Scientific Understanding of Consciousness
Olfactory Stimuli Discrimination
Science 21 March 2014: Vol. 343 no. 6177 pp. 1370-1372
Humans Can Discriminate More than 1 Trillion Olfactory Stimuli
C. Bushdid, et.al.
Laboratory of Neurogenetics and Behavior, The Rockefeller University, 1230 York Avenue, Box 63, New York, NY 10065, USA.
Laboratory of Mathematical Physics, The Rockefeller University, 1230 York Avenue, Box 212, New York, NY 10065, USA.
Howard Hughes Medical Institute, 1230 York Avenue, Post Office Box 63, New York, NY 10065, USA.
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.
To determine how many stimuli can be discriminated, one must know the range and resolution of the sensory system. Color stimuli vary in wavelength and intensity. Tones vary in frequency and loudness. We can therefore determine the resolution of these modalities along those axes and then calculate the number of discriminable tones and colors from the range and resolution. Humans can detect light with a wavelength between 390 and 700 nm and tones in the frequency range between 20 and 20,000 Hz. Working within this range, researchers carried out psychophysical experiments with color or tone discrimination tasks in order to estimate the average resolution of the visual and auditory systems. From these experiments, they estimated that humans can distinguish between 2.3 million and 7.5 million colors and ~340,000 tones. In the olfactory system, it is more difficult to estimate the range and resolution because the dimensions and physical boundaries of the olfactory stimulus space are not known. Further, olfactory stimuli are typically mixtures of odor molecules that differ in their components. Therefore, the strategies used for other sensory modalities cannot be applied to the human olfactory system. In the absence of a straightforward empirical approach, theoretical considerations have been used to estimate the number of discriminable olfactory stimuli.
Natural olfactory stimuli are almost always mixtures of large numbers of diverse components at different ratios. The characteristic scent of a rose, for example, is produced by a mixture of 275 components, although typically, only a small percentage of components contribute to the perceived smell. We reduced the complexity by investigating only mixtures of 10, 20, or 30 components drawn from a collection of 128 odorous molecules.
To generate each mixture, we combined these components together at equal ratios. The 128 components can be combined into 2.27 × 1014 different mixtures of exactly 10, 1.20 × 1023 different mixtures of exactly 20, and 1.54 × 1029 different mixtures of exactly 30. The most salient difference between two mixtures with the same number of components is the percentage of components in which they differ. We therefore performed psychophysical testing to determine the resolution of the human olfactory system along this axis. We asked by what percentage two mixtures must differ on average so that they can be discriminated by the average human nose. This percentage difference in components is the resolution of the olfactory system.
Subjects performed forced-choice discrimination tests to determine the discriminability of pairs of mixtures (referred to here as “mixture A” and “mixture B”) that varied in the percentage of shared components.
Pairs of mixtures are more difficult to distinguish the more they overlap. At least half of the tested subjects could discriminate mixture pairs that overlapped by less than 75% of their components. Some could also discriminate mixture pairs that overlapped by 75 and 90%, but none could discriminate mixture pairs with more than 90% overlap.
The resolution of the visual and auditory system is defined as the difference in frequency between two stimuli that is required for reliable discrimination. In the olfactory system, resolution can be defined as the highest percentage overlap in components between two mixtures at which those mixtures can be distinguished.
In hearing, for example, frequency resolution is much better at low than at high frequencies. In vision, wavelength discrimination is best near 560 nm, at which a difference in wavelength of 0.2 nm can be discriminated under optimal conditions.
Our results show that there are several orders of magnitude more discriminable olfactory stimuli than colors or tones. Colors are spatially arranged to create a large number of visual objects that are the building blocks of visual experiences, and tones can be combined to form a large number of chords that form auditory objects. The number of visual and auditory objects is much larger than the number of colors and tones, but it is unknown how many of these objects humans can discriminate.
The difference between the number of discriminable olfactory stimuli and colors or tones is even larger if one considers that the number of discernible tones and colors includes stimuli that differ in loudness or brightness. We focused here only on stimulus quality and ruled out intensity-based discrimination.
Our estimate of 1.72 × 1012 discriminable olfactory stimuli is a conservative one yet is still several orders of magnitude higher than previous estimates. The actual number of distinguishable olfactory stimuli is likely to be even higher than 1.72 × 1012 for three reasons. First, there are considerably more possible odorous molecules than the 128 different components that we used. Second, components can be combined in mixtures of more than 30 components. Third, even mixtures with the same components can be distinguished if the components are mixed at different ratios. Our results therefore establish only a lower limit of the number of discriminable olfactory stimuli.This lower limit of greater than 1 trillion is several orders of magnitude more than distinguishable colors or tones.
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