Odors as cognitive constructs: history of odor classification and attempts to map odor percepts to physical and chemical parameters.

Attempts to map odor percepts to physical and chemical parameters have a long and challenging history. In contrast to color vision, where three classes of photoreceptors respond to the same stimulus property (wavelength), ~400 classes of olfactory receptors are available to respond in a non-linear non-additive fashion to ~5000 different chemical parameters. Theoretically, millions of permutations between structural elements of chemicals and their smells are possible, and some chemicals with different structures have the same odor and vice versa. Importantly, the same odor quality can come from multiple environmental objects and most odors depend upon the synthesis of a complex melody of volatile chemicals that individually can have dissimilar smells. At the individual receptor level, both agonists and antagonists within a mixture can impact receptor function. Hence, it is perhaps not surprising that no universal relationship between odor quality and underlying physical or chemical dimensions analogous to spectral wavelength for vision or air pressure waves for hearing has yet been identified. This review provides a historical account of psychological odor categorization, attempts to map odor percepts to physiochemical parameters, and attendant pitfalls. It concludes that perceived odor qualities may be best viewed as cognitive constructs with attendant variability due to individual experiences, linguistic processes, and biologic factors that do not map well to universal physiochemical dimensions.