There is a quality to smell that the other senses do not quite share. A sound reaches you and you identify it. A sight enters your field of vision and you place it. But a scent can arrive and pull you, without warning and without apparent reason, into a room you have not thought about in thirty years. The mechanism behind this is anatomical, and it is well established: olfactory signals take a route through the brain that other sensory signals do not. They bypass the thalamus, the relay station through which vision, hearing, and touch pass before reaching higher cortical regions, and project directly to the amygdala and the hippocampal complex, the brain structures most closely associated with memory and emotional encoding. The pathway is not metaphorical. It is structural.
What a team of neuroscientists at the University of California, Irvine wanted to know was whether that structural proximity could be put to use. Specifically: could passively exposing older adults to varied scents while they slept improve cognitive performance? Their answer, published in Frontiers in Neuroscience in July 2023, was cautiously yes. This is one study, not settled consensus, and the figure at the centre of its coverage requires careful unpacking before it can be used honestly.
What the study actually did
The paper, led by project scientist Cynthia Woo and co-authored by Michael Yassa and Michael Leon at UCI’s Center for the Neurobiology of Learning and Memory, enrolled 43 men and women between the ages of 60 and 85, all with no prior diagnosis of cognitive impairment. Participants were randomly assigned to one of two groups. Those in the enriched group received a diffuser and seven cartridges containing individual natural essential oils, rotating through one scent per night over the course of a week: eucalyptus, lavender, lemon, orange, peppermint, rose, and rosemary. The diffuser ran for two hours during sleep. The control group received the same diffuser setup but with only trace amounts of odorant, insufficient to produce a meaningful sensory experience. Neither group was asked to perform any cognitive training or to change their behaviour in any other way.
At the start and again after six months, all participants completed a battery of neuropsychological assessments. They also underwent fMRI scans. The primary cognitive outcome was performance on the Rey Auditory Verbal Learning Test (RAVLT), a standard instrument that evaluates verbal learning, recall, and recognition using a list of words read aloud across multiple trials.
The study was funded by Procter & Gamble. Two of the senior authors, Leon and Yassa, have received travel expenses and compensation in connection with presentations at the company. The paper discloses this. It does not make the findings invalid, but readers should know the funding source, particularly as a commercial product based on the study was being anticipated for home use at the time of publication.
What the 226% figure does and does not mean
The headline number is real, in the sense that it appears in the paper and is statistically significant (p = 0.02, Cohen’s d = 1.08, which the authors characterise as a large effect size). But it describes something more specific than most of the coverage suggested, and the way it is often reported can mislead.
The 226 per cent is the proportional difference between the two groups in their change scores on the last learning trial of the RAVLT (the A5 subscale) over the six-month period. It is not a measure of how much any individual improved from their own baseline. Nor is it a measure of improvement across the full cognitive battery.
Critically, both groups shifted over the six months, in opposite directions. In the control group, 7 of 11 participants scored worse at the end of the study than at the beginning. In the enriched group, 6 of 12 improved and 5 stayed roughly the same. So the large percentage difference is partly a product of the enriched group holding steady or improving, while the control group declined. The researchers are measuring a difference in trajectory, not a lift from a single fixed baseline. This is an important and legitimate finding, but “226 per cent improvement in cognitive capacity” as a summary does not quite capture it.
What the study does appear to show, on this limited dataset of 43 participants, is that passive nightly olfactory enrichment may help preserve verbal memory performance in healthy older adults over six months, at a time when some natural decline would otherwise be expected. The finding is worth taking seriously, but it should not be read as the final word, and the finding belongs to this sample, this test, and this six-month window.
The uncinate fasciculus, and what the imaging showed
The paper’s second notable finding is structural. fMRI data showed that the enriched group had better integrity in a brain pathway called the left uncinate fasciculus, as measured by mean diffusivity, compared with the control group. The uncinate fasciculus connects the medial temporal lobe, which includes key memory structures, to the prefrontal cortex, which is involved in decision-making and working memory. This pathway is known to deteriorate with age, and its degradation has been associated with Alzheimer’s disease progression.
The implication the authors draw is that olfactory enrichment may not only be correlated with better memory performance on tests, but may also be associated with measurable changes in the relevant neural architecture. This is a hypothesis worth pursuing. But the imaging component of the study involved a subset of the already small sample, and mean diffusivity as an fMRI measure reflects changes in white matter microstructure that do not straightforwardly translate into clinical outcomes. The authors are careful here, describing the result as a pointer toward mechanism rather than a confirmed explanation.
Why smell occupies this particular position in the brain
The anatomical argument behind olfactory enrichment rests on a well-documented feature of how smell is processed. Unlike vision, hearing, touch, and taste, which all pass through the thalamus before reaching the cortex, olfactory signals travel from the olfactory bulb directly to primary olfactory cortex and then to limbic structures including the amygdala and the entorhinal cortex, which feeds into the hippocampus. The hippocampus is the structure most closely associated with encoding and retrieving episodic memories.
This is not a new observation. The connection has been discussed in neuroscience for decades and is the proposed basis for what is sometimes called the Proustian effect, the well-documented tendency for odours to trigger autobiographical memories with unusual vividness and emotional weight. What the UCI team was testing was whether that proximity could be turned into an intervention: not simply that smell evokes memory, but that sustained olfactory stimulation might support the health of the memory system itself.
The broader rationale draws on animal research showing that environmental enrichment, typically achieved in rodents by giving them varied stimuli, larger spaces, and conspecific company, reliably improves memory performance and promotes neuroplasticity. The question was whether a simplified, passive version of olfactory enrichment might produce analogous effects in humans, at a scale that would be practical for older adults living at home.
The olfactory deficit hypothesis, and what it adds
The UCI team’s work sits within a wider research context that the study’s coverage has not always acknowledged. The loss of olfactory function is one of the earlier detectable signs of several neurodegenerative conditions, including Alzheimer’s and Parkinson’s disease, typically predating cognitive symptoms by years. This association has been documented across a body of research and is not a claim specific to this group.
Leon and colleagues have proposed a complementary hypothesis: that modern living, particularly in urban environments with reduced exposure to varied natural odours, may represent a form of chronic olfactory underuse, and that this underuse may contribute to cognitive vulnerability in ageing. The phrasing Michael Leon used in public commentary after the study’s publication captured the claim: the human brain evolved at a time when olfactory stimulation was abundant and unavoidable, and what we experience now may be a significant departure from that norm. This is a hypothesis, not an established fact, and the study does not directly test it. But it provides the intellectual frame within which the intervention was designed.
A separate 2024 paper from the same UCI group, published in Frontiers in Molecular Neuroscience, surveyed 139 medical conditions associated with both olfactory loss and elevated inflammation, proposing that anti-inflammatory pathways activated by pleasant scents may be one mechanism by which olfactory enrichment supports brain health. This is speculative science at this stage, but it opens a mechanistic direction beyond simple neural stimulation.
What the study cannot settle
The sample was 43 people, further reduced in the fMRI analysis. Both groups were cognitively healthy at baseline, so the findings say nothing about whether olfactory enrichment helps people who already have diagnosed cognitive impairment. The study excluded participants with asthma or smell-related allergies, which is a reasonable methodological constraint but limits generalisation. Several participants were excluded to reduce confounding from the COVID-19 pandemic, which further shrank the analysed groups. The Yale Scientific noted the small sample size among the concerns raised after publication.
The study also did not test which, if any, of the seven scents was driving the effect. The authors’ prior research and the broader olfactory enrichment literature suggest that variety matters more than the specific identity of any given odour, but this remains an open question in the human context. And the study ran for six months with cognitively normal participants; whether effects persist or accumulate over longer periods is unknown.
There is also the question of the control condition. Participants in both groups had a diffuser running while they slept. Whether any performance difference could be explained by sleep quality changes, rather than olfactory stimulation specifically, is not fully resolved. The paper notes that enriched participants also reported sleeping more soundly, which adds an interesting complication. Sleep quality is itself associated with memory consolidation, and the relationship between the olfactory stimulation and the sleep improvement is not cleanly separated in the design.
What to watch next
The UCI team has indicated that follow-up work will focus on people with diagnosed cognitive loss, which is where the intervention would have the most practical relevance. Replication in larger samples by independent groups would substantially strengthen the picture. The broader olfactory enrichment field is active enough that the UCI finding is unlikely to remain the sole data point for long: a 2025 paper in the journal Alzheimer’s & Dementia from researchers at Deakin University in Australia tested olfactory memory training in older adults with subjective cognitive decline, finding mixed results that suggest the picture will be more complicated than a single study implies.
What the Woo et al. paper does establish is a plausible and well-framed direction for further inquiry. The olfactory pathway’s proximity to the hippocampal complex is anatomically real. The effect size in this study is large by conventional standards. The intervention is low-cost and imposes minimal burden. Whether all of that translates into a robust and generalisable intervention for cognitive decline in ageing requires considerably more evidence than one peer-reviewed trial of 43 people can provide.
