Representation of natural stimuli in the rodent main olfactory bulb.

Natural odorants are complex mixtures of diverse chemical compounds. Monomolecular odorants are represented in the main olfactory bulb by distinct spatial patterns of activated glomeruli. However, it remains unclear how individual compounds contribute to population representations of natural stimuli, which appear to be unexpectedly sparse. We combined gas chromatography and intrinsic signal imaging to visualize glomerular responses to natural stimuli and their fractionated components. While whole stimuli activated up to 20 visible glomeruli, each fractionated component activated only one or few glomeruli, and most glomeruli were activated by only one component. Thus, responses to complex mixtures reflected activation by multiple components, with each contributing only a small part of the overall representation. We conclude that the population response to a complex stimulus is largely the sum of the responses to its individual components, and activation of an individual glomerulus independently signals the presence of a specific component.