SPONTANEOUS VARIABILITY AND INDUCED INCREASE OF OLFACTORY SENSITIVITY TO DILUTED ODORANTS

The remarkably high variability of olfactory psychophysical thresholds could be interpreted as a transitory ability to detect, under certain, still undefined conditions, highly diluted odorants. One of the factors contributing to this phenomenon might be represented by Central modulation of detection thresholds. The present experiments deal with a particular case of tuning the threshold toward low concentrations of an odorant (amyl acetate, resembling banana smell). It is characterized by a decrease by about one or two orders of magnitude of ascending limit thresholds (estimated by gradually increasing stimulus concentrations from subthreshold values in a sequence of trials) in comparison with descending trials (estimated in the opposite way). Two factors seem to be relevanti:unidirectional stimulus ordering from sub-threshold toward supra-threshold values, probably activating physiological processes underlying stimulus expectation;and the lack of additional references a concerning stimulus intensity (other than the ordered sequence of preceding ascending concentrations).