Fig. 8

Model for spatial summation in the TL-PyrN circuit. A Dorsal view image of a Tg(HuC:lynTagRFP-t) larval brain. In this transgenic, all axon tracts and neuropil areas are fluorescently labeled. Image grayscale was inverted for clarity. Overlayed on the midbrain are reconstructions of a single SMTL (orange), a single RGC axon (cyan), and a single PyrN (red, blue, and green). B Magnified view of SMTL, RGC, and PyrN morphologies. Note large, sparsely branched SMTL axon that forms excitatory inputs (+) onto the SM PyrN dendrite (red) and the compact RGC axon that forms excitatory input (+) onto the SFGS PyrN dendrite (blue). C Model of how spatial summation in the TL-tectal circuit could be mediated by neural convergence at the SMTL-PyrN synapse. Direct RGC input to PyrNs specifies response class. In parallel, the same RGCs provide input to TLPNs that drive visual responses in TL. Three SMTLs with intermediate sized RFs converge onto the same dendrite. Spatial summation of inputs with partially overlapping RFs results in a large, compound RFs in PyrNs