Orthonasal and retronasal but not oral-cavity-only discrimination of vapor-phase fatty acids.

Discrimination of vapor-phase linoleic, oleic, and stearic fatty acids was studied using triangle tests. For each trial, 2 of the 3 modified odorant delivery containers (MODCs) had the same content and 1 was different. Contents were either mineral oil-diluted linoleic or oleic acids, with mineral oil in the other MODC (blanks) or undiluted stearic acid with NaCl in the other MODC (blanks). The task was to indicate which of the 3 MODC had the most different odor. Vapor-phase fatty acids and blanks were presented orthonasally, retronasally, or oral-cavity-only. It was found that all 3 fatty acids were discriminated from the blanks both orthonasally and retronasally, P 0.05 (30% of 30 participants discriminated linoleic acid from blanks, P = 0.71; 47%, oleic and stearic acids, P = 0.09). These results demonstrate that human participants can discriminate linoleic, oleic, and stearic fatty acids both orthonasally and retronasally, confirming that humans can smell fatty acids.

Metallic taste and retronasal smell.

A series of experiments investigated the nature of metallic taste reports and whether they can be attributed to the development of a retronasal smell. Two studies showed that the metallic sensation reports following oral stimulation with solutions of FeSO4 were reduced to baseline when the nose was occluded. No such reduction was seen for CuSO4 or ZnSO4, which were more bitter and astringent, respectively, and less metallic. A discrimination test based on weak but equi-intense levels of FeSO4 and CuSO4 showed that FeSO4 could be discriminated from water with the nose open but not when occluded, but that discrimination of CuSO4 from water was not impaired by nasal occlusion. A discrimination test demonstrated that the headspace over solutions of FeSO4 was not different from water, although some subjects could discriminate FeSO4 solutions from water in the mouth when the nose was occluded, perhaps by tactile or astringent cues. These results confirm that metallic taste reports following oral stimulation with FeSO4 are likely due to development of a retronasal smell, possibly following a lipid oxidation reaction in the mouth. However, metallic taste reports may arise from different mechanisms with copper and zinc salts.