Odor Identity: Measurement, Theory, and Neural Codes

The olfactory system maps millions of volatile molecules onto perception using a few hundred receptor types. For color vision, the mapping from stimulus (light spectrum) through receptors (three cone types) to perception (color space) was solved in the last century; for olfaction it remains open. To address this problem, we use the mouse as a model system. First, we developed a behavioral paradigm to measure perceptual distances between odors and reconstruct a perceptual odor space. Second, we developed a theory of odor coding based on the primacy model, in which odor identity is carried by the earliest-activated receptors; we describe the geometry of odor representations in receptor space and propose experimental tests. Finally, comparing several candidate codes at the level of olfactory bulb glomeruli, we find that a ranking code, the order of receptor activation, best represents concentration-invariant odor identity.