Infrequent strong connections constrain connectomic predictions of neuronal function

How does circuit wiring constrain neural computation? Recent work has leveraged connectomic datasets to predict the functions of cells and circuits in the brains of multiple species. However, many of these hypotheses have not been compared with physiological measurements, obscuring the limits of connectome-based functional predictions. To explore these limits, we characterized the visual responses of 43 cell types in the fruit fly and quantitatively compared them with connectomic predictions. We show that these predictions are accurate for some response properties, such as orientation tuning, but are surprisingly poor for other properties, such as receptive field size. Importantly, strong synaptic inputs are more functionally homogeneous than expected by chance and exert a disproportionately large influence on postsynaptic responses. Finally, we quantitatively define the subset of connections that best describe the functional differences between cell types. Our results establish a powerful set of constraints for improving the accuracy of connectomic predictions.