Among researchers of human olfaction, there is no consensus definition of adaptation yet, because few studies have clearly defined the time-course transition of perceived intensity regarded as adaptation. In order to address this situation, we defined adaptation as a situation in which perceived intensity decreases exponentially with the passage of time. In this study, we performed real-time evaluation of perceived intensity of continuously presented odors, using nine odorants. In order to develop a quantitative method for classifying perceived-intensity curves as ‘adaptation’ or ‘non-adaptation’, each perceived-intensity curve was approximated by an exponential model. We classified perceived-intensity curves as ‘adaptation’, if they fulfilled the following two criteria : (1) theoretical values of perceived intensity correlated strongly with measured values of perceived intensity, and (2) theoretical values of perceived intensity decreased below the detection threshold during odor presentation. Furthermore, we calculated the ratio of the numbers of ‘adaptation’ and ‘non-adaptation’ perceived-intensity curves, for each odor. We called this value the ‘adaptation index’, and proposed it as a quantitative index of the facility with which adaptation to each continuously presented odor occurs.
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