In the olfactory bulb of vertebrates and the antennal lobe of insects, precise connections between sensory receptor cells and olfactory glomeruli form the basis of a highly organized chemotopic map at the first stage of central processing in the brain. Beyond this basic level of organization, the olfactory system is typically separated into two subsystems: a 'main' olfactory pathway that detects and processes information about most environmental odorants, and an 'accessory' olfactory pathway that is devoted to information about social signals such as sex pheromones. A growing number of studies show, however, that it is not always possible to draw clear functional distinctions between the two subsystems. These findings have led some to speculate that the organizational principles by which olfactory stimuli are represented across glomeruli may be more similar in these two olfactory subsystems than previously thought.