<P> The history of investigation of the optic tectum has been marked by several large shifts in opinion . Before about 1970, most studies involved non-mammals--fish, frogs, birds - that is, species in which the tectum is the dominant structure that receives input from the eyes . The general view then was that the tectum, in these species, is the main visual center in the non-mammalian brain, and, as a consequence, is involved in a wide variety of behaviors . From the 1970s to 1990s, however, neural recordings from mammals, mostly monkeys, focused primarily on the role of the superior colliculus in controlling eye movements . This line of investigation came to dominate the literature to such a degree that the majority opinion was that eye - movement control is the only important function in mammals, a view still reflected in many current textbooks . </P> <P> In the late 1990s, however, experiments using animals whose heads were free to move showed clearly that the SC actually produces gaze shifts, usually composed of combined head and eye movements, rather than eye movements per se . This discovery reawakened interest in the full breadth of functions of the superior colliculus, and led to studies of multisensory integration in a variety of species and situations . Nevertheless, the role of the SC in controlling eye movements is understood in much greater depth than any other function . </P> <P> Behavioral studies have shown that the SC is not needed for object recognition, but plays a critical role in the ability to direct behaviors toward specific objects, and can support this ability even in the absence of the cerebral cortex . Thus, cats with major damage to the visual cortex cannot recognize objects, but may still be able to follow and orient toward moving stimuli, although more slowly than usual . If one half of the SC is removed, however, the cats will circle constantly toward the side of the lesion, and orient compulsively toward objects located there, but fail to orient at all toward objects located in the opposite hemifield . These deficits diminish over time but never disappear . </P> <P> In primates, eye movements can be divided into several types: fixation, in which the eyes are directed toward a motionless object, with eye movements only to compensate for movements of the head; smooth pursuit, in which the eyes move steadily to track a moving object; saccades, in which the eyes move very rapidly from one location to another; and vergence, in which the eyes move simultaneously in opposite directions to obtain or maintain single binocular vision . The superior colliculus is involved in all of these, but its role in saccades has been studied most intensively . </P>

What is not present at the level of the superior colliculus