Carbon chain length and the stimulus problem in olfaction

Understanding how odour quality perception is encoded in its molecular properties arguably poses one of the most significant problems in olfaction. Determining the odour structure-quality relationships of structurally similar odorants could provide a key tool to this problem. We tentatively explored whether a mixture of two molecules, differing only in carbon chain length (C), would yield the same percept as a single odorant with an intermediate carbon chain length, akin to colour vision, or be perceived as a different quality. Ability to discriminate between pairs of iso-intense solutions of n-butanol (4C), n-propanol (3C), n-pentanol (5C), and an intermediate 50/50 molecular weight mixture of n-propanol and n-pentanol (3C/5C) was assessed in 20 healthy young adults. We found that participants were able to discriminate 4C from the 50/50 molecular weight mixture of n-propanol and n-pentanol (3C/5C), and also from the other alcohols. In conclusion, we successfully replicated previous data demonstrating that participants are able to discriminate between structurally similar alcohols, and, more importantly, the present study shows that an odour mixture of two molecules differing only in carbon chain length is clearly distinguishable from a single odorant with an intermediate carbon chain length. These findings suggest that although carbon chain length matters to odour quality, carbon chain length is not a physical continuum within homologous series of substances that corresponds to a single qualitative dimension akin to the wavelength-hue relation for monochromatic light.