Modular representations of odorants in the glomerular layer of the rat olfactory bulb and the effects of stimulus concentration

To study the mechanism whereby odorants are encoded in the nervous system, we studied the glomerular-layer activity patterns in the rat olfactory bulb evoked by closely related odorants from different chemical families. These odorants had a common straight-chain hydrocarbon structure, but differed s ystematically in their functional groups. Neural activity was mapped across the entire glomerular layer by using the [C-14]2-deoxyglucose method. Grou p responses were averaged and compared by using data matrices. The glomerul ar activity patterns that resulted from this analysis were comprised of mod ules. Unique combinations of modules were activated by each odorant, demons trating what may be part of the neural code for odorants. Most of the modul es were clustered together in the bulb, perhaps providing for enhanced cont rast between related chemicals by means of lateral inhibition. We also dete rmined whether changes in odorant concentration would affect spatial patter ns of glomerular activity. Two odorants, pentanal and 2-hexanone, evoked di fferent patterns at increased concentrations, with additional glomeruli bei ng recruited at a great distance from glomeruli in which activity was evoke d at lower concentrations. Humans report that both of these odorants change in perceived odor with increasing concentration. Three other odorants (pen tanoic acid, methyl pentanoate, and pentanol) did not recruit new areas of glomerular activation with increasing concentration, and humans do not repo rt a changed odor across concentrations of these odorants. The results sugg est that changes in modular glomerular activity patterns could underlie alt ered odor perception across odorant concentrations, and they provide additi onal support for a combinatorial, spatially based code in the olfactory sys tem. (C) 2000 Wiley-Liss, Inc.