Quantification of odor quality

The relationship between odor quality and molecular properties is arguably the most: important issue in olfaction. Despite sophistication in the chemi cal characterization of molecules, accompanying perceptual characterization has had little quantitative usefulness, relying mostly on enumerative desc ription. As a result of weak interest in the topic outside industry and lit tle agreement regarding how to measure quality, the field of olfactory psyc hophysics has failed to develop a substantial database for odor quality and has offered little help to other researchers, e.g. neurobiologists, in cho ice of stimuli, interpretation of outcome or testable hypotheses. This revi ew scrutinizes how psychophysicists and others have measured quality and of fers criteria for useful techniques. Most measures have had a subjective co mponent that makes them anachronistic with modern methodology in experiment al behavioral science, indeterminate regarding the extent of individual dif ferences, unusable with infrahumans and of unproved ability to discern smal l differences. Techniques based upon performance, rather than on the more c ommon reporting of mental content, offer firmer possibilities for growth. T hese techniques inevitably lap the discriminative basis of perception. The nonsubjective techniques have high sensitivity, can have counterparts in in frahuman research, are suitable to examine individual differences and yield non-negotiable answers with potential archival value. Discriminative techn iques have their limitations, too-principally excess sensitivity that abrid ges their use to comparisons between similar-smelling stimuli. Research has begun to extend that range and may overcome the limitation. Application of discriminative methods may have the side-effect of shifting focus in struc ture-activity research from searches for molecular least common denominator s that underlie often vague similarity to the search for molecular properti es of importance in discrimination of small differences.