Insights into vision from interpretation of a neuronal wiring diagram

What insects can see has been probed by over a century of behavioral experiments. Motion and color vision have also been studied through neurophysiology in insect brains. Here I study form vision by interpreting a neuronal wiring diagram of the Drosophila optic lobe. The Dm3 "line amacrine" cells are shown to divide into three cell types with oriented dendrites, and to be connected with three TmY cell types, also with oriented dendrites. All six cell types are predicted to respond selectively to oriented visual stimuli, with preferred orientation defined by dendrite orientation. Their receptive fields are predicted by mapping input from other cell types that chiefly convey information from single facets of the compound eye. Dm3 to Dm3 and TmY connectivity is approximated by cross-orientation inhibition and TmY to TmY connectivity by iso-orientation excitation. Both connectivity motifs were previously hypothesized for mammalian visual cortex. Two of the TmY types target a novel type of LC10 cell, which leads by multiple pathways to brain regions that support learning of visual form. Based on the spatial organization of TmY to TmY and LC10 connectivity, I conjecture that flies may see illusory contours and corners.