Psychophysical evaluation of responses to pleasant and mal-odour stimulation in human subjects; adaptation, dose response and gender differences.

A psychophysical detection test was used to measure the response of human subjects to 'good' and 'bad' smells. Different intensities and frequencies of odour stimulation were delivered by an olfactometer and the responses to a group of malodours (valeric acid, skatol, butyric acid) and pleasant smells (amyl acetate, cis-3-hexenol, linalool) were compared. A mathematical model, a power equation, was used to fit the three-dimensional data plots (dose vs. stimulus frequency vs. response). The model was able to distinguish between malodours and pleasant odours on the basis of the values of parameters describing dose-response and adaptation/habituation. We show that the olfactory system adapts/habituates more rapidly to malodours than to pleasant smells, but is much more sensitive to changes in stimulation by malodours than pleasant odours. The degree of adaptation is inversely proportional to stimulus strength. The response profile for women was different to that of men for certain odours, in particular valeric acid, skatol and cis-3-hexenol. The difference lay in their sensitivity and the slope and range of the dose response. Thus, we have shown for the first time that the olfactory system adapts more readily to 'bad' smells than 'good' smells, and that it has a broader range of adaptation for bad smells. As a consequence, the olfactory system is specially responsive to changes in potential olfactory warning signals.