Elucidating neural circuit mechanisms that drive complex behavior and cognition requires a multidisciplinary approach combining neuroanatomy with neurophysiology and behavioral science. This integration of different disciplines works particularly well in the model organism Drosophila melanogaster, as it benefits from the rich arsenal of molecular and genetic tools that have been developed over the last century. A focus of research are the circuit elements that drive local computation in the optic lobes. However, considerably less is known about the pathways that leave the optic lobes and converge in the central complex, which is considered the navigation center in Drosophila. In this thesis I present the complete synaptic pathway connecting the optic lobes to the central complex. Starting from the terminals of inner photoreceptors in the medulla all the way to the dendrites of EPG neurons, the Drosophila counterpart of head-direction cells in mammals. Initially, by studying light microscopically the distribution of putative presynaptic sites of inner photoreceptors I turned to connectomic reconstructions as the first complete EM volume of an adult female brain (full adult fly brain FAFB) became available. First, I manually reconstructed a set of inner photoreceptors and all pre- and postsynaptic partners as well as annotated all synapses among them. In a second step, recent advances in automatic image segmentation of the FAFB volume allowed me to proofread the complete set of projection neurons connecting the photoreceptors to the central complex across multiple neurons and neuropils. I found that the neurons along the pathway fall into different subchannels that most likely carry information from qualitatively different visual modalities into the central complex. By mapping putative visual fields to the input neurons of EPG neurons I discovered a diverse mapping of visual space in the central complex. As a population the entirety of the fly’s visual field is covered, however the visual fields of individual neurons can differ drastically. Some neurons possess diffuse visual fields while others have discrete spot- or vertical bar-like visual fields supporting the idea of combinatorial integration of different visual modalities in the navigation center of Drosophila. These findings form the basis for analyzing the integration of visual modalities within the central complex, which could not only accelerate research in this area, but also underscore the crucial role of vision in spatial navigation.
