UC San Diego

Single sub-threshold odorants can, according to a few reports, become perceptible in concert. That is, they can exhibit mixture additivity. The present study measured thresholds for mixture of 1-butanol, 2-pentanone, and n-butyl acetate, and for each of these separately. 'Complete' additivity resulted, in that the threshold concentration of each compound in the mixture (measured by gas chromatography) turned out to be one-third of each component separately. Earlier threshold studies also claimed complete additivity and even hyper-additivity (synergism); in this regard they, like this study, differ from the more numerous studies of suprathreshold mixtures, which yield imperfect additivity (hypo-additivity). Mixtures not only enhance sensitivity, but they may also promote stability; intersubject variability was smaller for the mixture than for the components. Subjects were 20 young (18–26 years) and 20 elderly (69–91 years) persons, who gave two sets of thresholds on 2 days. Concordant with earlier findings, the elderly's thresholds averaged higher (≈ 20 times) than the young's, but complete additivity nevertheless held, even though they operated over different portions of the concentration continuum. The study affords a look at inter-test reliability of thresholds by comparing correlations between same and different compounds examined on same and different days.