Form informs function: innervation patterns of projection neurons correspond to response profiles of mushroom body output neurons during olfactory-visual integration.

Several studies have shown that honeybees have a remarkable ability to detect, learn, and discriminate different floral qualities such as colour patterns and odour bouquets. However, how they integrate this multi-sensory information at the neuronal level is less understood. The sites for multi-sensory convergence in honeybees and other insects are the mushroom bodies (MB). In this study, we focus on MB output neurons (MBON) to examine how different MBONs process olfactory, visual, and olfactory-visual compound stimuli using extracellular multi-unit recordings. By analysing functional responses, we defined three MBON subpopulations: "bimodal", "unimodal light", and "unimodal odour". All three groups exhibit compound-mediated modulation in both directions, meaning that compared to the unimodal response, the response strength to the compound can be either higher or lower. This suggests the presence of a separate compound-activated pathway, especially for unimodal MBONs. Comparing our results to the morphology of the MB input regions, the calyces, which are both divided into the lip (olfaction), collar (vision), and the basal ring (bimodal), we provide evidence that the latter region might play a key role in compound signalling. By establishing a model based on information flow through the MB circuitry, we reproduced the functional phenotypes exhibited by the different MBON types.