Perceptual convergence of multi-component mixtures in olfaction implies an olfactory white

In vision, two mixtures, each containing an independent set of many different wavelengths, may produce a common color percept termed white. In audition, two mixtures, each containing an independent set of many different frequencies, may produce a common perceptual hum termed white noise. Visual and auditory whites emerge upon two conditions: when the mixture components span stimulus space, and when they are of equal intensity. We hypothesized that if we apply these same conditions to odorant mixtures, whiteness may emerge in olfaction as well. We selected 86 molecules that span olfactory stimulus space and individually diluted them to a point of about equal intensity. We then prepared various odorant mixtures, each containing various numbers of molecular components, and asked human participants to rate the perceptual similarity of such mixture pairs. We found that as we increased the number of nonoverlapping, equal-intensity components in odorant mixtures, the mixtures became more similar to each other, despite not having a single component in common. With similar to 30 components, most mixtures smelled alike. After participants were acquainted with a novel, arbitrarily named mixture of similar to 30 equal-intensity components, they later applied this name more readily to other novel mixtures of similar to 30 equal-intensity components spanning stimulus space, but not to mixtures containing fewer components or to mixtures that did not span stimulus space. We conclude that a common olfactory percept, olfactory white, is associated with mixtures of similar to 30 or more equal-intensity components that span stimulus space, implying that olfactory representations are of features of molecules rather than of molecular identity.