In neuroanatomy, the pretectal area, or pretectum, is a midbrain structure composed of seven nuclei and comprises part of the subcortical visual system. Through reciprocal bilateral projections from the retina, it is involved primarily in mediating behavioral responses to acute changes in ambient light such as the pupillary light reflex, the optokinetic reflex, and temporary changes to the circadian rhythm. In addition to the pretectum's role in the visual system, the anterior pretectal nucleus has been found to mediate somatosensory and nociceptive information. Location and structure The pretectum is a bilateral group of highly interconnected nuclei located near the junction of the midbrain and forebrain. The pretectum is generally classified as a midbrain structure, although because of its proximity to the forebrain it is sometimes classified as part of the caudal diencephalon (forebrain). Within vertebrates, the pretectum is located directly anterior to the superior colliculus and posterior to the thalamus. It is situated above the periaqueductal grey and nucleus of the posterior commissure. Several nuclei have been identified within the pretectum, although their borders can be difficult to define and there has been debate over which regions should be included and their precise names. The five primary nuclei are: the olivary pretectal nucleus (ON), the nucleus of the optic tract (NOT), and the anterior (NPA), medial (NPM), and posterior (NPP) pretectal nuclei. The NOT consists of relatively large cells and is located between the superior colliculi. The ON is located medial to the NOT and has a tail that extends between the NOT and NPP, which is ventral to the ON. Two additional nuclei have also been identified: the posterior limitans (PLi) and the commisural pretectal area (CPA). While these two regions have not been examined to the same extent as the five primary nuclei, research has shown both the PLi and CPA to receive retinal input, which suggests a role in processing visual information. Inputs The pretectum receives significant binocular input from photosensitive ganglion cells in the retina. In primates these afferents are bilateral while in rodents they project from the contralateral retina. The majority of these retino-pretectal projections go to the ON and NOT while other pretectal nuclei receive minor retinal input in mammals including the posterior, medial, and anterior pretectal nuclei. The NOT receives input from several regions. From the thalamus the NOT receives inhibitory projections from GABA-producing neurons in the ipsilateral lateral geniculate nucleus and bilateral