Tuning and topography in an odor map on the rat olfactory bulb

The sense of smell originates in a diverse array of receptor neurons, compr ising up to 1000 different types. To understand how these parallel channels encode chemical stimuli, we recorded the responses of glomeruli in the olf actory bulbs of the anesthetized rat, by optical imaging of intrinsic signa ls. Odor stimulation produced two kinds of optical responses at the surface of the bulb: a broad diffuse component superposed by discrete small spots. Histology showed that the spots correspond to individual glomeruli, and th at similar to 400 of them can be monitored in this way. Based on its wavele ngth-dependence, this optical signal appears to derive from changes in ligh t scattering during neural activity. Pure odorants generally activated seve ral glomeruli in a bilaterally symmetric pattern, whose extent varied great ly with concentration. A simple formalism for ligand binding accounts quant itatively for this concentration dependence and yields the effective affini ty with which a glomerulus responds to an odorant. When tested with aliphat ic molecules of increasing carbon chain length, many glomeruli were sharply tuned for one or two adjacent chain lengths. Glomeruli with similar tuning properties were located near each other, producing a systematic map of mol ecular chain length on the surface of the olfactory bulb. Given local inhib itory circuits within the olfactory bulb, this can account for the observed functional inhibition between related odors. We explore several parallels to the function and architecture of the visual system that help interpret t he neural representation of odors.