Are there structural and functional modules in the vertebrate olfactory bulb?

A number of different recording methods have shown that odorants elicit patterns of neuronal activity widely distributed across cells of the olfactory receptor epithelium, olfactory bulb, and piriform cortex in the vertebrate olfactory system. These findings suggest that the physicochemical properties of odorant molecules are processed by distributed coding mechanisms activated in parallel in olfactory circuits in order to characterize a single, "monomolecular" odorant. These findings also suggest that the response patterns seen at higher levels are set up by differential responses in peripheral receptor cells of the olfactory epithelium. One requirement for understanding the details of this proposed encoding scheme is correlation of odor-generated patterns with the components of these circuits. In this paper, results from 2-deoxyglucose and voltage-sensitive dye studies suggest that certain components of these responses may relate to patterns established in reproducibly identifiable aggregates of bulbar cells. These findings are consistent with previous observations suggesting that columnar groups of periglomerular, mitral/tufted and granule cells, oriented perpendicular to the laminae of the bulb, are functionally related to one another. Such cell groups or modules, when activated in parallel, could serve as building block components of the complete ensemble response. According to this hypothesis, different sets of such modules would be activated with different odorant stimuli and modules could be shared to the degree to which the physicochemical properties of the different stimuli overlap.