The flow of air is greater into one nostril than into the other because there is a slight turbinate swelling in one1,2,3. The nostril that takes in more air switches from the left to the right one and back again every few hours4,5,6, but the effect of this switching on the sense of smell has been unclear7,8. Here we show that this difference in airflow between the nostrils causes each nostril to be optimally sensitized to different odorants, so that each nostril conveys a slightly different olfactory image to the brain.
The slight swelling that obstructs each nostril (Fig. 1a) causes odorants to be drawn into the nostrils at different rates. But for an odorant to act on the olfactory receptors, it must first cross the olfactory mucosa. Different odorants sorb to and cross the mucosa at different rates9. In the bullfrog, for example, a specific odorant's sorption rate interacts with the rate of airflow across the mucosa to produce varying amplitudes of response in the olfactory nerve10. A high-sorption odorant induces a smaller response when airflow is low and a larger one when it increases. In contrast, a low-sorption odorant induces a smaller response at a high airflow rate and a larger response when there is less airflow (Fig. 1b).
This occurs because, when a high-sorption odorant has a low airflow rate, the odorant molecules sorb to the mucosa before moving very far along it. Only a small portion of the epithelium is involved in the response, which is small. When the same odorant flows at a high airflow rate, it spreads across a larger mucosal area before sorbing, so the response is larger. When a low-sorption odorant flows quickly, it moves past the mucosa without sorbing so the epithelial response is small. When the same low-sorption-odorant flows slowly, it has time to sorb across the mucosa and the response is larger10.
We therefore investigated whether the nostril with the higher airflow in humans is the more sensitive to high-sorption odorants and the nostril with lower airflow more sensitive to low-sorption odorants. We used an olfactometer to produce an equally proportioned mixture of the high-sorption odorant L-carvone and the low-sorption odorant octane. The mixture was always the same but subjects were told that it was slightly different for every trial.
Subjects sampled the mixture by sniffing with one nostril (the other nostril was occluded) and made a judgement about the relative proportion of the two components in the mixture (for example, 55% octane and 45% L-carvone). The task was repeated for the second nostril and the judgements compared. The rate of airflow for each sniff was measured by anterior rhinomenometry. We found that 17 of 20 subjects (binomial, P=0.001) thought the mixture contained more octane when they used the low-airflow nostril, and more L-carvone when they used the high-airflow nostril (Fig. 1c).
The nostril with the higher airflow reverses periodically4,5,6, so we tested eight subjects after the nostril with greater airflow had switched, and found that the perception of the same mixture reversed in seven of the eight subjects (binomial, P=0.035). Odorant perception was therefore dependent on airflow rate, not on whether the odorant was smelled by the left or right nostril.
The different airflow between the nostrils results in a disparity of olfactory perception. Providing the olfactory system with two disparate images of the olfactory world with each sniff in this way may improve olfactory acuity by expanding the range of odorants that are within optimal sensitivity in a given sniff.
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Sobel, N., Khan, R., Saltman, A. et al. The world smells different to each nostril. Nature 402, 35 (1999). https://doi.org/10.1038/46944
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