Coding of odour quality: roles of convergence and inhibition

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 most efficient coding is yielded when the projection of first-order neurons into glomeruli is regular and the local neurons exert actions governed by a time-dependent algorithm due to J. Herault and C. Jutten that maximizes the independence of principal neurons. In most cases the action is found to be inhibitory although its sign is not imposed by the algorithm. Fifth, the significance of these results for transduction neurons, identifiability of glomeruli and resistance to damage of olfactory systems is discussed.