Connections between the OT and three divisions of the bird nucleus isthmi: Ipc, SLu, and Imc. Radial tectal neurons (blue) in the tectal layer 10b project topographically on Imc, Ipc, and SLu. Imc-Is neurons (a) and Imc-Te neurons (b) project widely on Ipc/SLu and OT, respectively. Ipc and SLu axons (green) terminate in narrow columnar arrays within certain tectal layers. The gray shading of the OT indicates retino-recipient tectal layers. The blue shading within Imc, Ipc, and SLu, indicates tectal termination.