Inhaling a pleasant aroma during sleep has been linked to a "dramatic" improvement in memory, early research suggests.
In
a small, randomized control trial researchers found that when
cognitively normal individuals were exposed to the scent of an essential
oil for 2 hours every night over 6 months, they experienced a 226%
improvement in memory compared with a control group who received only a
trace amount of the diffused scent.
In addition, functional
magnetic resonance imaging (fMRI) showed that those in the enriched
group had improved functioning of the left uncinate fasciculus, an area
of the brain linked to memory and cognition, which typically declines
with age.
"To my knowledge, that level of [memory] improvement is
far greater than anything that has been reported for healthy older
adults and we also found a critical memory pathway in their brains
improved to a similar extent relative to unenriched older adults,"
senior investigator Michael Leon, PhD, professor emeritus, University of
California, Irvine, told Medscape Medical News.
The study was published online July 24 in Frontiers of Neuroscience.
The Brain's "Superhighway"
Olfactory
enrichment "involves the daily exposure of individuals to multiple
odorants" and has been shown in mouse models to improve memory and
neurogenesis, the investigators note.
A previous study showed that
exposure to individual essential oils for 30 minutes a day over 3
months induced neurogenesis in the olfactory bulb and the hippocampus.
"The olfactory system is the only sense that has a direct
'superhighway' input to the memory centers areas of the brain; all the
other senses have to reach those brain areas through what you might call
the 'side streets' of the brain, and so consequently, they have much
less impact on maintaining the health of those memory centers."
When
olfaction is compromised, "the memory centers of the brain start to
deteriorate and, conversely, when people are given olfactory enrichment,
their memory areas become larger and more functional," he added.
Olfactory dysfunction is the first symptom of Alzheimer's disease (AD) and is also found in virtually all neurological and psychiatric disorders.
"I've counted 68 of them — including anorexia, anxiety, [attention-deficit/hyperactivity disorder], depression, epilepsy and stroke. In fact, by mid-life, your all-cause mortality can be predicted by your ability to smell things," Leon said.
Leon and colleagues previously developed an effective treatment for autism
using environmental enrichment that focused on odor stimulation, along
with stimulating other senses. "We then considered the possibility that
olfactory enrichment alone might improve brain function."
Rose, Orange, Eucalyptus…
For
the study, the researchers randomly assigned 43 older adults, aged 60 -
85 years, to receive either nightly exposure to essential oil scents
delivered via a diffuser (n = 20; mean [SD] age, 70.1 [6.6] years) or to
a sham control with only trace amounts of odorants (n = 23; mean age,
69.2 [7.1] years) for a period of 6 months.
The
intervention group was exposed to a single odorant, delivered through a
diffuser, for 2 hours nightly, rotating through seven pleasant aromas
each week. They included rose, orange, eucalyptus, lemon, peppermint, rosemary, and lavender scents.
All
participants completed a battery of tests at baseline, including the
Mini-Mental State Examination (MMSE), which confirmed normal cognitive
functioning. At baseline and after a 6-month follow-up, participants
completed the Rey Auditory Verbal Learning Test (RAVLT) as well as three
subsets of the Wechsler Adult Intelligence Scale–Third Edition
(WAIS-III).
Olfactory system function was assessed using "Sniffin Sticks," allowing the researchers to determine if olfactory enrichment enhanced olfactory performance.
Participants underwent fMRI at baseline and again at 6 months.
Brain
imaging results showed a "clear, statistically significant 226%
difference between enriched and control older adults in performance on
the RAVLT, which evaluates learning and memory (timepoint × group
interaction; F = 6.63; P = .02; Cohen's d = 1.08; a "large effect size").
They
also found a significant change in the mean diffusivity of the left
uncinate fasciculus in the enriched group compared with the controls
(timepoint × group interaction; F = 4.39; P = .043; h 2 p = .101; a "medium-size effect").
The
uncinate fasciculus is a "major pathway" connecting the basolateral
amygdala and the entorhinal cortex to the prefrontal cortex. This
pathway deteriorates in aging and in AD and "has been suggested to play a
role in mediating episodic memory, language, socio-emotional
processing, and selecting among competing memories during retrieval."
No significant differences were found between the groups in olfactory ability.
Limitations
of the study include its small sample size. The investigators hope the
findings will "stimulate larger scale clinical trials systematically
testing the therapeutic efficacy of olfactory enrichment in treating
memory loss in older adults."
Exciting but Preliminary
Commenting for Medscape Medical News,
Donald Wilson, PhD, professor of child and adolescent psychiatry and of
neuroscience and physiology, the Child Study Center, NYU Langone
Medical Center, New York, said that multiple studies have "demonstrated
that problems with sense of smell are associated with and sometimes can
precede other symptoms for many disorders, including AD, Parkinson's disease, and depression."
Recent
work has suggested that this relationship can be "bidirectional" — for
example, losing one's sense of smell might promote depression, while
depressive disorder might lead to impaired smell, according to Wilson,
also director and senior research scientist, the Emotional Brain
Institute, Nathan Kline Institute for Psychiatric Research, who wasn't
involved with the study.
This "two-way interaction" may raise the possibility that "improving olfaction could impact non-olfactory disorders."
This
paper "brings together" previous research findings to show that odors
during bedtime can improve some aspects of cognitive function and
circuits that are known to be important for memory and cognition — which
Wilson called "a very exciting, though relatively preliminary,
finding."
A caveat is that several measures of cognitive function were assessed and only one (verbal memory) showed clear improvement.
Nevertheless,
there's "very strong interest now in the olfactory and nonolfactory
aspects of odor training and this training expands the training
possibilities to sleep. This could be a powerful tool for cognitive
improvement and/or rescue if follow-up studies support these findings,"
Wilson said.
Front Neurosci. Published online July 24, 2023. Full text