Is there convergence of sensory axons in the olfactory bulb?
In situ hybridization on coronal sections of the rat olfactory bulb shows that sensory neurons expressing the same odorant receptor gene, and therefore sensitive to the same odorants, converge on one or a few glomeruli (Figure 4). Probes detecting 22 genes by Southern hybridization label only 19 of the 3000 glomeruli in the olfactory bulb. Moreover, the positions of labeled glomeruli are bilaterally symmetric and are conserved in all animals.16 A similar hybridization pattern has been described in the mouse by Linda Buck’s laboratory at Harvard.17 These data provide physical evidence that the central representation of odors is spatially organized, such that exposure to an odorant is associated with a unique pattern of depolarization in the olfactory bulb. In the visual system, every level of sensory processing must preserve an accurate spatial representation of the world. Neural space is thus constrained to mimic physical space. In contrast, it appears that the olfactory system, having
