Definitions

Superior colliculus

Superior colliculus

The superior colliculus (Latin, higher hill) is a paired structure that is part of the brain's tectal area. Superior Colliculus neurons respond to visual, auditory and somatosensory stimuli.

Structure and relations

The two superior colliculi sit below the thalamus and surround the pineal gland in the mesencephalon of vertebrate brains. It comprises the caudal 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, quadruplet bodies).

Function

In humans, the superior colliculus (SC) is involved in the generation of saccadic (fast) eye movements and eye-head coordination. Afferents to the SC originate in the cerebral cortex, inferior colliculus, retina, basal ganglia, and spinal cord. Efferents project to the paramedian pontine reticular formation, spinal cord, and elsewhere. In humans, as in most larger vertebrates, sensory information that goes to the mesencephalon will be relayed via the thalamus to the cerebral cortex for interpretation. However, the SC can also mediate some oculomotor movements without cortical involvement.

The SC only receives visual inputs in its superficial layers, whereas the deeper layers of the colliculus receive also auditory and somatosensory inputs and are connected to many sensorimotor areas of the brain. The colliculus as a whole is thought to help orient the head and eyes toward something seen and heard.

Diversity

Primates

It is usually accepted that the primate superior colliculus is unique among mammals, in that it does not contain a complete map of the visual field seen by the contralateral eye. Instead, like the visual cortex and lateral geniculate nucleus, each colliculus only represents the contralateral half of the visual field, up to the midline, and excludes a representation of the ipsilateral half. This functional characteristic is explained by the absence, in primates, of anatomical connections between the retinal ganglion cells in the temporal half of the retina and the contralateral superior colliculus. In other mammals, the retinal ganglion cells throughout the contralateral retina project to the contralateral colliculus. This distinction between primates and non-primates has been one of the key lines of evidence in support of the flying primates theory proposed by Australian neuroscientist Jack Pettigrew in 1986, after he discovered that flying foxes (megabats) resemble primates in terms of the pattern of anatomical connections between the retina and superior colliculus.

Other vertebrates

In echolocating bats, the SC has been shown to influence vocalization parameters and ear movements, both orienting components of the bat's biosonar system, thereby assisting in orienting movements.

The comparable area of the mesencephalon of non-mammalian vertebrates is called the optic tectum. In amphibians, reptiles, and fish, the optic tectum is the largest visual processing area, though its function remains largely unknown. It seems to be required for predator/prey discrimination leading to escape or hunting behavior respectively. In contrast, the role of the SC for visual discrimination is less prominent in more complex vertebrates.

In snakes with infrared vision like pythons and pit vipers though the initial neural input is through trigeminal nerve instead of optic nerve, the rest of the processing circuits for this form of sense is quite similar to visual sense and thus involves the superior colliculi.

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