M. sleep made no difference to the students’ scores. It may be that the hues, horrors and hilarity of dreams during R.E.M. reflect the brain’s efforts to digest and integrate emotional, rather than factual, memories, Dr. Stickgold said.
“Extracting patterns and rules and what we call the gist of a memory might turn out to be antithetical to the process of nailing down the facts themselves,” Dr. Stickgold said. “So, for instance, you might use R.E.M. to integrate one, and slow-wave sleep for the other.”
The new findings do not close the book on how memories are formed and consolidated during sleep. Other scientists have found evidence that rather than reactivation, the brain’s slow-wave state induces an overall weakening of neuron-to-neuron signaling, making recently recorded memories look bolder by reducing the background neural “noise.” And it may be, Dr. Born said, that both processes are occurring during sleep: a pruning away of the noise of the day’s irrelevant observations, and a replaying of its important ones.
Either way, the researchers said, the new findings are likely to prompt some creative thinking on the part of students facing the terror of final exams. The German research group has preliminary evidence that acrid smells might be even better in enhancing memory.
“We use an apparatus to sense the onset of slow-wave sleep and deliver the odor” in short, alternating bursts, Dr. Born said, adding, “I suppose for some students it would not be too difficult to develop something like this.”
That’s what engineering departments are for. http://www.nytimes.com/2007/03/08/science/08cnd-sleep.html?em&ex=1173589200&en=ca355ce84aa95616&ei=5070
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