Olfactory discrimination ability of human subjects for ten pairs of enantiomers

We tested the ability of human subjects to distinguish between enantiomers, i.e. odorants which are identical except for chirality. In a forced-choice triangular test procedure 20 subjects were repeatedly presented with 10 en antiomeric odor pairs and asked to identify the bottle containing the odd s timulus. We found (i) that as a group, the subjects were only able to signi ficantly discriminate the optical isomers of alpha-pinene, carvone and limo nene, whereas they failed to distinguish between the (+)- and (-)-forms of menthol, fenchone, rose oxide, camphor, alpha-terpineol, beta-citroneliol a nd 2-butanol; (ii) marked individual differences in discrimination performa nce, ranging from subjects who were able to significantly discriminate betw een 6 of the 10 odor pairs to subjects who failed to do so with 9 of the 10 tasks; (iii) that with none of the 10 odor pairs were the antipodes report ed to differ significantly in subjective intensity when presented at equal concentrations; and (iv) that error rates were quite stable and did not dif fer significantly between sessions, and thus, we observed a lack of learnin g or training effects. Additional tests of the degree of trigeminality and threshold measurements of the optical isomers of a-pinene, carvone and limo nene suggest that the discriminability of these three enantiomeric odor pai rs is indeed due to differences in odor quality, These findings support the assumption that enantioselective molecular odor receptors may only exist f or some but not all volatile enantiomers and thus that chiral recognition o f odorants may not be a general phenomenon but is restricted to some substa nces.