Neuronal wiring diagram of an adult brain

So far we have mentioned neurons that connect the optic lobes with each other, or with the central brain. The intricate circuitry within each optic lobe is also included in the FlyWire brain connectome. Matsliah et al. analysed and typed all neurons intrinsic to the right optic lobe. Photoreceptor axons terminate in the lamina and medulla, neuropils of the optic lobes (Fig. 5a,b). Each eye contains approximately 800 ommatidia that map to columns in the lamina that are arranged in a hexagonal lattice (Fig. 5c). This structure repeats in subsequent neuropils from lamina to medulla to lobula to lobula plate. The neuropils have been finely subdivided into layers that are perpendicular to the columns. The 2D visual field is mapped onto each layer and any given cell type tends to synapse in some subset of the layers. Cell types vary greatly in size, with uni-columnar cell types being the smallest; (for example, Mi4); at the other extreme are large cells that span almost all columns (for example, Dm17); in between the extremes are many multi-columnar cell types (for example, Dm12) (Fig. 5c).

Mi4 is a true ‘tiling’ type—that is, its arbors cover the visual field with little or no overlap, and have similar size and shape (Fig. 5c). Dm12 arbors overlap with each other, but the spatial arrangement is still regular. These and other distal medullary cell types were previously characterized by multicolour light microscopy. Our electron microscopy reconstructions reveal even more detailed information about the spatial patterning of these types (for example, co-fasciculation of neurites of neighbouring Dm12 cells; Fig. 5c). More importantly, the FlyWire reconstruction encompasses all multi-columnar cell types, including those outside the medulla. Judging from the many examples we have studied throughout the optic lobe, it seems that regular coverage of the visual field without gaps is a defining criterion for most cell types, similar to mammalian retina. There are, however, exceptional cell types that cover the visual field in an irregular manner. For example, there are exactly two LPi14 cells per optic lobe. The shapes of each pair are complementary, as if they were created by cutting the visual field into two pieces with a jigsaw (Fig. 5d); this tiling was not evident when reconstructing only a portion of an optic lobe.

Much of the existing research on wide-field visual motion processing has relied on the simplifying idea that the computations are mostly in columnar circuits, and the columnar outputs are finally integrated by large tangential cells in the lobula plate. This research has been aided by wiring diagrams containing connections between cells in the same column or neighbouring columns. In previous studies, an absence of information across columns has necessitated treating each column as identical in simulations of the optic lobe. The FlyWire brain connectome contains not only the columnar neurons, but also all neurons that extend across columns (Fig. 5c). These neurons are both excitatory and inhibitory, and can support interactions between even distant columns. This opens the possibility of a much richer understanding of optic lobe computations and is further explored by Christenson et al. in investigating hue selectivity.