Neither pleasant nor foul-smelling, and in no way overwhelming: this is how researchers sum up the smell they are calling “olfactory white”.
The smell was uncovered during experiments that mixed aroma molecules from across the scent spectrum. Even if two mixtures had no components in common, they tended towards having a similar scent as more aromas were added. By the time they contained about 30 components, most mixtures smelled alike, and could mask other distinctive smells. The researchers say that the resulting smell, which is unlikely to occur naturally, has parallels with both white light and 'white noise'. These are produced by combining the wavelengths of the visible spectrum and different sound frequencies, respectively.
Given that our noses contain hundreds of different odour receptors, the phenomenon is counterintuitive.
Noam Sobel, an neuroscientist at the Weizmann Institute of Science in Rehovot, Israel, and his team selected 86 single-molecule odorants, representative of the known scent spectrum, and diluted each to the same perceived intensity. They assembled 191 pairs of non-overlapping aroma mixtures, each containing up to 43 of the molecules spaced out over the olfactory spectrum, and asked 56 volunteers to rate the similarity of a selection of the pairs. The results showed that the more components each of the two mixtures contained, the more alike they smelled. Volunteers consistently began to find the mixtures similar at 20 or more components, and by 30 components the mixtures were “highly similar” — a trend that implies there is an end point of perceptual convergence, olfactory white.
The phenomenon was confirmed by a further experiment with a different series of four newly generated 40-component mixtures labelled 'laurax'. Participants were familiarized with one of them. When smelling odorant mixtures they had not smelled before, volunteers were more likely to identify the scent as laurax if a mixture contained 20 or more components. Participants applied the laurax label to novel 40-component mixtures 58% of the time, and a follow-up study confirmed that most could still identify — and so remember — the laurax smell six months later.