psychophysical scales, and for olfactory sensations the reference points on such scales consist of the perceptions experienced on exposure to standard odorants. odor identification depends on accessing olfactory information in memory as well as in perception. Fluctuations in performance have been attributed to problems with semantic memory, that is, forming the association between the odor and its label(2). The situation becomes more complicated when odorants in mixtures. Mixing odorants alters per ceived quality as well as intensity. The perceived intensity of a mixture has been found to closely approximate that of the stronger of the separate components, whereas the quality of a mixture composed of equally strong unmixed components lies between those of the components G) There may be active suppression as well as hypoadditivity. 2.4 Decision Processes and Thresholds of odor Perception