Systematic analysis of the neural circuits in the higher visual centers of the Drosophila brain

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In spite of its importance in the visual information processing, relatively little is known about the visual projection neurons that connect the lower visual centers in the optic lobe and higher visual centers in the central brain of insects. By analyzing the neuropil structures of the putative higher visual centers of Drosophila melanogaster and sysmatically identifying and classifying the morphology of the visual projection neurons from the screening of about 4,000 GAL4 enhancer-trap strains, we analyzed how neural pathways that are associated with the lobula and lobula plate of the optic lobe terminate in the central brain. We also analyzed the direction of information in each identified neurons by selectively visualizing the distribution of the presynaptic (output) sites. Among the 24 pathways we identified, all the columnar pathways were centripetal, whereas 70% of the tangential/tree-like pathways were centrifugal. Centripetal VPNs convey visual information to three higher visual centers: the optic tubercle, ventrolateral protocerebrum, and posteriorlateral protocerebrum. On the other hand, most centrifugal VPNs arise from other areas of the brain, suggesting that the signals that regulate the optic lobe activity are not generated by simple feedback loops.