The functional organisation of the moth antennal lobe was studied using two species, Manduca sexta and Spodoptera littoralis, as model organisms. Glomerular activity was investigated at the population level by means of optical imaging techniques. In the male-specific macroglomerular complex (MGC), responses to pheromone compounds corroborated earlier results obtained from single cell recordings, i.e. different compartments of the MGC responded specifically to one component of the pheromone. Among the sexually isomorphic glomeruli the responses to plant-associated compounds were more distributed but odour-specific and similar across individuals of both sexes. Several glomeruli were activated by a single compound and each glomerulus was activated by several different compounds. Thus, broad tuning and overlapping responses suggest an across-glomerular coding mechanism for non-pheromones. Using series of homologous aliphatic compounds revealed that the highest correlation between activity patterns was always found for compounds with the same functional group and with minimal difference in carbon chain length. A concentration-dependence of glomerular activity was found. Increased concentration resulted in a recruitment of activated glomeruli. The single most activated glomerulus was often not the same across concentrations. The movement, however, was generally restricted to neighbouring glomeruli. Furthermore, activity patterns elicited by different odorants were more similar at high than at low stimulus doses. A method to selectively stain a large population of projection neurons with a Ca2+ sensitive dye was applied to S. littoralis. Also at the output level glomeruli were broadly tuned and activity patterns evoked by different odours overlapped each other. Temporal differences were found both between stimuli and between glomeruli. Glomerular activity patterns evoked by different odours became less similar as a function of time
