(A) An example LC6 neuron found in Serial Section Transmission Electron Microscopy volume of the complete female brain. (B) All LC6 neurons on one side of the brain (by convention, the right side, but displayed here on left) were traced into the glomerulus. Two example cells are highlighted in red and blue (same cells in C, D, and E). All cells have dendrites mainly in lobula layer 4 (orange), and project to the LC6 glomerulus (cyan). Cell bodies are rendered as gray spheres. The projections of two Tm5 neurons are shown coming from the medulla side of the lobula. They were selected to identify the center (brown Tm5) and central meridian (both Tm5s) of the eye in C. (C) Projection view of the lobula layer. Traced LC6 dendrites (gray) are projected onto a surface fit through all dendrites (light orange). Blue disks represent centers-of-mass of each LC6 dendrites. The vertical line is the estimated central meridian, the line that partitions the eye between anterior and posterior halves, mapped onto the eye coordinate in D. (D) Estimate of dendritic field coverage of visual space (basis for anatomical receptive fields) for all LC6 neurons. Polygons are the estimated visual fields, and the boundary indicates the estimated boundary of the lobula (layer 4). The red example cell RF corresponds to posterior-viewing parts of the eye while the blue example cell RF corresponds to fronto-ventral viewing direction. In agreement with previous characterization of LC6 neurons (Wu et al., 2016), the dendrites appear to cover the entire visual field with significant overlap. This overlap is most prominent in the region of the lobula corresponding to the fronto-ventral part of the eye. Note that the central meridian is not shown in D and is not exactly the same as the line indicated by ‘side 90°’. (E) Four groups of five LC6 neurons, corresponding to different regions of visual space, used for exploring retinotopy in F. The filled regions are examples of the anatomical receptive field estimates, for the red and blue neurons in (B), (C), and (D). (F) Central figure shows the skeletons of the four groups of LC6 neurons with four cross-sections: (1) outside the lobula, (2) before the entrance of the glomerulus and (3 , 4) within the glomerulus. The disks in the four cross-section plots represent LC6 axons with the same color code as in (E). The positions of these axons show significant mixing, and do not generally mark the boundary of the group of neurons, as they do in the Lobula. The gray dots correspond to the 45 LC6 neurons not identified by one of the four groups; for compactness of this visualization, up to one gray dot per panel is omitted. (G) An example illustrating the Retinotopy Index, RI, that quantifies the amount of rearrangement occurring when a set of points is mapped from one space to another. On the left side is an example for four points, in the original space (such as the lobula), below is the (Euclidean) distance-based ranking of the points, using each point as a reference. In the middle are example transformations and the resulting distance-ranked lists. As detailed in the text and in the Materials and methods, the quantification is based on counting the number of swaps required to re-order the transformed list of points back to the original list. The RI for a point set and mapping is the averaged RI over all reference points. For mappings that preserve the order, the RI will be close to 1, while RI ~ 0, corresponds to (on average) a random mapping of the points (and therefore a random re-ordering of retinotopy). (H) Simulated examples of 2 mappings applied to a (jittered) grid of 66 points. In the first example (‘block-randomized’), the mapping maintains most of the global organization, but within the local blocks, the order has been randomized, resulting in an RI that is close to, but reduced from 1. In the second example, the order of the points has been completely randomized and the resulting RI for this mapping is ~ 0. (I) Retinotopy Indices for all 65 LC6 neurons (shown as individual points) at four cross-sections defined in (F) (plotted with their corresponding colors), with the lobula being the original space (RI = 1 at that location). The black dot and horizontal bars denote the mean ± SEM. Asterisks represent the result of the Mann-Whitney U test, comparing the population to 0; *p<0.05, ***p<0.001.