SUPERIOR COLLICULUS

The superior colliculi are the anterior pair of four bumps on the dorsal surface of a mammalian midbrain. The other, more posterior pair are called the inferior colliculi. In the normal human anatomical position the anterior pair are somewhat above, as well as in front of the posterior pair, hence the names superior and inferior. Colliculi is Latin for small hills or bumps. The four colliculi collectively are known as the corpora quadrigemina, or body with four parts. They make up the roof, or tectum, of the midbrain.

The superior colliculi are a major terminus of fibers of the optic nerve. Like many structures in the visual system, the colliculi present a laminated appearance. In placental and marsupial mammals, the entering fibers of the optic nerve, which are axons of retinal ganglion cells from the eye, make up the second layer of the colliculus. The more superficial first layer is composed largely of cell bodies. Beneath these top two layers are four more layers of cells and fibers in alternation.

Monotreme mammals, and most non-mammalian vertebrates, do not have the superficial top layer of cell bodies: the fibers of the optic nerve are the most superficial part of the colliculus.

Many vertebrates do not have prominent inferior colliculi, which are part of the auditory system, and in these animals the superior colliculi are more commonly known as the optic tectum. In most vertebrates the optic tectum is the primary site of input from the eyes.

Many visual reflexes depend upon pathways through the superior colliculi. These visual midbrain structures also have substantial connections through the thalamus to the cerebral cortex, and may play other important roles in visual perception.