Outline of a theory of olfactory processing and its relevance to humans.

Two traditional beliefs about olfaction are that the system is poorly understood and that it functions poorly in humans. An outline of a theory of olfactory processing has in fact been emerging for many years (Rall and Shepherd, 1968; Stewart et al., 1979; Haberly, 1985; Lancet, 1986; Buck and Axel, 1991; Shepherd, 1991; Imamura et al., 1992; Mori and Shepherd, 1994; Mombaerts, 2004; Wilson, 2004). I will first briefly update the theory based on evidence from many recent studies, a consensus that is not widely appreciated because it is multilevel, multidisciplinary and cross-phylogenetic. I will then consider new evidence that evolution has produced in humans an excellent overall sense of smell and, combined with taste and somatosensation and other inputs, the best sense of flavor in the animal world. This should give a new emphasis to the importance of these senses for sensory physiology, human nutrition, and human evolution. Our focus will be on mammals, while recognizing the crossphylogenetic application of many of the basic principles (Hildebrand and Shepherd, 1997).