Humans can discriminate more than 1 trillion olfactory stimuli

Abstract

Humans can discriminate several million different colors and almost half a million different tones, but the number of discriminable olfactory stimuli remains unknown. The lay and scientific literature typically claims that humans can discriminate 10,000 odors, but this number has never been empirically validated. We determined the resolution of the human sense of smell by testing the capacity of humans to discriminate odor mixtures with varying numbers of shared components. On the basis of the results of psychophysical testing, we calculated that humans can discriminate at least 1 trillion olfactory stimuli. This is far more than previous estimates of distinguishable olfactory stimuli. It demonstrates that the human olfactory system, with its hundreds of different olfactory receptors, far outperforms the other senses in the number of physically different stimuli it can discriminate.

Fig. 1

Odor mixtures used to test the resolution of the human olfactory system. (A) Illustration of sample mixtures with exactly 10, 20, or 30 components (green squares) picked from a collection of 128 odorous molecules (gray squares) and the number of possible mixtures of each type. (B) Example of one mixture pair. (C-E) Schematics of each of the 13 types of odor pairs used for discrimination tests along with the total number of possible mixture pairs of each type.

Fig. 2

An empirical investigation of the resolution of the human olfactory system. (A) Schematic of the discrimination tests carried out for mixtures of 10, 20, or 30 odorous molecules. (B-C) Results of discrimination tests with 26 subjects asked to discriminate mixtures of 10 (left), 20 (middle), or 30 (right) components with decreasing overlap from left to right. The dotted line represents the chance detection level (33.3%). For (B), dots represent performance of individual subjects across 20 mixture pairs. For (C), dots represent average performance of all 26 subjects for a given mixture pair. Statistically significant discriminability (red dots) was assessed with a Chi-square test; P value<0.05.

Fig. 3

The number of discriminable olfactory stimuli. (A) Discrimination capacity of subjects according to % mixture overlap. (B) Discriminability of mixture pairs according to % mixture overlap. (C) Extrapolation of the number of discriminable mixtures derived from A. (D) Extrapolation of the number of discriminable mixtures derived from B. (E) Summary of discriminable sensory stimuli across sensory modalities. Data are curated from the following sources: smells (previous estimates) (5-7), tones (3), and colors (1, 2).