SURE, why not? The SUbstitution-REciprocity method for measurement of odor quality discrimination thresholds: replication and extension to nonhuman primates.

Recently, Olsson and Cain (2000, Chem. Senses, 25: 493) introduced a psychometric method which, for the first time, allows the standardized determination of odor quality discrimination (OQD) thresholds. The method defines a threshold value that is an average fraction by which one odorant has to be substituted with another to reach a criterion level of discrimination. This measure of discrimination is reciprocal in the sense that it is a result of two separate psychometric functions involving two different standards but the same comparison stimuli. Using the same odor stimuli as Olsson and Cain, with six human subjects but adopting a slightly different experimental design, we were able to replicate their finding that the proportion of correct discriminations changes monotonically with the proportion of adulterant in mixtures of eugenol and citral. As the SURE (SUbstitution-REciprocity) method is based on discriminative responses, it should also be applicable with nonhuman species which can be trained to give unequivocal discriminative responses at the behavioral level. Using an olfactory conditioning paradigm, we therefore trained four squirrel monkeys to discriminate between exactly the same pairs of odor stimuli as our human subjects. We found the psychometric functions of the monkeys to be similar to those of the human subjects. Our results show that the SURE method can successfully be employed with nonhuman primates and thus offers a new approach to study the odor spaces of nonhuman species. Future studies should elucidate whether the SURE method allows for direct comparisons of OQD thresholds and of similarities and differences between odor quality perception of different species.