The superior colliculus is a set of two bumps on the dorsal side of the midbrain. A larger region, the optic tectum, is formed by this structure and the inferior colliculus. Sometimes, the superior colliculus is referred to simply as the tectum. Unlike the inferior colliculus, which is involved in hearing, the tectum plays a role in processing vision.
This structure is involved with visual reflexes. Both the visual cortex and the retina of the eye itself project information to the outer layers of the tectum. Intermediate layers also receive sensory input from both visual and auditory neurons, as well as input from motor centers. The deepest layers receive mainly motor input, and can even direct eye movement and other motor actions. This wide variety of input types helps this structure to move the head and eyes toward sensory stimuli.
In each layer, neurons of the superior colliculus are arranged in a map. This representational map is aligned with retinal cells. Functionally, this allows activation of different retinal cells, triggering a corresponding response on the map. The tectum can then orient the eyes and head in the same direction the stimuli appeared.
To accomplish the orienting motions, the superior colliculus sends connections from its deeper layers to the cervical spinal tract. These projections extend all the way to the cervical cord, passing through the brainstem on their way. Signals sent through this pathway help the body orient the neck and head in the direction of stimuli. Not only visual stimuli will cause this orientation reflex to occur. Auditory and somatosensory, or touch, stimuli will also activate this pathway.
Intermediate and deep neurons also send motor projections directly to the eyes. Visual signals from the right side of the retinas cross into the left hemisphere of the brain. There is a similar crossover with the superior colliculus' control of eye movement. Cells in the left tectum will orient eye movement to the right.
Head movement and eye orientation are the main reactions this structure controls in human beings, along with some arm movements. Other animals have their own unique reflex behaviors that are controlled by the tectum, too. Frogs use this structure to rapidly direct their tongue flicks to catch prey, and rats can turn their entire bodies based on tectal activity. Humans have major cortical areas that expand the visual and motor centers, which is why this structure is not as proportionally as large as it is in other species.