Do Fragrance Experiments Change Fatigue and Accuracy in the Same Way?

Even when people perform better on a task in an experiment involving fragrance, they do not necessarily feel less tired. A 2018 study used olfactory receptor responses to develop a new odorant mixture and presented it during a fatigue-inducing task. Although favorable differences were reported in changes in some accuracy measures, no improvement was confirmed in subjective fatigue or reaction time.

However, the original paper contains inconsistent statistical results for its central accuracy finding. The study is interesting not only for its performance results, but also for asking whether different-smelling substances might have similar effects if they stimulate the same receptors.

Stimulating the receptors for a green-leaf scent with different molecules

The researchers started with Hex-Hex Mix, a mixture of cis-3-hexenol and trans-2-hexenal with a green-leaf odor. Earlier research had reported effects on performance under fatigue and other outcomes, prompting them to investigate whether receptors in the nose were involved.

First, they tested 392 human olfactory receptors in cultured cells and identified six that responded to the mixture. They then screened 177 fragrance materials for responses in the same receptor group and developed a four-component odorant mixture called MCMP.

MCMP consists of methyl β-naphthyl ketone, l-carvone, methyl isoeugenol, and phenylethyl acetate. Unlike the original green-leaf mixture, it is described as having a honey-like floral odor.

In the cell experiments, MCMP stimulated ten receptors in total, including all six stimulated by the original mixture. The mixtures therefore shared some receptor responses, but their response patterns were not identical.

Seventeen participants experienced both the fragrance and water-only conditions

Seventeen healthy men participated in the human trial. On two consecutive days, each experienced the MCMP condition and a condition in which the same equipment delivered water alone, with the order randomly assigned.

Fatigue was induced using a 40-minute 2-back task. Participants repeatedly judged whether a letter appearing on a screen matched the letter shown two presentations earlier. During this task, MCMP was delivered using a diffuser and periodic spraying. The odor was detectable: this was not an experiment with subliminal stimulation.

Immediately before and after the task, participants completed a separate nine-minute cognitive task, rated their subjective fatigue, and underwent measurement of autonomic-related indicators using a fingertip pulse waveform. The main cognitive task required responses based on the meanings of the words “red” and “blue,” whether or not those meanings matched the color of a traffic-light signal.

Favorable accuracy results, but inconsistent statistical reporting

The researchers highlighted the pre-to-post change in accuracy on trials in which the color and word matched. The figure showed a more favorable change under MCMP than under water alone, and a test comparing the changes between conditions was reported as significant even after correction for multiple comparisons.

However, an analysis of the condition-by-time interaction for the same accuracy measure was reported as nonsignificant, with p = .40. Although both analyses ask whether the pre-to-post change differs between conditions, the change-score test was significant at t = 2.93, whereas the interaction was nonsignificant at F = 0.86. These values are difficult to reconcile.

It is therefore reasonable to describe the favorable accuracy pattern that was reported, but the analyses need to be checked before concluding strongly that the effect is statistically established. A similar performance improvement was not confirmed on trials in which the color and word did not match.

Neither feeling tired nor reaction time improved

Subjective fatigue increased after the 40-minute task regardless of fragrance exposure. There was no between-condition difference showing that MCMP reduced feelings of fatigue.

There was also no significant fragrance effect on cognitive-task reaction time, and the pulse-wave-derived indicators showed no clear changes indicating improvement. The authors suggest that, if MCMP has an effect, it may appear in performance on a particular attention task rather than in subjective fatigue or broader bodily responses.

The remaining gap between receptors and human functioning

The novel aspect of this study was designing a different odorant mixture using receptor responses rather than trying to imitate an odor’s perceptual character. However, the human trial compared MCMP with water; it did not include a simultaneous comparison with the original Hex-Hex Mix.

The design also cannot establish whether any of the six shared receptors contributed to performance or whether the four additional receptors stimulated by MCMP were involved. There was no control condition using another odor, so the effects of expectations about an odor’s presence or its pleasantness were not separated. Nor did the study directly measure brain activity.

Given the short experiment with 17 men and the inconsistency in the accuracy statistics, this work should be viewed as an exploratory attempt at receptor-based fragrance design, not as an established approach to fatigue recovery or a treatment for chronic fatigue.

Summary

Using six olfactory receptors responsive to a green-leaf mixture as a guide, the researchers developed the differently scented MCMP and confirmed responses in ten receptors, including those six. In the human trial, changes in accuracy on a color-word congruent task were reported as more favorable than under water alone, but inconsistent statistical reporting remains for that result.

No improvement was confirmed in subjective fatigue, reaction time, or autonomic-related indicators. The study illustrates why receptor responses to odors, task performance, and the experience of tiredness need to be examined separately.

The study disclosed support from Kao and stated that two authors were company employees at the time of the research. It also listed Kao’s related patent, P6283579. The paper states that the funder had no role in study design, analysis, or publication decisions, among other activities, and that the tested odorant mixture was not a marketed Kao product.

Reference

Saito N, et al. (2018). Involvement of the olfactory system in the induction of anti-fatigue effects by odorants. PLOS ONE, 13(3), e0195263. Original paper and open full text