Human Trigeminal and Olfactory Chemosensitivity: Detection of Single Chemicals and Binary Mixtures
The study of anosmic subjects has allowed to separate the trigeminal from the olfactory input in the detection of airborne chemicals by the nose. Systematic testing of homologous series of chemicals has allowed to probe into the role that physicochemical properties play to make a substance a potent (i.e., low threshold) or weak (i.e., high threshold) odorant or irritant. This combined approach has served to define the gap between olfactory and chemesthetic detection of volatile organic compounds (VOCs), with smell leading in sensitivity by as little as one and as much as five orders of magnitude. It has also paved the way to the description and prediction of the chemesthetic potency of VOCs27-29 (see M. Abraham article, this volume) and to the recognition of the important role of lipophilicity in chemesthesis18.
Additional indices of trigeminal sensitivity such as eye irritation and nasal localization (or lateralization), that can be measured in normosmics devoid of olfactory influence, have rendered a similar picture of absolute and relative chemesthetic potency of VOCs to that rendered by testing nasal pungency in anosmics, validating this approach.
The study of olfactory and trigeminal detection of mixtures of VOCs constitutes an important topic for future studies. Initial work looking broadly at the chemosensory detection of mixtures of up to 9 components, and very recent detailed studies of one particular binary mixture have indicated the existence of sensory agonism among individual components. Such agonism seems more robust or, perhaps, more complete for chemesthesis than for olfaction but only further studies could confirm the generality of these findings.