Rapid Olfactory Adaptation Induced by Perithreshold Odorant Concentrations in Human Observers

A recent study from our laboratory described the development and use of a novel psychophysical method to characterize the onset time course of rapid olfactory adaptation (Smith et al., Chem Senses 35:717-725 2010). In the present study, we used this same approach to measure adaptation-induced changes in olfactory sensitivity following exposure to different self-adapting perithreshold-level stimuli. We used a custom-built liquid-dilution olfactometer to estimate two-odor discrimination thresholds for 600-ms presentations of a vanilla odorant alone and 500 ms after the onset of a 1,500-ms simultaneous vanilla adapting stimulus. Our previous findings suggest that a 500-ms exposure to a suprathreshold stimulus is sufficient to induce an asymptotic level of rapid olfactory adaptation in most participants. Twenty normosmic college-aged volunteers (ages 18-24; 14 females) served as subjects in this experiment. Thresholds were initially estimated for the 600-ms vanilla target alone, then compared in the presence of a simultaneous adapting stimulus set to 0.25, 0.5, 1.0, or 2.0 times the participant's initial threshold to assess the effects of adapting odorant level on adaptation magnitude. The results suggest that significant decreases in odorant sensitivity were evident even with subthreshold adapting odorant levels, but that the magnitude of adaptation was unaffected by adapting odorant concentration.