Abstract
Odours are represented as unique combinations of activated glomeruli in the antennal lobes of insects. Receptor neurons arborizing in the glomeruli are not only qualitatively selective, but in addition respond to variations in stimulus concentration. As each glomerulus likely represents a single receptor neuron type, optical recordings of calcium changes in insect antennal lobes show how concentration variations affect a large population of afferents. We measured the glomerular responses in the moth Spodoptera littoralis to different concentrations of plant-related odorants. Localized calcium responses were shown to correspond to individual glomeruli. We found that the dynamic range of glomerular responses spanned 3-4 log units of concentration and the most strongly responding glomeruli often reached a plateau at high stimulus doses. Further, we showed that the single most active glomerulus was often not the same across concentrations. However, if the principal glomerulus moved, it was generally to an adjacent or proximal glomerulus. As concentration increased, a higher number of glomeruli became activated. Correlations of glomerular representations of the same compound at different doses decreased as the difference in concentration increased. Moreover, representations evoked by different odorants were more correlated at high than at low doses, which means that the uniqueness of activity patterns decreased with increasing concentration. Thus, if odours are coded as spatial patterns of glomerular activity, as has been suggested, these olfactory codes are not persistent across concentrations.
