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Superior colliculus
Diagram of the superior colliculus (L) of the human midbrain (shown in red) and surrounding regions. The superior colliculus is surrounded by a red ring and transparent red circle to indicate its location.
Section through midbrain at the level of the superior colliculus showing path of oculomotor nerve
Details
Part ofTectum
Identifiers
Latincolliculus superior
MeSHD013477
NeuroNames473
NeuroLex IDbirnlex_1040
TA98A14.1.06.015
TA25912
THH3.11.03.3.01002
TEcolliculus_by_E5.14.3.3.1.4.4 E5.14.3.3.1.4.4
FMA62403
Anatomical terms of neuroanatomy

In neuroanatomy, the superior colliculus (from Latin 'upper hill') is a structure lying on the roof of the mammalian midbrain.[1] In non-mammalian vertebrates, the homologous structure is known as the optic tectum or optic lobe.[1][2][3] The adjective form tectal is commonly used for both structures.

In mammals, the superior colliculus forms a major component of the midbrain. It is a paired structure and together with the paired inferior colliculi forms the corpora quadrigemina. The superior colliculus is a layered structure, with a pattern that is similar in all mammals.[4] The layers can be grouped into the superficial layers (stratum opticum and above) and the deeper remaining layers. Neurons in the superficial layers receive direct input from the retina and respond almost exclusively to visual stimuli. Many neurons in the deeper layers also respond to other modalities, and some respond to stimuli in multiple modalities.[5] The deeper layers also contain a population of motor-related neurons, capable of activating eye movements as well as other responses.[6] In other vertebrates the number of layers in the homologous optic tectum varies.[4]

The general function of the tectal system is to direct behavioral responses toward specific points in body-centered space. Each layer contains a topographic map of the surrounding world in retinotopic coordinates, and activation of neurons at a particular point in the map evokes a response directed toward the corresponding point in space. In primates, the superior colliculus has been studied mainly with respect to its role in directing eye movements. Visual input from the retina, or "command" input from the cerebral cortex, creates a "bump" of activity in the tectal map which, if strong enough, induces a saccadic eye movement. Even in primates, however, the superior colliculus is also involved in generating spatially directed head turns, arm-reaching movements,[7] and shifts in attention that do not involve any overt movements.[8] In other species, the superior colliculus is involved in a wide range of responses, including whole-body turns in walking rats. In mammals, and especially primates, the massive expansion of the cerebral cortex reduces the superior colliculus to a much smaller fraction of the whole brain. It remains nonetheless important in terms of its function as the primary integrating center for eye movements.

In non-mammalian species the optic tectum is involved in many responses including swimming in fish, flying in birds, tongue-strikes toward prey in frogs, and fang-strikes in snakes. In some species, including fish and birds, the optic 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

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See also: Nuclei colliculi superioris
Section of mid-brain at level of superior colliculi.
Hind- and mid-brains; postero-lateral view. Superior colliculus labeled in blue.

The superior colliculus is a paired structure of the dorsal midbrain and is part of the midbrain tectum. The two superior colliculi are situated inferior/caudal to the pineal gland and the splenium of corpus callosum. They are overlapped by the pulvinar of the thalamus, and a medial geniculate nucleus of the thalamus is situated lateral to either superior colliculus.[9] The two inferior colliculi are situated immediately inferior/caudal to the superior colliculi; 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.[10]

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.[citation needed]

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.[11]

The superior colliculus is a synaptic layered structure.[12] 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, seven layers are identified[13] 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 layers together. It is often subdivided into "upper" and "lower" parts.
  • Lamina V or SAI, the stratum album intermedium ("intermediate white layer"), consists mainly of fibers from various sources.

Finally come the two deep layers:

  • Lamina VI or SGP, the stratum griseum profundum ("deep gray layer"), consists of loosely packed neurons and myelinated fibers.
  • Lamina VII or SAP, the stratum album profundum ("deep white layer"), lying directly above the periaqueductal gray, consists entirely of fibers.

Neural circuitry

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The superficial layers receive input mainly from the retina, vision-related areas of the cerebral cortex, and two tectal-related structures called the pretectum and parabigeminal nucleus. The retinal input encompasses the entire superficial zone, and is bilateral, although the contralateral portion is more extensive. The cortical input comes most heavily from the primary visual cortex (area 17, V1), the secondary visual cortex (areas 18 and 19), and the frontal eye fields. The parabigeminal nucleus plays a very important role in tectal function that is described below.

In contrast to the vision-dominated inputs to the superficial layers, the intermediate and deep layers receive inputs from a very diverse set of sensory and motor structures. Most areas of the cerebral cortex project to these layers, although the input from "association" areas tends to be heavier than the input from primary sensory or motor areas.[14] However, the cortical areas involved, and the strength of their relative projections, differ across species.[15] Another important input comes from the substantia nigra, pars reticulata, a component of the basal ganglia. This projection uses the inhibitory neurotransmitter GABA, and is thought to exert a "gating" effect on the superior colliculus. The intermediate and deep layers also receive input from the spinal trigeminal nucleus, which conveys somatosensory information from the face, as well as the hypothalamus, zona incerta, thalamus, and inferior colliculus.

In addition to their distinctive inputs, the superficial and deep zones of the superior colliculus also have distinctive outputs. One of the most important outputs goes to the pulvinar and lateral intermediate areas of the thalamus, which in turn project to areas of the cerebral cortex that are involved in controlling eye movements. There are also projections from the superficial zone to the pretectal nuclei, lateral geniculate nucleus of the thalamus, and the parabigeminal nucleus. The projections from the deeper layers are more extensive. There are two large descending pathways, traveling to the brainstem and spinal cord, and numerous ascending projections to a variety of sensory and motor centers, including several that are involved in generating eye movements.

Both colliculi also have descending projections to the paramedian pontine reticular formation and spinal cord, and thus can be involved in responses to stimuli faster than cortical processing would allow.

Mosaic structure

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On detailed examination, the collicular layers are actually not smooth sheets, but divided into a honeycomb arrangement of discrete columns.[16] The clearest indication of columnar structure comes from the cholinergic inputs arising from the parabigeminal nucleus, whose terminals form evenly spaced clusters that extend from top to bottom of the tectum.[17] Several other neurochemical markers including calretinin, parvalbumin, GAP-43, and NMDA receptors, and connections with numerous other brain structures in the brainstem and diencephalon, also show a corresponding inhomogeneity.[18] The total number of columns has been estimated at around 100.[16] The functional significance of this columnar architecture is not clear, but it is interesting that recent evidence has implicated the cholinergic inputs as part of a recurrent circuit producing winner-take-all dynamics within the tectum, as described in more detail below.

All species that have been examined—including mammals and non-mammals—show compartmentalization, but there are some systematic differences in the details of the arrangement.[17] In species with a streak-type retina (mainly species with laterally placed eyes, such as rabbits and deer), the compartments cover the full extent of the SC. In species with a centrally placed fovea, however, the compartmentalization breaks down in the front (rostral) part of the SC. This portion of the SC contains many "fixation" neurons that fire continually while the eyes remain fixed in a constant position.

Function

[edit]

The history of investigation of the optic tectum has been marked by several large shifts in opinion. Before about 1970, most studies involved non-mammals—fish, frogs, birds—that is, species in which the optic tectum is the dominant structure that receives input from the eyes. The general view then was that the optic tectum, in these species, is the main visual center in the non-mammalian brain, and, as a consequence, is involved in a wide variety of behaviors.[19] From the 1970s to 1990s, however, neural recordings from mammals, mostly monkeys, focused primarily on the role of the superior colliculus in controlling eye movements. This line of investigation came to dominate the literature to such a degree that the majority opinion was that eye-movement control is the only important function in mammals, a view still reflected in many current textbooks.

In the late 1990s, however, experiments using animals whose heads were free to move showed clearly that the SC actually produces gaze shifts, usually composed of combined head and eye movements, rather than eye movements per se. This discovery reawakened interest in the full breadth of functions of the superior colliculus, and led to studies of multisensory integration in a variety of species and situations. Nevertheless, the role of the SC in controlling eye movements is understood in much greater depth than any other function.

Behavioral studies have shown that the SC is not needed for object recognition, but plays a critical role in the ability to direct behaviors toward specific objects, and can support this ability even in the absence of the cerebral cortex.[20] Thus, cats with major damage to the visual cortex cannot recognize objects, but may still be able to follow and orient toward moving stimuli, although more slowly than usual. If one half of the SC is removed, however, the cats will circle constantly toward the side of the lesion, and orient compulsively toward objects located there, but fail to orient at all toward objects located in the opposite hemifield. These deficits diminish over time but never disappear.

Eye movements

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Main article: Eye movement

In primates, eye movements can be divided into several types: fixation, in which the eyes are directed toward a motionless object, with eye movements only to compensate for movements of the head; smooth pursuit, in which the eyes move steadily to track a moving object; saccades, in which the eyes move very rapidly from one location to another; and vergence, in which the eyes move simultaneously in opposite directions to obtain or maintain single binocular vision. The superior colliculus is involved in all of these, but its role in saccades has been studied most intensively.[21][22][23]

Each of the two colliculi—one on each side of the brain—contains a two-dimensional map representing half of the visual field. The fovea—the region of maximum sensitivity—is represented at the front edge of the map, and the periphery at the back edge. Eye movements are evoked by activity in the deep layers of the SC. During fixation, neurons near the front edge—the foveal zone—are tonically active. During smooth pursuit, neurons a small distance from the front edge are activated, leading to small eye movements. For saccades, neurons are activated in a region that represents the point to which the saccade will be directed. Just prior to a saccade, activity rapidly builds up at the target location and decreases in other parts of the SC. The coding is rather broad, so that for any given saccade the activity profile forms a "hill" that encompasses a substantial fraction of the collicular map: The location of the peak of this "hill" represents the saccade target.[24]

The SC encodes the target of a gaze shift, but it does not seem to specify the precise movements needed to get there.[25] The decomposition of a gaze shift into head and eye movements and the precise trajectory of the eye during a saccade depend on integration of collicular and non-collicular signals by downstream motor areas, in ways that are not yet well understood. Regardless of how the movement is evoked or performed, the SC encodes it in "retinotopic" coordinates: that is, the location of the SC 'hill" corresponds to a fixed location on the retina. This seems to contradict the observation that stimulation of a single point on the SC can result in different gaze shift directions, depending on initial eye orientation. However, it has been shown that this is because the retinal location of a stimulus is a non-linear function of target location, eye orientation, and the spherical geometry of the eye.[26]

There has been some controversy about whether the SC merely commands eye movements, and leaves the execution to other structures, or whether it actively participates in the performance of a saccade. In 1991, Munoz et al., on the basis of data they collected, argued that, during a saccade, the "hill" of activity in the SC moves gradually, to reflect the changing offset of the eye from the target location while the saccade is progressing.[27] At present, the predominant view is that, although the "hill" does shift slightly during a saccade, it does not shift in the steady and proportionate way that the "moving hill" hypothesis predicts.[28] However, moving hills may play another role in the superior colliculus; more recent experiments have demonstrated a continuously moving hill of visual memory activity when the eyes move slowly while a separate saccade target is retained.[29]

The output from the motor sector of the SC goes to a set of midbrain and brainstem nuclei, which transform the "place" code used by the SC into the "rate" code used by oculomotor neurons. Eye movements are generated by six muscles, arranged in three orthogonally-aligned pairs. Thus, at the level of the final common path, eye movements are encoded in essentially a Cartesian coordinate system.

Although the SC receives a strong input directly from the retina, in primates it is largely under the control of the cerebral cortex, which contains several areas that are involved in determining eye movements.[30] The frontal eye fields, a portion of the motor cortex, are involved in triggering intentional saccades, and an adjoining area, the supplementary eye fields, are involved in organizing groups of saccades into sequences. The parietal eye fields, farther back in the brain, are involved mainly in reflexive saccades, made in response to changes in the view. Recent evidence[31][32] suggests that the primary visual cortex (V1) guides reflexive eye movements, according to V1 Saliency Hypothesis, using a bottom-up saliency map of the visual field generated in V1 from external visual inputs.[33]

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.[8][34][35][36]

The superior colliculus also receives auditory information from the inferior colliculus. This auditory information is integrated with the visual information already present to produce the ventriloquism effect.

Distractibility

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As well as being related to eye movements, the SC appears to have an important role to play in the circuitry underpinning distractibility. Heightened distractibility occurs in normal aging [37] and is also a central feature in a number of medical conditions, including attention deficit hyperactivity disorder (ADHD).[38] Research has shown that lesions to the SC in a number of species can result in heightened distractibility[39][40] and, in humans, removing the inhibitory control on the SC from the pre-frontal cortex, therefore increasing activity in the area, also increases distractibility.[41] Research in an animal model of ADHD, the spontaneously hypertensive rat, also shows altered collicular-dependent behaviours[42][43] and physiology.[43] Furthermore, amphetamine (a mainstay treatment for ADHD) also suppresses activity in the colliculus in healthy animals.[44]

Other animals

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Other mammals

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Primates

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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 represents only the contralateral half of the visual field, up to the midline, and excludes a representation of the ipsilateral half.[45] 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.[46]

Cats

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In the cat the superior colliculus projects through the reticular formation and interacts with motor neurons in the brainstem.[47]

Bats

[edit]

Bats are not, in fact, blind, but they depend much more on echolocation than vision for navigation and prey capture. They obtain information about the surrounding world by emitting sonar chirps and then listening for the echoes. Their brains are highly specialized for this process, and some of these specializations appear in the superior colliculus.[48] In bats, the retinal projection occupies only a thin zone just beneath the surface, but there are extensive inputs from auditory areas, and outputs to motor areas capable of orienting the ears, head, or body. Echoes coming from different directions activate neurons at different locations in the collicular layers,[49] and activation of collicular neurons influences the chirps that the bats emit. Thus, there is a strong case that the superior colliculus performs the same sorts of functions for the auditory-guided behaviors of bats that it performs for the visual-guided behaviors of other species.

Bats are usually classified into two main groups: Microchiroptera (the most numerous, and commonly found throughout the world), and Megachiroptera (fruit bats, found in Asia, Africa and Australasia). With one exception, Megabats do not echolocate, and rely on a developed sense of vision to navigate. The visual receptive fields of neurons in the superior colliculus in these animals form a precise map of the retina, similar to that found in cats and primates.

Rodents

[edit]

The superior colliculus in rodents have been hypothesized to mediate sensory-guided approach and avoidance behaviors.[50][51] Studies employing circuit analysis tools on mouse superior colliculus have revealed several important functions.[12] In a series of studies, researchers have identified a set of Ying-Yang circuit modules in the superior colliculus to initiate prey capture and predator avoidance behaviors in mice.[52][53][54][55] In mice it has been found to map the touch control of tongue for handling food and water when chewing and swallowing.[56] By using single-cell RNA-sequencing, researchers have analyzed the gene expression profiles of superior colliculus neurons and identified the unique genetic markers of these circuit modules.[57]

Other vertebrates

[edit]

Optic tectum

[edit]
Schematic circuit diagram of topographic connections between the optic tectum and the two parts of nucleus isthmii.
H&E stain of chicken optic tectum at E7 (embryonic day 7) showing the generative zone (GZ), the migrating zone (MZ) and the first neuronal lamina (L1). Scale bar 200 μm. From Caltharp et al., 2007.[58]

The optic tectum is the visual center in the non-mammalian brain which develops from the alar plate of the mesencephalon. In these other vertebrates the connections from the optic tectum are important for the recognition and reaction to various sized objects which is facilitated by excitatory optic nerve transmitters like L-glutamate.[59]

Disrupting visual experience early on in zebrafish development results in a change in tectal activity. Changes in tectal activity resulted in an inability to successfully hunt and capture prey.[60] Hypothalamus inhibitory signaling to the deep tectal neuropil is important in tectal processing in zebrafish larvae. The tectal neuropil contains structures including periventricular neuronal axons and dendrites. The neuropil also contains GABAergic superficial inhibitory neurons located in stratum opticum.[61] Instead of a large cerebral cortex, zebrafish have a relatively large optic tectum that is hypothesized to carry out some of the visual processing that the cortex performs in mammals.[62]

Recent lesion studies have suggested that the optic tectum has no influence over higher-order motion responses like the optomotor response or the optokinetic response,[63] but may be more integral to lower-order cues in motion perception like in the identification of small objects.[64]

The optic tectum is one of the fundamental components of the vertebrate brain, existing across a range of species.[65] Some aspects of the structure are very consistent, including a structure composed of a number of layers, with a dense input from the optic tracts to the superficial layers and another strong input conveying somatosensory input to deeper layers. Other aspects are highly variable, such as the total number of layers (from 3 in the African lungfish to 15 in the goldfish[66]), and the number of different types of cells (from 2 in the lungfish to 27 in the house sparrow[66]).

The optic tectum is closely associated with an adjoining structure called the nucleus isthmi, which has drawn a lot of interest because it evidently makes a very important contribution to tectal function.[67] (In the superior colliculus the like structure is termed the parabigeminal nucleus). The nucleus isthmii is divided into two parts, called isthmus pars magnocellularis (Imc; "the part with the large cells") and isthmus pars parvocellularis (Ipc); "the part with the small cells"). Connections between the three areas—optic tectum, Ipc, and Imc—are topographic. Neurons in the superficial layers of the optic tectum project to corresponding points in the Ipc and Imc. The projections to the Ipc are tightly focused, while the projections to the Imc are somewhat more diffuse. Ipc gives rise to tightly focused cholinergic projections both to Imc and the optic tectum. In the optic tectum, the cholinergic inputs from Ipc ramify to give rise to terminals that extend across an entire column, from top to bottom. Imc, in contrast, gives rise to GABAergic projections to Ipc and optic tectum that spread very broadly in the lateral dimensions, encompassing most of the retinotopic map. Thus, the tectum-Ipc-Imc circuit causes tectal activity to produce recurrent feedback that involves tightly focused excitation of a small column of neighboring tectal neurons, together with global inhibition of distant tectal neurons.

The optic tectum is involved in many responses including swimming in fish, flight in birds, tongue-strikes toward prey in frogs, and fang-strikes in snakes. In some species, including fish and birds, the optic tectum, also known as the optic lobe, is one of the largest components of the brain.

In hagfish, lamprey, and shark it is a relatively small structure, but in teleost fish it is greatly expanded, in some cases becoming the largest structure in the brain. In amphibians, reptiles, and especially birds it is also a very significant component.[66]

In snakes that can detect infrared radiation, such as pythons and pit vipers, the initial neural input is through the trigeminal nerve instead of the optic tract. The rest of the processing is similar to that of the visual sense and, thus, involves the optic tectum.[68]

Fish

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The brain of a cod, with the optic tectum highlighted

Lamprey

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The lamprey has been extensively studied because it has a relatively simple brain that is thought in many respects to reflect the brain structure of early vertebrate ancestors. Inspired by the pioneering work of Carl Rovainen that began in the 1960s (see [69]), since the 1970s Sten Grillner and his colleagues at the Karolinska Institute in Stockholm have used the lamprey as a model system to try to work out the principles of motor control in vertebrates, starting in the spinal cord and working upward into the brain.[70] In common with other systems (see [71] for a historical perspective of the idea), neural circuits within the spinal cord seem capable of generating some basic rhythmic motor patterns underlying swimming, and that these circuits are influenced by specific locomotor areas in the brainstem and midbrain, that are in turn influenced by higher brain structures including the basal ganglia and tectum. In a study of the lamprey tectum published in 2007,[72] they found that electrical stimulation could elicit eye movements, lateral bending movements, or swimming activity, and that the type, amplitude, and direction of movement varied as a function of the location within the tectum that was stimulated. These findings were interpreted as consistent with the idea that the tectum generates goal-directed locomotion in the lamprey as shown in other species.

Birds

[edit]
Drawing by Ramon y Cajal of several types of Golgi-stained neurons in the optic tectum of a sparrow.

In birds the optic tectum is involved in flight and is one of the largest brain components. The study of avian visual processing has enabled a greater understanding of that in mammals including humans.[73]

See also

[edit]
This article uses anatomical terminology.

Additional images

[edit]
  • Scheme showing central connections of the optic nerves and optic tracts. (Superior colliculus visible near center.)
    Scheme showing central connections of the optic nerves and optic tracts. (Superior colliculus visible near center.)
  • Superior colliculus (coronal section)
    Superior colliculus (coronal section)
  • Brainstem. Posterior view.
    Brainstem. Posterior view.

Notes

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Superior colliculus

View on Grokipedia
from Grokipedia
The superior colliculus is a paired, layered located on the dorsal aspect of the rostral in mammals, serving as a key sensorimotor hub that integrates multisensory inputs—primarily visual, but also auditory and somatosensory—to detect salient stimuli and initiate rapid orienting responses, such as eye and head movements toward or away from environmental cues. Homologous to the optic tectum in non-mammalian vertebrates, it processes optical stimuli to orient attention and coordinate gaze, functioning as a counterpart that receives direct projections and cortical feedback to mediate reflexive behaviors. Anatomically, the superior colliculus is located on the dorsal surface of the posterior , rostral to the , caudal to the , and dorsal to the , forming part of the tectum with a topographic organization that mirrors the . It consists of seven alternating neuronal and fibrous layers, broadly divided into superficial layers (stratum zonale, griseum superficiale, and opticum) dedicated to , particularly vision, and deeper intermediate and deep layers that handle motor outputs and . In mice, approximately 90% of cells project to the superficial layers, while in like macaques, this is reduced to about 10%, highlighting species-specific adaptations in visual reliance. Embryologically, it arises from the mesencephalon during weeks 4–5 of development, regulated by genes such as Pax7 and engrailed, which control and laminar formation. Its blood supply derives from branches of the , including the collicular and posteromedial choroidal arteries, with potential contributions from the . Functionally, the superior colliculus plays a pivotal role in generating saccadic eye movements, modulating , and eliciting innate behaviors such as defensive responses to threats or predatory sequences, by transforming sensory maps into motor commands via projections to nuclei, the , and . The superficial layers primarily relay visual information from the and to subcortical targets like the and pretectum, while deeper layers receive non-visual inputs from the , somatosensory areas, , and , enabling multisensory convergence for enhanced stimulus localization. It also contributes to cognitive processes, including and saliency assessment, with and neurons in these layers driving behaviors like fear via connections to the or prey capture through links to the . Clinically, lesions or tumors such as tectal gliomas can impair control, leading to deficits in visual orientation, , or increased , underscoring its integration within broader sensorimotor networks implicated in disorders like autism and .

Anatomy

Location and gross anatomy

The superior colliculus consists of a pair of dome-shaped structures located in the dorsal aspect of the tectum, forming the rostral portion of the corpora quadrigemina. These paired elevations are positioned inferior to the and superior (rostral) to the inferior colliculi, with their posterior surface visible on the dorsal . The two superior colliculi are separated by the shallow cruciform sulcus in the midline, contributing to the quadrigeminal plate's characteristic appearance. It is prominently visible on midsagittal sections of the , where it appears as a rounded prominence anterior to the pineal recess and posterior to the aqueduct. The blood supply to the superior colliculus is derived primarily from branches of the , including the collicular artery and posteromedial choroidal artery, with potential contributions from the . In terms of gross relations, the superior colliculus lies dorsal to the and , ventral to the and splenium of the , and lateral to the midline structures such as the trochlear nucleus at transitional levels; ventrally, it relates to the cerebral peduncles via the expanse. The (cranial nerve IV) emerges from the just caudal to the but relates proximally to the superior colliculus via its in the anterior medullary velum.

Layered organization

The superior colliculus exhibits a highly organized laminar structure consisting of seven alternating fiber-rich () and cell-rich (gray matter) layers, extending from superficial to deep: the stratum zonale (SZ), stratum griseum superficiale (SGS), stratum opticum (SO), stratum griseum intermedium (SGI), stratum album intermedium (SAI), stratum griseum profundum (SGP), and stratum album profundum (SAP). These layers form a topographic that supports the integration of sensory and motor functions, with the overall structure varying slightly by species but maintaining this core seven-layer pattern in mammals. The superficial layers, comprising the SZ, SGS, and SO, are primarily involved in visual processing and are dominated by small GABAergic neurons that constitute approximately 30-45% of cells in these regions across species like mice and cats. These layers feature a precise retinotopic organization mirroring the contralateral visual field, with the SGS containing fusiform and pyramidal cells that receive direct retinal inputs via the optic tract. Approximate thicknesses for the superficial layers total around 0.5 mm in rodents, though this can vary in larger mammals. Distinct cell morphologies in the SGS and SO include horizontal cells with laterally extending dendrites spanning up to 500 μm, vertical cells with radially oriented processes, and stellate cells providing local excitatory connections. In the intermediate layers, the SGI and SAI house wide-field burst neurons critical for generating motor commands, with the SGI featuring a mix of multipolar and cells that exhibit broad receptive fields for sensorimotor transformation. These layers integrate inputs to support orienting behaviors, and their approximate thickness reaches about 1 mm, allowing for complex local circuitry among and projection cells. Stellate and vertical cell types persist here, contributing to inhibitory feedback loops that modulate burst activity. The deep layers, including the SGP and SAP (sometimes referred to as the stratum multiforme and stratum griseum centralis in certain nomenclatures), contain larger projection neurons with diverse morphologies such as multipolar and triangular cells that facilitate multisensory convergence and descending outputs. These regions exhibit expanded receptive fields and include a higher proportion of neurons marked by VGLUT2 expression, enabling integration of visual, auditory, and somatosensory signals. Approximate deep layer thickness is around 1.5 mm, supporting extensive dendritic arbors of wide-field vertical cells that span multiple laminae. Horizontal and stellate cells are less prominent but contribute to local inhibition within these deeper zones.

Neural circuitry

Afferent inputs

The superior colliculus (SC) receives a diverse array of afferent inputs that integrate sensory information across modalities, primarily targeting its layered structure. The superficial layers, including the stratum zonale (SZ), stratum griseum superficiale (SGS), and stratum opticum (SO), are dominated by visual afferents, while the intermediate layers (stratum griseum intermedium, SGI; stratum album intermedium, SAI) and deep layers (stratum griseum profundum, SGP; stratum album profundum, SAP) incorporate multisensory and modulatory signals. Primary visual inputs originate from the via the optic tract, projecting predominantly to the superficial layers with a strong contralateral dominance; in such as mice, approximately 85-90% of retinal ganglion cells (RGCs) project to the SC overall. These projections establish a retinotopic map of the , with nasal serving the contralateral hemifield. Additional visual inputs arrive from primary (V1) and other visual areas, also targeting the superficial layers to refine spatial processing. Auditory afferents converge on the intermediate and deep layers from the and , forming an audiotopic representation of auditory space that aligns with visual maps. Somatosensory inputs, arising from the and , project to the intermediate layers, maintaining a somatopic organization that maps body surfaces onto the SC's rostrocaudal axis. Cortical projections provide higher-order integration, with visual areas (V1 and extrastriate regions like V3 and MT) targeting superficial and intermediate layers, parietal areas such as the lateral intraparietal area (LIP) and somatosensory cortices innervating intermediate layers, and frontal eye fields (FEF) projecting mainly to intermediate layers for attentional modulation. Subcortical modulatory inputs include GABAergic inhibition from the pars reticulata (SNr) to the intermediate layers, which gates sensory responses, and cholinergic projections from the parabigeminal nucleus to superficial and intermediate layers, enhancing visual saliency detection. Serotonergic projections from the target various layers, adjusting response thresholds to promote approach or avoidance behaviors via 5-HT receptors in a context-dependent manner. Overall, these afferents preserve topographic organization: retinotopic in superficial layers, audiotopic and somatopic in intermediate/deep layers, enabling multisensory alignment for orienting behaviors.

Efferent outputs

The superior colliculus (SC) sends descending efferent projections primarily through the predorsal bundle to key brainstem centers, enabling the coordination of rapid orienting movements. These outputs target the omnipause neurons in the nucleus raphe interpositus, which pause during saccades to facilitate burst firing in -related circuits. The bundle also projects to the horizontal center in the paramedian pontine reticular formation (PPRF), which drives conjugate horizontal eye movements via abducens motor neurons. Additionally, connections extend to the vertical center in the rostral interstitial nucleus of the (riMLF), supporting torsional and vertical saccades. Tectobulbar and tectospinal tracts form additional descending pathways from the SC, particularly from its intermediate and deep layers, to influence head and orientation. The tectobulbar tract targets reticular formation nuclei, such as the gigantocellular and tegmental regions, to mediate postural adjustments and orienting behaviors. In parallel, the tectospinal tract descends contralaterally through the , synapsing on cervical motor neurons to control head turning and muscles. Ascending efferents from the SC relay sensory-motor information to thalamic nuclei for cortical integration. Projections target the pulvinar nucleus, facilitating visual attention and relaying to extrastriate cortex. Outputs also reach the lateral posterior nucleus, which serves as a visual relay in rodents and primates, and the mediodorsal thalamus, influencing prefrontal areas for cognitive processing. The SC maintains crossed and uncrossed tecto-olivary projections to the inferior olive, contributing to cerebellar coordination of smooth pursuit and adaptive motor learning. These pathways, originating from intermediate gray layers, provide climbing fiber inputs that refine gaze stability during orienting. Efferent outputs exhibit layer-specific , reflecting the SC's functional segregation. Superficial layers project to the pretectal nuclei, driving pupillary reflexes and near-response adjustments. In contrast, deep layers send outputs to the pars reticulata of the , modulating inhibition, and to the , influencing motor timing and error correction.

Intrinsic connections

The superior colliculus is organized into a structure comprising discrete modules of approximately 100-200 μm in diameter, forming vertical columns that span its layered and align with specific retinotopic representations of the . These compartments, identified through neurochemical staining and afferent terminal clustering, create an intermingled meshwork where sensory and motor processing is segregated yet integrated vertically, as demonstrated in studies using mapping and retrograde tracing. Within these columns, recurrent collaterals from projection neurons in the intermediate and deep layers provide excitatory feedback to superficial layers, enhancing sensory-motor integration and sustaining activity bursts associated with orienting behaviors. Electrophysiological recordings and anatomical tracing in reveal these collaterals terminate locally, often on , to modulate feedback loops without extensive horizontal spread. Horizontal connections across modules are primarily mediated by wide-field inhibitory in the superficial layers, which generate surround suppression to sharpen receptive fields and filter irrelevant stimuli. These , pharmacologically identified as bicuculline-sensitive, produce a center-excitation/surround-inhibition profile akin to a "Mexican hat" function, as shown in slice preparations from where electrical stimulation evoked over hundreds of micrometers. Feedforward excitation from superficial to deep layers is driven by vertical cells, such as narrow-field vertical neurons, which relay visual signals via and NMDA receptor-mediated synapses to premotor circuits. photostimulation experiments in rat slices confirm this pathway evokes postsynaptic currents in intermediate layer cells, with maximal responses confined to columnar zones approximately 500 μm wide, supporting visuomotor transformation. Tracing studies, including viral and HRP injections, consistently show clustered innervation patterns that respect the mosaic boundaries, with afferents and efferents terminating in discrete patches to reinforce columnar specificity and prevent cross-talk between modules.

Functions

Visual processing

The superficial layers of the superior colliculus (SC) organize visual information into a precise retinotopic , which directly mirrors the topographic projections from retinal ganglion cells (RGCs) to the SC via the retinotectal pathway. This map ensures that adjacent regions of the are represented in adjacent SC locations, facilitating spatial correspondence between retinal input and collicular processing. A key feature of this organization is foveal magnification, where the central foveal region of the retina, responsible for high-acuity vision, occupies a disproportionately larger area in the SC map compared to the peripheral field, emphasizing detailed processing of central stimuli. Within these superficial layers, SC neurons specialize in detecting salient visual features that signal potential behavioral relevance, including motion, abrupt luminance changes, and expanding (looming) stimuli. Direction-selective neurons in the SC respond preferentially to stimuli moving in specific directions, contributing to the early analysis of motion trajectories and enabling rapid detection of approaching objects. This feature detection is particularly tuned for ecologically important cues, such as looming patterns that mimic predator approaches, with neurons exhibiting heightened responses to size expansion over translation or contraction. The SC's visual processing is dominated by inputs from the magnocellular (M) pathway, which supports fast conduction speeds and low , ideal for transient, low-contrast stimuli like motion or flicker. In contrast, the parvocellular (P) pathway, which favors high-resolution color and form processing, plays a lesser role in the SC compared to the ventral cortical stream. This M-pathway bias allows the SC to prioritize rapid, coarse-grained analysis over fine details, processing visual transients in as little as 50-100 ms post-stimulus onset. In the superficial layers of the SC, binocular inputs from both eyes are integrated to extract depth information through , where neurons respond optimally to specific horizontal offsets between left- and right-eye images, providing cues for relative depth and 3D structure. This disparity sensitivity enhances the SC's ability to represent spatial locations in depth, complementing the superficial layers' 2D . Recent reviews and studies from 2023-2025 highlight the SC's role in encoding object-related visual saliency, where wide-field neurons compute de novo saliency maps based on local feature contrasts, independent of behavioral preferences. Additionally, these layers support mechanisms for dynamic scenes, with neurons adapting responses to expected visual loom during locomotion or object motion, suppressing redundant signals while amplifying novel or unexpected changes. Such processing in the SC provides a foundational saliency signal that briefly informs downstream motor outputs for orienting behaviors.

Orienting movements

The superior colliculus plays a central role in initiating reflexive orienting movements, transforming sensory signals into motor commands that direct the eyes, head, and body toward salient stimuli in the environment. Neurons in the intermediate gray layer (SGI) of the superior colliculus generate burst activity that serves as a motor command for these behaviors, particularly , by encoding the vector of the desired shift in a where the location of peak activity determines the direction and amplitude of the . This burst firing descends via descending pathways to premotor centers, triggering rapid, ballistic eye movements that bring the stimulus into foveal view. For saccadic eye movements, burst neurons in the SGI specifically encode the vector—its and direction—allowing for precise, rapid shifts toward unexpected visual or other stimuli. These neurons exhibit high-frequency bursts just prior to and during the , with the spatial distribution of activity across the collicular motor map computing the "center of gravity" to specify the movement metrics. In , this mechanism supports both reflexive and voluntary , though the colliculus is particularly crucial for the former. The superior colliculus also coordinates orienting movements of the head with eye saccades, ensuring efficient shifts through integrated motor outputs. Projections from the SGI to the tectocervical and tectoreticulospinal pathways influence motoneurons and premotor circuits, facilitating head turns that complement ocular movements. The amplitude of these head movements scales with stimulus eccentricity: larger deviations from the current direction elicit greater head contributions to the overall , optimizing the combined eye-head shift while minimizing excessive ocular excursion. In head-restrained conditions, this coordination adjusts such that pure saccades compensate, but in freely moving animals, the colliculus dynamically allocates the shift between eye and head components. Collicular neurons employ both fixed-vector and dynamic gain field mechanisms to represent orienting commands, adapting to varying contexts. Fixed-vector coding generates saccades with invariant metrics relative to the current eye position, as seen in microstimulation studies where evokes consistent displacement vectors. In contrast, dynamic gain fields modulate neuronal activity based on eye position signals, allowing the colliculus to compute desired shifts in head- or space-centered coordinates rather than purely eye-centered ones; this is evident in neurons whose firing rates vary multiplicatively with orbital position, enabling flexible transformations for combined eye-head movements. These mechanisms ensure robust orienting across different starting positions and stimulus locations. Suppression of unwanted or competing movements is achieved through interactions with the , particularly via tonic inhibition from the pars reticulata (SNr) to the superior colliculus. SNr projections maintain constant inhibition of collicular output neurons, preventing premature or erroneous saccades; during appropriate orienting, phasic pauses in SNr activity release this inhibition, disinhibiting SGI burst neurons to permit movement initiation. This "brake-release" model underlies the colliculus's role in gating reflexive responses, suppressing ipsilateral or irrelevant directions while facilitating contralateral orienting. Primate studies provide direct evidence for the superior colliculus's motor role in orienting. Electrical microstimulation of the SGI in monkeys evokes contralateral saccades whose direction and correspond to the stimulated site's on the motor , confirming its command function independent of sensory inputs. Conversely, lesions or reversible inactivation of the superior colliculus impair express saccades—short-latency reflexive eye movements (around 100 ms)—resulting in increased latencies and reduced frequency, while longer voluntary saccades remain relatively spared, highlighting the structure's specificity for rapid orienting. These deficits underscore the colliculus's essential contribution to reflexive shifts in .

Attention and cognition

The superior colliculus (SC) receives top-down modulatory inputs from frontal and parietal cortical areas, which bias saliency maps to enhance selection of behaviorally relevant stimuli. These projections, particularly from the and lateral intraparietal area, amplify neuronal responses in the SC's superficial layers to attended visual targets while suppressing irrelevant distractors, thereby refining the prioritization of spatial information for orienting. Such modulation is crucial for voluntary , as demonstrated in studies where inactivation of these cortical inputs disrupts SC-mediated attentional shifts. In the deep layers of the SC, multisensory integration occurs through convergent audiovisual inputs, where the combination of auditory and visual cues produces superadditive responses that enhance detection and orienting speed compared to unisensory stimulation. This integration is particularly robust for spatiotemporally aligned stimuli, leading to enhanced neural firing rates that facilitate rapid behavioral responses, analogous to perceptual illusions like the in speech processing but applied to non-verbal cues. For instance, neurons in these layers show heightened activity when visual motion is paired with localized sounds, improving saliency computation and reducing reaction times in dynamic environments. The SC contributes to covert attention by modulating visual processing without overt movements, as evidenced by neuronal recordings in mice showing enhanced responses to expected target locations in the . This process involves distractor suppression, where SC activity inhibits responses to irrelevant stimuli, thereby sharpening al focus and reducing distractibility in cluttered scenes. Studies in further indicate that SC inactivation impairs the ability to filter distractors, leading to increased errors in attentional tasks. Recent research from 2024-2025 highlights the SC's role in higher cognition, including rapid object detection where SC neurons preferentially respond to salient real-world objects within 100 ms of stimulus onset, supporting efficient visual search. In predictive processing, the SC integrates prior expectations with incoming sensory data via interactions with primary visual cortex, enabling anticipatory adjustments in perceptual representations. Additionally, the SC coordinates whole-brain dynamics during sudden insights, synchronizing activity across cortical and subcortical networks to facilitate generative problem-solving and abstract reasoning traditionally ascribed to neocortical regions. These findings underscore the SC's involvement in causal higher-order cognitive operations, such as pattern recognition and adaptive decision-making. Noradrenergic modulation from the (LC) to the SC regulates arousal-dependent filtering of sensory inputs, enhancing SC responsiveness during heightened vigilance states. LC projections release norepinephrine onto SC neurons, increasing gain for salient stimuli while suppressing noise, as shown in arousal manipulations that alter SC firing patterns to prioritize task-relevant information. This modulation is particularly evident in conditions of elevated , where it supports sustained by dynamically adjusting the SC's .

Development

Embryonic origins

The superior colliculus originates from the alar plate of the mesencephalon, the vesicle, during early embryogenesis. This structure emerges as part of the dorsal roof, known as the tectum, with initial formation occurring between gestational weeks 4 and 6, coinciding with the differentiation of the primary vesicles into secondary vesicles. Neuroblasts from the alar plate migrate to form the tectal primordium, establishing the foundational layering that will develop into the superior colliculus bilaterally. Its development is induced by key signaling centers at the midbrain boundaries. The isthmic organizer at the mid-hindbrain junction secretes 8 (Fgf8), which patterns the midbrain and promotes tectal identity by inducing expression of mesencephalic markers such as En1 and En2. Experimental implantation of Fgf8-soaked beads in chick embryos transforms adjacent diencephalic tissue into ectopic tectum-like structures, confirming its inductive role. Complementarily, sonic hedgehog (Shh) from the zona limitans intrathalamica at the di-mesencephalic boundary contributes to rostrocaudal patterning, helping delineate the tectal domain from thalamic regions, though its influence is more pronounced in structures. Early in the superior colliculus involves progenitors in the ventricular zone generating both excitatory and inhibitory neurons. and progenitors arise around embryonic days 11-14 in (corresponding to weeks 6-8), undergoing radial migration along glial scaffolds from the ventricular zone toward the pial surface to populate the nascent tectal layers. This migration is characterized by spindle-shaped cells oriented perpendicular to the pial surface, as observed in live of GAD67-GFP slices, ensuring proper laminar organization. Initial retinotopic organization begins during weeks 7-10 through mechanisms involving and . Gradients of in the tectum, combined with EphA receptors on axons, direct topographic mapping, with nasal axons terminating rostrally and temporal axons caudally. Opposing EphA and refine this projection, as demonstrated in mutants lacking ephrin-A2/A5, which disrupt orderly innervation. Genetic factors such as Pax7 and Engrailed (En1/2) transcription factors further define tectal identity; Pax7 specifies dorsal fate and cytoarchitecture, while En1/2 establish rostrocaudal polarity and regulate ephrin expression for map refinement. Ectopic En1/2 expression shifts boundaries and induces tectal markers like Pax7 in non-tectal regions.

Postnatal maturation

The segregation of layered circuits in the superior colliculus, distinguishing visual inputs in the superficial layers from somatosensory and auditory inputs in deeper layers, is largely completed at birth in mammals. This process ensures the initial organization of sensory-specific domains, transitioning from intermingled embryonic projections to unimodal responses. Postnatally, the retinotopic map in the superior colliculus refines through activity-dependent competition among axons, peaking in the first 2-3 postnatal months in . Spontaneous retinal waves provide the instructive signals for this remodeling, with disruption leading to enlarged, imprecise target zones in the colliculus. In mice, this critical refinement window spans the first postnatal week, after which arbors stabilize into sharp topographic alignments. Visual inputs exhibit sensitive s in the superior colliculus, where monocular deprivation induces shifts and retinotopic map distortions akin to those in . In mice, binocular neurons display robust plasticity during an early postnatal (approximately postnatal days 19-34), with deprivation strengthening responses from the non-deprived eye and altering motion processing. This experience-dependent reorganization underscores the colliculus's role in integrating balanced binocular signals for orienting behaviors. Synaptic pruning and myelination in the superior colliculus peak during infancy, driven by sensory experience to sculpt efficient circuits. Pruning eliminates excess synapses formed early postnatally, with dark rearing from birth delaying refinement and reducing synapse-to-neuron ratios by later stages in rats. Myelination begins around postnatal day 15 in rats, with initial fibers in the stratum opticum, progressing to full by postnatal day 30-45; activity deprivation slows this process, impacting signal conduction. These changes enhance the precision of as environmental inputs mature. In adulthood, superior colliculus circuits retain plasticity through (LTP) and depression (LTD) at retinocollicular synapses, supporting associative learning and adaptation. LTP strengthens excitatory transmission following high-frequency stimulation, while LTD weakens it, enabling refinements in response to behavioral demands like cross-modal cue pairing. Multisensory neurons further adapt via short-term experience, enhancing integration for tasks such as orienting to novel stimuli. Human functional MRI studies indicate that superior colliculus responses to salient visual stimuli mature by ages 5-7 years, aligning with the development of reflexive attention and gaze control. Early activation patterns shift from diffuse to focal, reflecting refined saliency detection as subcortical pathways integrate with cortical networks. This timeline corresponds to improved performance in visual search tasks, highlighting the colliculus's conserved role in early perceptual development.

Comparative anatomy

Mammalian variations

The superior colliculus exhibits notable variations in structure and function across mammalian species, reflecting adaptations to diverse sensory ecologies and behavioral demands. In , such as mice and rats, the superior colliculus occupies a relatively large proportion of the , receiving projections from 85–90% of cells, which underscores its central role in visual processing compared to other mammals. This structure emphasizes , particularly in the intermediate and deep layers, where visual, auditory, and somatosensory inputs converge to facilitate rapid orienting responses to environmental stimuli. Its organization features a of functional modules, approximately 50 μm in diameter, that segregate specific sensory and motor computations, enabling precise spatial mapping and behavioral coordination. In carnivores like cats, the superior colliculus plays a prominent role in prey capture behaviors, integrating sensory cues to guide orienting movements toward potential targets. Auditory maps are particularly enhanced, with precise alignment of visual and auditory receptive fields in the intermediate layers, allowing for effective localization of moving prey through combined acoustic and visual signals. Primates, including humans, display a distinct configuration with expanded superficial layers dedicated to high-acuity foveal vision, supporting detailed visual analysis and saccadic eye movements. In contrast to , only about 10% of cells project to the superior colliculus, resulting in reduced multisensory convergence and a greater reliance on cortical visual pathways for processing. Among chiropterans, echolocating bats exhibit specializations in the superior colliculus tailored to sonar-based , with auditory dominance in the deep layers where neurons selectively respond to echolocation calls and echoes for spatial targeting. These adaptations prioritize acoustic processing over visual inputs, reflecting the bats' reliance on biosonar for prey detection and obstacle avoidance. Evolutionary trends across mammals show an increase in cortical inputs to the superior colliculus in higher species, such as , enabling greater cognitive modulation of orienting behaviors through influences from association areas involved in and .

Non-mammalian homologs

In non-mammalian vertebrates, the optic tectum functions as the homolog of the mammalian superior colliculus, serving as a key structure for integrating sensory inputs and generating orienting responses. This conserved region receives direct projections from the and other sensory modalities, forming layered circuits that map external space retinotopically across species. Unlike in mammals, where cortical influences predominate, the optic tectum in , amphibians, birds, and reptiles acts as the primary visual center, directly driving visuomotor behaviors with minimal oversight. In birds, the optic tectum is particularly elaborate, comprising up to 15 distinct layers that support complex visual processing. Superficial layers receive retinotopic inputs, while deeper layers integrate multisensory signals and output to motor pathways, enabling precise orienting of the head and eyes toward stimuli. This structure dominates visual function without heavy reliance on telencephalic structures, facilitating rapid responses in flight and foraging. exhibit a similarly prominent optic tectum, often forming an inflated, twin-lobed canopy over the ventricle in teleosts, where it processes visual and electrosensory information for and predation. The , a basal , possesses a simpler optic tectum with direct retinotectal projections that preserve spatial mapping and trigger escape responses to looming threats. cells target superficial tectal layers, eliciting rapid bends or undulatory swimming when stimuli approach from the posterior , demonstrating early evolutionary roles in behaviors. In amphibians and reptiles, the optic tectum displays intermediate complexity, with layered organization supporting tectal dominance in visuomotor reflexes such as strikes in frogs or infrared-guided strikes in pit vipers. These species retain a high proportion of axons projecting directly to the tectum, emphasizing its central role in reflexive orienting over higher cognitive modulation. Functional conservation is evident in the retinotopic mapping and orienting capabilities preserved across classes, from lampreys to birds, where tectal activation aligns sensory representations with motor outputs for goal-directed actions. However, adaptations occur, such as advanced motion processing in birds, where tectal neurons in intermediate layers respond selectively to directional stimuli, enhancing detection of moving prey or predators. Evolutionarily, the optic tectum predates the mammalian superior colliculus as the ancestral hub for sensory-motor integration, with mammalian forms reflecting adaptations to expanded structures.

Clinical significance

Lesion effects

Unilateral lesions of the (SC) lead to deficits in orienting toward stimuli in the contralateral , including contralateral impairments and hemianopic , where affected individuals show reduced and fewer fixations to the contralesional side. These lesions also disrupt express saccades, the rapid, short-latency eye movements essential for quick visual orienting, resulting in prolonged saccadic latencies (50-250 ms longer) and increased need for corrective saccades in . In cats, unilateral SC ablation induces forced circling toward the ipsilateral side and persistent neglect of contralateral visual space, with asymmetries in vestibulo-ocular responses that partially resolve over weeks. Bilateral SC lesions produce more global impairments, including widespread deficits, diminished orienting to novel visual stimuli, and slowed reaction times across both visual fields. In rats, such lesions severely disrupt visually guided orientation tasks and discrimination, with greater initial deficits following simultaneous bilateral ablations compared to staged procedures, though some recovery occurs post-recovery period. Animal models highlight these effects: in cats, SC ablation causes circling and visual inattention to contralateral stimuli, underscoring the structure's role in spatial orienting. studies demonstrate loss of saliency detection, with lesioned monkeys showing reduced saccades to peripheral distractors, impaired in the periphery, and deficits in threat responsiveness, particularly in capuchins where orienting to stimuli is compromised. Human cases of SC lesions are rare due to the structure's deep location, but isolated damage, such as from a small , results in contralesional visual with fewer leftward fixations (mean 5.2 vs. healthy controls 10.9, p=0.002) and prolonged rightward bias. Surgical ablations involving the SC, occasionally performed in cases targeting nearby midbrain regions, reveal persistent deficits in scanning , including slowed detection of salient targets and incomplete recovery of . Recovery from SC lesions shows partial compensation, primarily through cortical pathways that maintain some visually guided behaviors; for instance, modality-specific orientation returns to near-normal levels after several weeks in lesioned animals, though multisensory deficits in deeper layers persist longer.

Role in disorders

The superior colliculus (SC) has been implicated in (PD) through mechanisms involving altered output, where hyperactivity of the pars reticulata (SNr) due to increases inhibition of the SC. In PD models, such as those induced by lesions, visual responses in the SC are enhanced as a compensatory mechanism, contributing to oculomotor slowing and lapses in as compensatory mechanisms fail to filter irrelevant stimuli effectively. In attention-deficit/hyperactivity disorder (ADHD), dysfunction in the SC is associated with hyper-responsiveness during tasks requiring saliency detection, as suggested by behavioral and oculomotor studies, which correlates with increased distractibility and impaired filtering of sensory inputs. This heightened activity disrupts the SC's role in prioritizing relevant environmental cues, exacerbating core symptoms of inattention and . Schizophrenia involves altered in the SC, leading to deficits in binding that impair the temporal and spatial alignment of sensory inputs. Electrophysiological and behavioral studies demonstrate that patients exhibit widened temporal binding windows, reflecting impaired collicular of convergent visual and auditory signals, which contributes to perceptual distortions and . In autism spectrum disorder (ASD), atypical gaze patterns stem from collicular-cortical dysconnectivity, disrupting the subcortical pathways that process social orienting cues such as eye contact. Neuroimaging reveals altered functional connectivity between the SC and cortical regions like the amygdala and prefrontal areas, resulting in reduced fixation on faces and heightened avoidance of direct gaze, which underlies social communication challenges. Recent studies (2025) emphasize the SC-ventral tegmental area (VTA) pathway in mediating social orienting deficits, where disruptions contribute to reduced responses to social cues like eye contact, further linking subcortical pathways to ASD symptomatology. Recent research from 2023 to 2025 has highlighted the SC's involvement in anxiety disorders through its processing of threats, where exaggerated collicular responses to approaching stimuli amplify via projections to the . Studies using high-resolution imaging show that the human SC encodes dynamic threat signals, potentially contributing to hypervigilance in conditions like . Additionally, the SC has emerged as a promising target for non-invasive stimulation techniques, demonstrating precise modulation of collicular activity to regulate oculomotor functions in preclinical models.

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