Even while smelling an odor at the same concentration, the course of its fading differed depending on whether people attended to the odor or counted sounds. A 2018 study found that initial intensity ratings fell faster when attention was divided with sounds. When participants focused on the odor, there were also later periods when it became more noticeable again.
The study examined how the focus of attention relates to becoming accustomed to an odor. Keeping the odor reaching the nose as constant as possible, the researchers tracked changes in perceived intensity for 10 minutes.
The same 19 people focused on odor one day and counted sounds another
Nineteen people who passed a smell test attended twice, one week apart. The stimulus was a pleasant blended fragrance with floral and green notes. An olfactometer delivered it near the nose at a constant flow rate. The odor was presented for 10 minutes, with one minute of odorless air before and after.
On both days, participants continuously rated odor intensity with a slider. The accompanying sounds were also identical in both conditions. On one day, however, they were told to ignore the sounds and focus on the odor; on the other, they counted occasional higher-pitched tones. The order of conditions was balanced across participants.
The design thus compared focusing on the odor with performing an auditory task while continuing to rate it. It did not attract attention by making the odor itself stronger or weaker.
During the first minute, dividing attention with sounds made the odor fade faster
In both conditions, ratings reached their maximum within about 10 seconds of odor onset, then declined over roughly two minutes. The initial maximum intensity did not differ clearly between conditions in statistical terms.
The difference emerged in the subsequent decline. Mean ratings from 15 to 65 seconds after onset were 3.12 in the odor-focused condition and 2.02 while counting sounds. An analysis correcting for the many time-point comparisons identified intervals with differences, and interval averages were then calculated from those same data.
When curves were fitted to the group means, a straight line representing a relatively steady decline fitted the odor-focused condition well, while an exponential function with a steep initial fall fitted the divided-attention condition. These describe the average time course, not a demonstration that every person followed the same curve.
After fading, the odor could become noticeable again in waves
The changes did not end with the initial decline. When participants focused on the odor, there were further periods around five minutes and around seven minutes after onset when it was perceived as stronger than while counting sounds. The reported intervals were 290–340 seconds and 420–470 seconds.
The researchers also analyzed fluctuations in intensity ratings. Slow fluctuations corresponding to cycles of about 65–125 seconds were larger when participants focused on the odor than when they divided their attention with sounds. The data analyzed here were changes in slider-rated intensity, not rhythms measured in brain waves.
The authors viewed olfactory desensitization not simply as a process of continuously declining responses, but as a dynamic process in which attention can bring odor perception back to the foreground. Which neural activity underlies this perceptual switching remains a question for future research.
Breathing and task demands also matter for interpretation
The study also recorded breathing and found no difference in respiratory rate between conditions over the reported comparison intervals. However, the association between fluctuations in respiratory rate and odor ratings was stronger when attention was divided with sounds.
There is a mismatch in the paper between the later comparison intervals used for breathing and the intervals reported to differ in intensity ratings. The results therefore do not rule out every effect of breathing. What was established was the absence of a clear respiratory-rate difference in the reported intervals.
Counting sounds also increases the demands of a simultaneous task, rather than changing only the focus of attention. The authors likewise noted the need to investigate cognitive load and working memory further. This experiment involved 19 people and one pleasant odor; whether unpleasant or strongly irritating odors follow the same course requires separate testing.
Summary
When participants focused on an odor, its initial decline in perceived intensity was slower than when they counted sounds while smelling it, and there were later periods when it became more noticeable again. Slow rating fluctuations of about one to two minutes were also larger in the focused condition.
Becoming accustomed to a constant odor relates not only to the stimulus reaching the nose but also to how attention is directed. That is the study’s central finding. It shows that, when evaluating an odor, what people attend to and what other tasks they are doing can matter.
The study was supported by Unilever, and one author was affiliated with its research and development division at the time of publication. No separate competing-interest statement was found.
Reference
Fallon, N., Giesbrecht, T., & Stancak, A. (2018). Attentional modulation of desensitization to odor. Attention, Perception, & Psychophysics, 80, 1064–1071. https://doi.org/10.3758/s13414-018-1539-2 (Open original-paper PDF)
