 The bulb therefore provides a spatial map that identifies which of the numerous receptors have been activated within the sensory epithelium so that the quality of an olfactory stimulus is encoded by specific combination of glomeruli activated by a given odorant (combinatorial code).While refinement of the bulbar topographic code relies on activity, the development occurs partly without apparent contribution from activity-dependent processes.Mice lacking the olfactory cyclic nucleotide-gated ion channel fail to exhibit odor-evoked electrophysiological responses in the sensory epithelium, but the pattern of convergence of like axons in the bulb is unaltered in these mutant mice, arguing strongly that olfactory experience is not necessary for the establishment or refinement of the topographic map.