tectum, also known as the optic lobe, is one of the largest components of the brain. Note on terminology: This article follows terminology established in the literature, using the term "superior colliculus" when discussing mammals and "optic tectum" when discussing either specific non-mammalian species or vertebrates in general. Structure The superior colliculus is a synaptic layered structure. The two superior colliculi sit below the thalamus and surround the pineal gland in the mammalian midbrain. It comprises the dorsal aspect of the midbrain, posterior to the periaqueductal gray and immediately superior to the inferior colliculus. The inferior and superior colliculi are known collectively as the corpora quadrigemina (Latin for quadruplet bodies). The superior colliculi are larger than the inferior colliculi, though the inferior colliculi are more prominent. The brachium of superior colliculus (or superior brachium) is a branch that extends laterally from the superior colliculus, and, passing to the thalamus between the pulvinar and the medial geniculate nuclei, is partly continued into an eminence called the lateral geniculate nucleus, and partly into the optic tract. The superior colliculus is associated with a nearby structure called the parabigeminal nucleus, often referred to as its satellite. In the optic tectum this nearby structure is known as the nucleus isthmi. Neural circuitry The microstructure of the superior colliculus and of the optic tectum, varies across species. As a general rule, there is always a clear distinction between superficial layers, which receive input primarily from the visual system and show primarily visual responses, and deeper layers, which receive many types of input and project to numerous motor-related brain areas. The distinction between these two zones is so clear and consistent that some anatomists have suggested that they should be considered separate brain structures. In mammals, neuroanatomists conventionally identify seven layers The top three layers are called superficial: Lamina I or SZ, the stratum zonale, is a thin layer consisting of small myelinated axons together with marginal and horizontal cells. Lamina II or SGS, the stratum griseum superficiale ("superficial gray layer"), contains many neurons of various shapes and sizes. Lamina III or SO, the stratum opticum ("optic layer"), consists mainly of axons coming from the optic tract. Next come two intermediate layers: Lamina IV or SGI, the stratum griseum intermedium ("intermediate gray layer"), is the thickest layer, and is filled with many neurons of many sizes. This layer is often as thick as all the other