Coding of odour quality: roles of convergence and inhibition

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Abstract
Coding of odour quality in the first two neuronal layers of olfactory systems is studied, taking especially insects as reference. First, odour quality is defined by K independent molecular properties that activate differentially the chemosensitive receptors of first-order neurons. Each odour quality discriminated is assumed to be coded by a distinct spatial activity pattern of the principal neurons (or equivalently glomeruli) of the second layer. Second, computer simulations show that the differential projections into glomeruli of several types of first-order neurons (convergence) is the main factor responsible for the generation of activity maps. These maps give a complete and unbiased representation of all odour qualities which make them suitable as an internal code. Third, lateral inhibition mediated by local neurons can significantly increase the difference of activity between the least and most active glomeruli for each odour, and consequently the total number of discriminated odours. Fourth, the mos...