OLFACTORY ADAPTATION AS AN ASPECT OF ODOR SIMILARITY

Two experiments examined self-adaptation within and cross-adaptation between two structurally different substances with almost identical bitter chocolate odors, trimethyl pyrazine (TMP) and 2-propionyl-3-methyl furan (PMF). The first experiment charted psychophysical functions for odor intensity under steady-state self- and cross-adaptation with adapting concentrations matched in perceived intensity at two different levels. Participants breathed an adapting concentration continuously for 2 min and then rated test concentrations every minute during momentary respites from the adapting stimulus. Both self-adaptation and cross-adaptation weakened the intensity of low test concentrations proportionally more than higher ones and thereby steepened the psychophysical functions. Adapting stimuli at matched levels caused equivalent self-adaptation (i.e. altered the functions equivalently) and equivalent, though weaker, cross-adaptation (i.e. exhibited symmetry of cross-adaptation). The second experiment examined whether cross-adaptation to TMP and PMF would prove relatively specific. At a matched perceived intensity of adapting stimulus, adaptation to TPM and PMF again produced equal amounts of self-adaptation and of cross-adaptation to each other, though not on three other test stimuli: anethole, ethyl butyrate and 2,3 pentanedione. Both TMP and PMF showed some generality as cross-adapting stimuli, with PMF showing a bit more than TMP. The present protocols and others discussed herein offer opportunities to consider functional criteria for the relatedness of similar-smelling substances.