the parabigeminal nucleus). The nucleus isthmii is divided into two parts, called isthmus pars magnocellularis (Imc; "the part with the large cells") and isthmus pars parvocellularis (Ipc); "the part with the small cells"). Connections between the three areas — optic tectum, Ipc, and Imc — are topographic. Neurons in the superficial layers of the optic tectum project to corresponding points in the Ipc and Imc. The projections to the Ipc are tightly focused, while the projections to the Imc are somewhat more diffuse. Ipc gives rise to tightly focused cholinergic projections both to Imc and the optic tectum. In the optic tectum, the cholinergic inputs from Ipc ramify to give rise to terminals that extend across an entire column, from top to bottom. Imc, in contrast, gives rise to GABAergic projections to Ipc and optic tectum that spread very broadly in the lateral dimensions, encompassing most of the retinotopic map. Thus, the tectum-Ipc-Imc circuit causes tectal activity to produce recurrent feedback that involves tightly focused excitation of a small column of neighboring tectal neurons, together with global inhibition of distant tectal neurons. The optic tectum is involved in many responses including swimming in fish, flight in birds, tongue-strikes toward prey in frogs, and fang-strikes in snakes. In some species, including fish and birds, the optic tectum, also known as the optic lobe, is one of the largest components of the brain. In hagfish, lamprey, and shark it is a relatively small structure, but in teleost fish it is greatly expanded, in some cases becoming the largest structure in the brain. In amphibians, reptiles, and especially birds it is also a very significant component. In snakes that can detect infrared radiation, such as pythons and pit vipers, the initial neural input is through the trigeminal nerve instead of the optic tract. The rest of the processing is similar to that of the visual sense and, thus, involves the optic tectum. Fish Lamprey The lamprey has been extensively studied because it has a relatively simple brain that is thought in many respects to reflect the brain structure of early vertebrate ancestors. Inspired by the pioneering work of Carl Rovainen that began in the 1960s (see ), since the 1970s Sten Grillner and his colleagues at the Karolinska Institute in Stockholm have used the lamprey as a model system to try to work out the principles of motor control in vertebrates, starting in