Saturday, November 14, 2009

Superior colliculus

The optic tectum or simply tectum is a paired structure that forms a major component of the vertebrate midbrain. In mammals this structure is more commonly called the superior colliculus (Latin, higher hill), but even in mammals, the adjective tectal is commonly used. The tectum is a layered structure, with a number of layers that varies by species. The superficial layers are sensory-related, and receive input from the eyes as well as other sensory systems. The deep layers are motor-related, capable of activating eye movements as well as other responses. There are also intermediate layers, with multi-sensory cells and motor properties.
The general function of the tectal system is to direct behavioral responses toward specific points in egocentric ("body-centered") space. Each layer of the tectum contains a topographic map of the surrounding world in retinotopic coordinates, and activation of neurons at a particular point in the map evokes a response directed toward the corresponding point in space. In primates, the tectum ("superior colliculus") has been studied mainly with respect to its role in directing eye movements. Visual input from the retina, or "command" input from the cerebral cortex, create a "bump" of activity in the tectal map, which if strong enough induces a saccadic eye movement. Even in primates, however, the tectum is also involved in generating spatially directed head turns, arm-reaching movements, and shifts in attention that do not involve any overt movements. In other species, the tectum is involved in a wide range of responses, including whole-body turns in walking rats, swimming fishes, or flying birds; tongue-strikes toward prey in frogs; fang-strikes in snakes; etc.
In some non-mammal species, including fish and birds, the tectum is one of the largest components of the brain. In mammals, and especially primates, the massive expansion of the cerebral cortex reduces the tectum ("superior colliculus") to a much smaller fraction of the whole brain. Even there, though, it remains functionally very important as the primary integrating center for eye movements.


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