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. the sc encodes the target of a gaze shift, but it does not seem to specify the precise movements needed to get there. 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. 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. 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. 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. 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. 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 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. 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. 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 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 and is also a central feature in a number of medical conditions, including attention deficit hyperactivity disorder (adhd). research has shown that lesions to the sc in a number of species can result in heightened distractibility and, in humans, removing the inhibitory control on the sc from the pre-frontal cortex, therefore increasing activity in the area, also increases distractibility. research in an animal model of adhd, the spontaneously hypertensive rat, also shows altered collicular-dependent behaviours and physiology. furthermore, amphetamine (a mainstay treatment for adhd) also suppresses activity in the colliculus in healthy animals. other animals 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 represents only 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. cats in the cat the superior colliculus projects through the reticular formation and interacts with motor neurons in the brainstem. bats 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. 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, 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 the superior colliculus in rodents have been hypothesized to mediate sensory-guided approach and avoidance behaviors. studies employing circuit analysis tools on mouse superior colliculus have revealed several important functions. 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. 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. optic tectum 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. 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. 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. 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. 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. 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, but may be more integral to lower-order cues in motion perception like in the identification of small objects. the optic tectum is one of the fundamental components of the vertebrate brain, existing across a range of species. 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), and the number of different types of cells (from 2 in the lungfish to 27 in the house sparrow). 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. (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. 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. fish the brain of a cod, with the optic tectum highlighted lamprey 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