Study Uncovers Memory Aid: A Scent During Sleep

March 9, 2007

Study Uncovers Memory Aid: A Scent During Sleep
By _BENEDICT CAREY_
(http://topics.nytimes.com/top/reference/timestopics/people/c/benedict_carey/index.html?inline=nyt-per)

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.

The new study, appearing today in the journal Science, is the first rigorous
test of the effect 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 learned material and help illuminate 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 certain cues — 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, an associate professor of
psychiatry at Harvard, who was not involved in the research. The new study, Dr.
Stickgold added, “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 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.
The study’s results could eventually help doctors improve patients’ memory
by devising treatments directed at deep sleep. As they age, people spend less
and less time each night in such sleep, and existing sleep medications do
not generally increase it. But pharmaceutical companies are investigating
compounds that do so.

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 in that communication, the study’s authors
argue, is that the cortex is telling the hippocampus to reactivate the same
neurons that fired when a particular fact was noticed or learned. The
hippocampus does so, encoding the firing sequence in the cortex and thereby
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,” said Dr. Jan Born, a neuroscientist at
Lübeck who undertook the study with Björn Rasch, Christian Büchel and Steffen
Gais.

Olfactory sensing pathways in the brain lead more directly to the
hippocampus than do visual and auditory ones. That may be why smells can so vividly
revive things past like forgotten joys or humiliations.

To check their reasoning, the researchers took M.R.I. images of some of the
students’ brains during their rose-scented slumber. As expected, 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. For example, the researchers found 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, given that the rose fragrance during REM sleep made no difference
to the students’ scores, it may be that the hues, horrors and hilarity of
dreams during REM reflect the brain’s efforts to integrate emotional, rather
than factual, memories, said Dr. Stickgold, of Harvard.

“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 REM to integrate
one, and slow-wave sleep for the other.”

The new findings hardly 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 may 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.