Psychometric functions for ternary odor mixtures and their unmixed components

Abstract

People are often able to reliably detect a mixture of 2 or more odorants, even if they cannot reliably detect the individual mixture components when presented individually. This phenomenon has been called mixture agonism. However, for some mixtures, agonism among mixture components is greater in barely detectable mixtures than in more easily detectable mixtures (level dependence). Most studies that have used rigorous methods have focused on simple, 2-component (binary) mixtures. The current work takes the next logical step to study detection of 3-component (ternary) mixtures. Psychometric functions were measured for 5 unmixed compounds and for 3 ternary mixtures of these compounds (2 of 5, forced-choice method). Experimenters used air dilution olfactometry to precisely control the duration and concentration of stimuli and used gas chromatography/mass spectrometry to verify vapor-phase concentrations. For 2 of the 3 mixtures, agonism was approximately additive in general agreement with similar work on binary mixtures. A third mixture was no more detectable than the most detectable component, demonstrating a lack of agonism. None of the 3 mixtures showed evidence of level dependence. Agonism may be common in ternary mixtures, but general rules of mixture interaction have yet to emerge. For now, detection of any mixture must be measured empirically.

Figure 1

Molecular structures of the odor materials used in the experiment.

Figure 2

Psychometric functions for single (unmixed) compounds. y axis: the log-odds ratio of chance-corrected pcorr. x axis: stimulus concentration in log ppm (by mass). The lines represent best-fit linear functions (least-squares regression). The functions for ML and octanoic acid (C₈) are depicted on a separate graph for clarity.

Figure 3

Psychometric functions for ternary mixtures (filled squares) and single compounds (open diamonds for C₂, open squares for C₄, open triangles for the third mixture component). y axis: log-odds ratio of chance-corrected pcorr and x axis: concentration step (2.2-fold dilutions, where the proportions of the 3 compounds in each mixture remain the same as concentration increases). Lines (solid for mixtures, dashed or dotted for single compounds) represent linear fits (least-squares regression). Error bars omitted for clarity (error bars for these functions appear in Figures 2 and 4).

Figure 4

Psychometric functions for 3 ternary mixtures: Acetic, butyric, and hexanoic acids (left); acetic, butyric, and octanoic acids (middle); and acetic acid, butyric acid, and ML (right). y axis: log-odds ratio of chance-corrected pcorr; x axis: concentration step (2.2-fold dilutions, where the proportions of the 3 compounds in each mixture remain the same as concentration increases). Filled symbols (solid lines) represent observed detection data. Open symbols (dashed lines) represent additivity predictions based on detection functions for individual components under an assumption of probability summation. Error bars represent ± standard error of the mean.