English: Circuit diagram showing connections between the two parts of nucleus isthmii and the optic tectum. Each area, tectum, Ipc, and Imc, contains a retinotopic map. The connections between them follow the topography. Each map sector in the tectum gives rise to excitatory projections to corresponding sectors of Ipc and Imc, although the ones to Ipc are more tightly focused. Ipc sends tightly focused cholingergic projections to tectum and Imc. Imc sends GABAergic projections both to tectum and Ipc, which diverge very broadly. It has been suggested that this architecture supports winner-take-all dynamics in the tectal circuit.