CODING OF ODOR QUALITY - ROLE OF CONVERGENCE AND INHIBITION

Coding of odour quality in the first two neuronal layers of olfactory systems is studied, taking especially insects as reference. First, odo ur quality is defined by K independent molecular properties that activ ate differentially the chemosensitive receptors of first-order neutron s. Each odour quality discriminated is assumed to be coded by a distin ct 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 ge neration of activity maps. These maps give a complete and unbiased rep resentation of all odour qualities which make them suitable as an inte rnal code. Third, lateral inhibition mediated by local neurons can sig nificantly increase the difference of activity between the least and m ost 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 algor ithm due to Herault and Jutten that maximizes the independence of prin cipal neurons. In most cases the action is found to be inhibitory alth ough its sign is not imposed by the algorithm. Fifth, the significance of these results for transduction and selectivity of first-order neur ons, identifiability of glomeruli and resistance to damage of olfactor y systems is discussed.