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V1 Saliency Hypothesis

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The V1 Saliency Hypothesis , or V1SH (pronounced ‘vish’) is a theory about V1, the primary visual cortex (V1) . It proposes that the V1 in primates creates a saliency map of the visual field to guide visual attention or gaze shifts exogenously.

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134-403: V1SH is the only theory so far to not only endow V1 a very important cognitive function, but also to have provided multiple non-trivial theoretical predictions that have been experimentally confirmed subsequently. According to V1SH, V1 creates a saliency map from retinal inputs to guide visual attention or gaze shifts. Anatomically, V1 is the gate for retinal visual inputs to enter neocortex , and

268-415: 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

402-420: A book. Note that this saliency map, which is constructed by a biological or natural brain, is not the same as the sort of saliency map that is engineered in artificial or computer vision, partly because the artificial saliency maps often include attentional guidance factors that are endogenous in nature. In this (biological) saliency map of the visual field, each visual location has a saliency value. This value

536-527: 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

670-465: A central band known as the visual streak. Around the fovea extends the central retina for about 6 mm and then the peripheral retina. The farthest edge of the retina is defined by the ora serrata . The distance from one ora to the other (or macula), the most sensitive area along the horizontal meridian , is about 32 mm. In section, the retina is no more than 0.5 mm thick. It has three layers of nerve cells and two of synapses , including

804-449: 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

938-579: A considered view that the bird retina depends for nutrition and oxygen supply on a specialized organ, called the "pecten" or pecten oculi , located on the blind spot or optic disk. This organ is extremely rich in blood vessels and is thought to supply nutrition and oxygen to the bird retina by diffusion through the vitreous body. The pecten is highly rich in alkaline phosphatase activity and polarized cells in its bridge portion – both befitting its secretory role. Pecten cells are packed with dark melanin granules, which have been theorized to keep this organ warm with

1072-448: 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 . 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

1206-578: 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 The top three layers are called superficial : Next come two intermediate layers : Finally come

1340-422: 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

1474-509: A linear model, this response profile is well described by a difference of Gaussians and is the basis for edge detection algorithms. Beyond this simple difference, ganglion cells are also differentiated by chromatic sensitivity and the type of spatial summation. Cells showing linear spatial summation are termed X cells (also called parvocellular, P, or midget ganglion cells), and those showing non-linear summation are Y cells (also called magnocellular, M, or parasol retinal ganglion cells), although

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1608-421: A low-level visual area that plays mainly a supporting role to other brain areas for their more important visual functions. Opinions started to change by a surprising piece of behavioral data: an item uniquely shown to one eye --- an ocular singleton --- among similarly appearing items shown to the other eye (using e.g. a pair of glasses for watching 3D movies ) can attract gaze or attention automatically. An example

1742-413: 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

1876-443: A neuron's response to a bar in its receptive field is higher when this bar is oriented in its preferred orientation. Analogously, many V1 neurons have their preferred colours. In this schematic, each input bar to the retina activates two (groups of) V1 neurons, one preferring its orientation and the other preferring its colour. The responses from neurons activated by their preferred orientations in their receptive fields are visualized in

2010-420: 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,

2144-436: A population of motor-related neurons, capable of activating eye movements as well as other responses. In other vertebrates the number of layers in the homologous optic tectum varies. 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

2278-474: 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

2412-473: A protein, retinochrome, that recycles retinal and replicates one of the functions of the vertebrate RPE, cephalopod photoreceptors are likely not maintained as well as in vertebrates, and that as a result, the useful lifetime of photoreceptors in invertebrates is much shorter than in vertebrates. Having easily replaced stalk eyes (some lobsters) or retinae (some spiders, such as Deinopis ) rarely occurs. The cephalopod retina does not originate as an outgrowth of

2546-399: 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. The optic tectum is

2680-513: 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

2814-415: 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

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2948-433: 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. Thus, cats with major damage to

3082-432: 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

3216-528: Is a lack of one or more of the cone subtypes that causes individuals to have deficiencies in colour vision or various kinds of colour blindness . These individuals are not blind to objects of a particular colour, but are unable to distinguish between colours that can be distinguished by people with normal vision. Humans have this trichromatic vision , while most other mammals lack cones with red sensitive pigment and therefore have poorer dichromatic colour vision. However, some animals have four spectral subtypes, e.g.

3350-402: Is a structure lying on the roof of the mammalian midbrain . In non-mammalian vertebrates , the homologous structure is known as the optic tectum or optic lobe . 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

3484-417: Is also available. Changes in retinal blood circulation are seen with aging and exposure to air pollution, and may indicate cardiovascular diseases such as hypertension and atherosclerosis. Determining the equivalent width of arterioles and venules near the optic disc is also a widely used technique to identify cardiovascular risks. The retina translates an optical image into neural impulses starting with

3618-404: Is also the largest cortical area devoted to vision. In the 1960s, David Hubel and Torsten Wiesel discovered that V1 neurons are activated by tiny image patches that are large enough to depict a small bar but not a discernible face. This work led to a Nobel prize, and V1 has since been seen as merely serving a back-office function (of image processing ) for the subsequent cognitive processing in

3752-406: Is because of iso-orientation suppression: when two V1 neurons are near each other and have the same or similar preferred orientations, they tend to suppress each other’s activities. Therefore, among the group of neurons that prefer and respond to the uniformly oriented background bars, each neuron receives iso-orientation suppression from other neurons of this group. Meanwhile, the neuron responding to

3886-465: Is because the texture bars at the border between the two textures evoke the highest V1 neural responses (since they are least suppressed by iso-orientation suppression), therefore, the border bars are the most salient in the image to attract attention to the border. However, the segmentation becomes much more difficult if the texture in B is superposed on the original image in A (the result is depicted in C). This

4020-459: Is because, at non-border texture locations, V1 neural responses to the horizontal and vertical bars (from B) are higher than those to the oblique bars (from A); these higher responses dictate and raise the saliency values at these non-border locations, making the border no longer as competitive for saliency. V1SH was proposed in late 1990's by Li Zhaoping . It was uninfluential initially since for decades it has been believed that attentional guidance

4154-537: Is called mesopic vision . At mesopic light levels, both the rods and cones are actively contributing pattern information. What contribution the rod information makes to pattern vision under these circumstances is unclear. The response of cones to various wavelengths of light is called their spectral sensitivity. In normal human vision, the spectral sensitivity of a cone falls into one of three subtypes, often called blue, green, and red, but more accurately known as short, medium, and long wavelength-sensitive cone subtypes. It

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4288-432: Is defined as the strength of this location to attract attention exogenously. So if location A has a higher saliency value than location B, then location A is more likely to attract visual attention or gaze shifts towards it than location B. In V1, each neuron can be activated only by visual inputs in a small region of the visual field. This region is called the receptive field of this neuron, and typically covers no more than

4422-441: Is essentially or only controlled by higher-level brain areas. These higher-level brain areas include the frontal eye field and parietal cortical areas in the frontal and more anterior part of the brain, and they are believed to be intelligent for attentional and executive control . In addition, the primary visual cortex, V1, located in occipital lobe in the back or posterior part of the brain, has traditionally been thought of as

4556-464: Is hyperpolarised. The amount of neurotransmitter released is reduced in bright light and increases as light levels fall. The actual photopigment is bleached away in bright light and only replaced as a chemical process, so in a transition from bright light to darkness the eye can take up to thirty minutes to reach full sensitivity. When thus excited by light, the photoceptor sends a proportional response synaptically to bipolar cells which in turn signal

4690-406: Is illustrated in this figure. Here, an image containing a single letter 'X' is shown to the right eye, and another image containing an array of the same 'X's and a letter 'O' is shown to the left eye. In such a situation, human observers normally perceive an image resembling a superposition of the two monocular images, such that they see an array of all the 'X's and the single 'O'. The 'X' arising from

4824-413: Is involved in all of these, but its role in saccades has been studied most intensively. 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

4958-403: Is most enhanced. The choroid supplies about 75% of these nutrients to the retina and the retinal vasculature only 25%. When light strikes 11-cis-retinal (in the disks in the rods and cones), 11-cis-retinal changes to all-trans-retinal which then triggers changes in the opsins. Now, the outer segments do not regenerate the retinal back into the cis- form once it has been changed by light. Instead

5092-438: Is not direct. Since about 150 million receptors and only 1 million optic nerve fibres exist, convergence and thus mixing of signals must occur. Moreover, the horizontal action of the horizontal and amacrine cells can allow one area of the retina to control another (e.g. one stimulus inhibiting another). This inhibition is key to lessening the sum of messages sent to the higher regions of the brain. In some lower vertebrates (e.g.

5226-488: Is now seen as one of the corner stones in the brain's network of attentional mechanisms, and its functional role in guiding visual attention is appearing in handbooks and textbooks. Zhaoping argues that If V1SH is correct, the ideas about how visual system works, and consequently questions to ask for future vision research, should be fundamentally changed. Retina The retina (from Latin rete  'net'; pl.   retinae or retinas )

5360-663: Is present (if it were absent in this example figure, all 'X's and the single 'O' would be shown to the left eye only). This observation was counter-intuitive, was easily reproduced by other vision researchers, and was uniquely predicted by V1SH. Since V1 is the only visual cortical area with neurons tuned to eye of origin of visual inputs, this observation strongly supports V1's role in guiding attention. More experiments followed to further investigate V1SH, and supporting data emerged from functional brain imaging, visual psychophysics, and from monkey electrophysiology (although see some conflicting data). V1SH has since become more popular. V1

5494-432: Is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs . The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then processes that image within the retina and sends nerve impulses along the optic nerve to the visual cortex to create visual perception . The retina serves a function which is in many ways analogous to that of

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5628-485: The blind spot . In contrast, in the cephalopod retina, the photoreceptors are in front, with processing neurons and capillaries behind them. Because of this, cephalopods do not have a blind spot. Although the overlying neural tissue is partly transparent, and the accompanying glial cells have been shown to act as fibre-optic channels to transport photons directly to the photoreceptors, light scattering does occur. Some vertebrates, including humans, have an area of

5762-414: The brain through the fibres of the optic nerve . Neural signals from the rods and cones undergo processing by other neurons, whose output takes the form of action potentials in retinal ganglion cells whose axons form the optic nerve. In vertebrate embryonic development , the retina and the optic nerve originate as outgrowths of the developing brain, specifically the embryonic diencephalon ; thus,

5896-420: The brainstem . 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

6030-433: 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

6164-515: The corpora quadrigemina . The superior colliculus is a layered structure, with a pattern that is similar in all mammals. 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. The deeper layers also contain

6298-449: The film or image sensor in a camera . The neural retina consists of several layers of neurons interconnected by synapses and is supported by an outer layer of pigmented epithelial cells. The primary light-sensing cells in the retina are the photoreceptor cells , which are of two types: rods and cones . Rods function mainly in dim light and provide monochromatic vision. Cones function in well-lit conditions and are responsible for

6432-439: 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 . The superior colliculus is a synaptic layered structure. The microstructure of the superior colliculus and of the optic tectum, varies across species. As

6566-457: 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. The two inferior colliculi are situated immediately inferior/caudal to the superior colliculi; the inferior and superior colliculi are known collectively as

6700-408: The ophthalmic artery bifurcates and supplies the retina via two distinct vascular networks: the choroidal network, which supplies the choroid and the outer retina, and the retinal network, which supplies the retina's inner layer. Although the inverted retina of vertebrates appears counter-intuitive, it is necessary for the proper functioning of the retina. The photoreceptor layer must be embedded in

6834-425: The outer plexiform layer and the inner plexiform layer . In the outer neuropil layer, the rods and cones connect to the vertically running bipolar cells , and the horizontally oriented horizontal cells connect to ganglion cells. The central retina predominantly contains cones, while the peripheral retina predominantly contains rods. In total, the retina has about seven million cones and a hundred million rods. At

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6968-412: The photosensitive ganglion cells ; and transmission along the optic nerve. At each synaptic stage, horizontal and amacrine cells also are laterally connected. The optic nerve is a central tract of many axons of ganglion cells connecting primarily to the lateral geniculate body , a visual relay station in the diencephalon (the rear of the forebrain). It also projects to the superior colliculus ,

7102-787: The pigeon ), control of messages is "centrifugal" – that is, one layer can control another, or higher regions of the brain can drive the retinal nerve cells, but in primates, this does not occur. Using optical coherence tomography (OCT), 18 layers can be identified in the retina. The layers and anatomical correlation are: From innermost to outermost, the layers identifiable by OCT are as follows: on OCT anatomical boundaries? references (unclear if it can be observed on OCT) b) Müller cell nuclei (obliquely orientated fibres; not present in mid-peripheral or peripheral retina) Poorly distinguishable from RPE. Previously: "cone outer segment tips line" (COST) homogenous region of variable reflectivity Retinal development begins with

7236-403: The receptive field of the cell. The receptive fields of retinal ganglion cells comprise a central, approximately circular area, where light has one effect on the firing of the cell, and an annular surround, where light has the opposite effect. In ON cells, an increment in light intensity in the centre of the receptive field causes the firing rate to increase. In OFF cells, it makes it decrease. In

7370-425: The retinal ganglion cells . The photoreceptors are also cross-linked by horizontal cells and amacrine cells , which modify the synaptic signal before it reaches the ganglion cells, the neural signals being intermixed and combined. Of the retina's nerve cells, only the retinal ganglion cells and few amacrine cells create action potentials . In the retinal ganglion cells there are two types of response, depending on

7504-431: The suprachiasmatic nucleus , and the nucleus of the optic tract . It passes through the other layers, creating the optic disc in primates. Additional structures, not directly associated with vision, are found as outgrowths of the retina in some vertebrate groups. In birds , the pecten is a vascular structure of complex shape that projects from the retina into the vitreous humour ; it supplies oxygen and nutrients to

7638-412: 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

7772-679: The 1960s (see ), 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. In common with other systems (see 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

7906-563: 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

8040-523: The N-T axis is coordinated by expression of the forkhead transcription factors FOXD1 and FOXG1 . Additional gradients are formed within the retina. This spatial distribution may aid in proper targeting of RGC axons that function to establish the retinotopic map. The retina is stratified into distinct layers, each containing specific cell types or cellular compartments that have metabolisms with different nutritional requirements. To satisfy these requirements,

8174-463: 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

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8308-502: 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 ,

8442-429: 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

8576-423: 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

8710-488: The absorption of stray light falling on the pecten. This is considered to enhance metabolic rate of the pecten, thereby exporting more nutritive molecules to meet the stringent energy requirements of the retina during long periods of exposure to light. The bifurcations and other physical characteristics of the inner retinal vascular network are known to vary among individuals, and these individual variances have been used for biometric identification and for early detection of

8844-422: The brain beyond V1. However, research progress to understand the subsequent processing has been much more difficult or slower than expected (by, e.g., Hubel and Wiesel). Outside the box of the traditional views, V1SH is catalyzing a change of framework to enable fresh progresses on understanding vision. See for where primary visual cortex is in the brain and relative to the eyes. V1SH states that V1 transforms

8978-424: The brain, as the vertebrate one does. This difference suggests that vertebrate and cephalopod eyes are not homologous , but have evolved separately. From an evolutionary perspective, a more complex structure such as the inverted retina can generally come about as a consequence of two alternate processes - an advantageous "good" compromise between competing functional limitations, or as a historical maladaptive relic of

9112-557: The brainstem and diencephalon, also show a corresponding inhomogeneity. The total number of columns has been estimated at around 100. 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

9246-409: 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, 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

9380-409: 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. On detailed examination,

9514-480: The central retina adapted for high-acuity vision. This area, termed the fovea centralis , is avascular (does not have blood vessels), and has minimal neural tissue in front of the photoreceptors, thereby minimizing light scattering. The cephalopods have a non-inverted retina, which is comparable in resolving power to the eyes of many vertebrates. Squid eyes do not have an analog of the vertebrate retinal pigment epithelium (RPE). Although their photoreceptors contain

9648-411: The centre of the macula is the foveal pit where the cones are narrow and long, and arranged in a hexagonal mosaic , the most dense, in contradistinction to the much fatter cones located more peripherally in the retina. At the foveal pit, the other retinal layers are displaced, before building up along the foveal slope until the rim of the fovea, or parafovea , is reached, which is the thickest portion of

9782-484: The collicular layers are actually not smooth sheets, but divided into a honeycomb arrangement of discrete columns. 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 . Several other neurochemical markers including calretinin, parvalbumin, GAP-43, and NMDA receptors, and connections with numerous other brain structures in

9916-418: The colliculus in healthy animals. 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

10050-485: The convoluted path of organ evolution and transformation. Vision is an important adaptation in higher vertebrates. A third view of the "inverted" vertebrate eye is that it combines two benefits - the maintenance of the photoreceptors mentioned above, and the reduction in light intensity necessary to avoid blinding the photoreceptors, which are based on the extremely sensitive eyes of the ancestors of modern hagfish (fish that live in very deep, dark water). A recent study on

10184-422: The correspondence between X and Y cells (in the cat retina) and P and M cells (in the primate retina) is not as simple as it once seemed. In the transfer of visual signals to the brain, the visual pathway , the retina is vertically divided in two, a temporal (nearer to the temple) half and a nasal (nearer to the nose) half. The axons from the nasal half cross the brain at the optic chiasma to join with axons from

10318-401: The details of the arrangement. 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

10452-532: The establishment of the eye fields mediated by the SHH and SIX3 proteins, with subsequent development of the optic vesicles regulated by the PAX6 and LHX2 proteins. The role of Pax6 in eye development was elegantly demonstrated by Walter Gehring and colleagues, who showed that ectopic expression of Pax6 can lead to eye formation on Drosophila antennae, wings, and legs. The optic vesicle gives rise to three structures:

10586-578: The evolutionary purpose for the inverted retina structure from the APS (American Physical Society) says that "The directional of glial cells helps increase the clarity of human vision. But we also noticed something rather curious: the colours that best passed through the glial cells were green to red, which the eye needs most for daytime vision. The eye usually receives too much blue—and thus has fewer blue-sensitive cones. Further computer simulations showed that green and red are concentrated five to ten times more by

10720-460: The external visual input rather than from internal factors such as animal’s expectations or goals (e.g., to read a book). Therefore, a saliency map is said to guide attention exogenously rather than endogenously . Accordingly, this saliency map is also called the bottom-up saliency map to guide reflexive or involuntary shifts of attention . For example, it guides our gaze shifts towards an insect flying in our peripheral visual field when we are reading

10854-407: The eye, and may also aid in vision. Reptiles have a similar, but much simpler, structure. In adult humans, the entire retina is about 72% of a sphere about 22 mm in diameter. The entire retina contains about 7 million cones and 75 to 150 million rods. The optic disc, a part of the retina sometimes called "the blind spot" because it lacks photoreceptors, is located at the optic papilla , where

10988-416: 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

11122-424: The eyes remain fixed in a constant position. 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

11256-445: The figure above, the orientation singleton, which evokes the highest V1 response to this image, attracts visual attention or gaze. V1SH can explain data on visual search , such as the short response times to find a uniquely red item among green items, or a uniquely vertical bar among horizontal bars, or an item uniquely moving to the right among items moving to the left. These kind of visual searches are called feature searches , when

11390-530: The glial cells, and into their respective cones, than blue light. Instead, excess blue light gets scattered to the surrounding rods. This optimization is such that color vision during the day is enhanced, while night-time vision suffers very little". The vertebrate retina has 10 distinct layers. From closest to farthest from the vitreous body: These layers can be grouped into four main processing stages—photoreception; transmission to bipolar cells ; transmission to ganglion cells , which also contain photoreceptors,

11524-445: 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

11658-502: The inferior colliculus. This auditory information is integrated with the visual information already present to produce the ventriloquism effect . 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

11792-437: 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. 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

11926-416: The input from "association" areas tends to be heavier than the input from primary sensory or motor areas. However, the cortical areas involved, and the strength of their relative projections, differ across species. 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

12060-427: 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

12194-412: 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. In the cat the superior colliculus projects through the reticular formation and interacts with motor neurons in

12328-502: 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,

12462-411: The light-sensing cells are in the back of the retina, so that light has to pass through layers of neurons and capillaries before it reaches the photosensitive sections of the rods and cones. The ganglion cells, whose axons form the optic nerve, are at the front of the retina; therefore, the optic nerve must cross through the retina en route to the brain. No photoreceptors are in this region, giving rise to

12596-431: 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

12730-406: The limited capacity of the optic nerve. Compression is necessary because there are 100 times more photoreceptor cells than ganglion cells . This is done by " decorrelation ", which is carried out by the "centre–surround structures", which are implemented by the bipolar and ganglion cells. Superior colliculus In neuroanatomy , the superior colliculus (from Latin  'upper hill')

12864-467: 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

12998-436: The neural mechanisms in V1 to generate the saliency map. In this example, the retinal image has many purple bars, all uniformly oriented (right-tilted) except for one bar that is oriented uniquely (left-tilted). This orientation singleton is the most salient in this image, so it attracts attention or gaze, as observed in psychological experiments. In V1, many neurons have their preferred orientations for visual inputs. For example,

13132-511: The neural retina, the retinal pigmented epithelium, and the optic stalk. The neural retina contains the retinal progenitor cells (RPCs) that give rise to the seven cell types of the retina. Differentiation begins with the retinal ganglion cells and concludes with production of the Muller glia. Although each cell type differentiates from the RPCs in a sequential order, there is considerable overlap in

13266-436: The non-mammalian brain, and, as a consequence, is involved in a wide variety of behaviors. 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,

13400-521: The onset of disease. The mapping of vascular bifurcations is one of the basic steps in biometric identification. Results of such analyses of retinal blood vessel structure can be evaluated against the ground truth data of vascular bifurcations of retinal fundus images that are obtained from the DRIVE dataset. In addition, the classes of vessels of the DRIVE dataset have also been identified, and an automated method for accurate extraction of these bifurcations

13534-402: 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. The superior colliculus is a paired structure of the dorsal midbrain and is part of

13668-445: The optic nerve are devoted to the fovea. The resolution limit of the fovea has been determined to be around 10,000 points. The information capacity is estimated at 500,000 bits per second (for more information on bits, see information theory ) without colour or around 600,000 bits per second including colour. When the retina sends neural impulses representing an image to the brain, it spatially encodes (compresses) those impulses to fit

13802-444: 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 ,

13936-478: The optic-nerve fibres leave the eye. It appears as an oval white area of 3 mm . Temporal (in the direction of the temples) to this disc is the macula , at whose centre is the fovea , a pit that is responsible for sharp central vision, but is actually less sensitive to light because of its lack of rods. Human and non-human primates possess one fovea, as opposed to certain bird species, such as hawks, that are bifoviate, and dogs and cats, that possess no fovea, but

14070-413: The orientation singleton does not belong to this group and thus escapes this suppression, hence its response is higher than the other neural responses. Iso-colour suppression is analogous to iso-orientation suppression, so all neurons preferring and responding to the purple colours of the input bars are under the iso-colour suppression. According to V1SH, the maximum response at each bar’s location represents

14204-747: The patterned excitation of the colour-sensitive pigments of its rods and cones, the retina's photoreceptor cells . The excitation is processed by the neural system and various parts of the brain working in parallel to form a representation of the external environment in the brain. The cones respond to bright light and mediate high-resolution colour vision during daylight illumination (also called photopic vision ). The rod responses are saturated at daylight levels and do not contribute to pattern vision. However, rods do respond to dim light and mediate lower-resolution, monochromatic vision under very low levels of illumination (called scotopic vision ). The illumination in most office settings falls between these two levels and

14338-478: The perception of colour through the use of a range of opsins , as well as high-acuity vision used for tasks such as reading. A third type of light-sensing cell, the photosensitive ganglion cell , is important for entrainment of circadian rhythms and reflexive responses such as the pupillary light reflex . Light striking the retina initiates a cascade of chemical and electrical events that ultimately trigger nerve impulses that are sent to various visual centres of

14472-431: 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

14606-432: 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

14740-408: The receptive field. Hence saliency value at each location depends on visual input context. This is as it should be since saliency depends on context. For example, a vertical bar is salient in an image in which all the other visual items surrounding it are horizontal bars, but this same vertical bar is not salient if these other items are all vertical bars instead. The figure above gives a schematics of

14874-464: The resting state the cell is depolarised. The photon causes the retinal bound to the receptor protein to isomerise to trans-retinal . This causes the receptor to activate multiple G-proteins . This in turn causes the Ga-subunit of the protein to activate a phosphodiesterase (PDE6), which degrades cGMP, resulting in the closing of Na+ cyclic nucleotide-gated ion channels (CNGs). Thus the cell

15008-457: The retina is considered part of the central nervous system (CNS) and is actually brain tissue. It is the only part of the CNS that can be visualized noninvasively . Like most of the brain, the retina is isolated from the vascular system by the blood–brain barrier . The retina is the part of the body with the greatest continuous energy demand. The vertebrate retina is inverted in the sense that

15142-450: The retina. The macula has a yellow pigmentation, from screening pigments, and is known as the macula lutea. The area directly surrounding the fovea has the highest density of rods converging on single bipolar cells. Since its cones have a much lesser convergence of signals, the fovea allows for the sharpest vision the eye can attain. Though the rod and cones are a mosaic of sorts, transmission from receptors, to bipolars, to ganglion cells

15276-514: The retinal is pumped out to the surrounding RPE where it is regenerated and transported back into the outer segments of the photoreceptors. This recycling function of the RPE protects the photoreceptors against photo-oxidative damage and allows the photoreceptor cells to have decades-long useful lives. The bird retina is devoid of blood vessels, perhaps to give unobscured passage of light for forming images, thus giving better resolution. It is, therefore,

15410-490: The retinal pigment epithelium (RPE), which performs at least seven vital functions, one of the most obvious being to supply oxygen and other necessary nutrients needed for the photoreceptors to function. The energy requirements of the retina are even greater than that of the brain. This is due to the additional energy needed to continuously renew the photoreceptor outer segments, of which 10% are shed daily. Energy demands are greatest during dark adaptation when its sensitivity

15544-450: The right-eye image will not appear distinctive. Nevertheless, even when they are doing a task to search (in their perceived image) for the unique and perceptually distinctive 'O' as quickly as possible, their gaze automatically or involuntarily shifts to the 'X' arising from the right-eye image, often before their gaze shifts to the 'O'. Attention capture by such an ocular singleton occurs even when observers fail to guess whether this singleton

15678-500: The rods and cones. Light is absorbed by the retinal pigment epithelium or the choroid (both of which are opaque). The white blood cells in the capillaries in front of the photoreceptors can be perceived as tiny bright moving dots when looking into blue light. This is known as the blue field entoptic phenomenon (or Scheerer's phenomenon). Between the ganglion-cell layer and the rods and cones are two layers of neuropils , where synaptic contacts are made. The neuropil layers are

15812-456: The saliency value at each bar’s location. This saliency value is thus highest at the location of the orientation singleton, and is represented by the response from neurons preferring and responding to the orientation of this singleton. These saliency values are sent to the superior colliculus , a midbrain area, to execute gaze shifts to the receptive field of the most activated neuron responding to visual input space. Hence, for this input image in

15946-455: 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

16080-411: The schematics by the black dots in the plane representing the V1 neural responses. Similarly, responses from neurons activated by their preferred colours in their receptive fields are visualized by the purple dots. The sizes of the dots visualize the strengths of the V1 neural responses. In this example, the largest response comes from the neurons preferring and responding to the uniquely oriented bar. This

16214-407: The search target is unique in a basic feature value like orientation, color, or motion direction. The shortness of the search response time manifests a higher saliency value at the location of the search target to attract attention. V1SH also explains why it takes longer to find a unique red-vertical bar among red-horizontal bars and green-vertical bars. This is an example of conjunction searches when

16348-434: The search target is unique only by the conjunction of two features, each of which is present in the visual scene. Furthermore, V1SH explains data that are difficult to be explained by alternative frameworks. The figure above illustrates an example: two neighboring textures in A, one made of uniformly left-tilted bars and another of uniformly right-tilted bars, are very easy to be segmented from each other by human vision. This

16482-423: 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

16616-431: The size of a coin at an arm’s length. Neighbouring V1 neurons have neighbouring and overlapping receptive fields. Hence, each visual location can simultaneously activate many V1 neurons. According to V1SH, the most activated neuron among these neurons signals the saliency value at this location by its neural activity. A V1 neuron’s response to visual inputs within its receptive field is also influenced by visual inputs outside

16750-421: The superior colliculus is also involved in generating spatially directed head turns, arm-reaching movements, and shifts in attention that do not involve any overt movements. 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

16884-450: The temporal half of the other eye before passing into the lateral geniculate body . Although there are more than 130 million retinal receptors, there are only approximately 1.2 million fibres (axons) in the optic nerve. So, a large amount of pre-processing is performed within the retina. The fovea produces the most accurate information. Despite occupying about 0.01% of the visual field (less than 2° of visual angle ), about 10% of axons in

17018-430: The timing of when individual cell types differentiate. The cues that determine a RPC daughter cell fate are coded by multiple transcription factor families including the bHLH and homeodomain factors. In addition to guiding cell fate determination, cues exist in the retina to determine the dorsal-ventral (D-V) and nasal-temporal (N-T) axes. The D-V axis is established by a ventral to dorsal gradient of VAX2 , whereas

17152-473: 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

17286-479: The trout adds an ultraviolet subgroup to short, medium, and long subtypes that are similar to humans. Some fish are sensitive to the polarization of light as well. In the photoreceptors, exposure to light hyperpolarizes the membrane in a series of graded shifts. The outer cell segment contains a photopigment . Inside the cell the normal levels of cyclic guanosine monophosphate (cGMP) keep the Na+ channel open, and thus in

17420-433: The two deep layers : 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),

17554-436: The unique ribbon synapse . The optic nerve carries the ganglion-cell axons to the brain, and the blood vessels that supply the retina. The ganglion cells lie innermost in the eye while the photoreceptive cells lie beyond. Because of this counter-intuitive arrangement, light must first pass through and around the ganglion cells and through the thickness of the retina, (including its capillary vessels, not shown) before reaching

17688-399: 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

17822-520: 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. In primates, eye movements can be divided into several types: fixation , in which

17956-413: The visual inputs into a saliency map of the visual field to guide visual attention or direction of gaze. Humans are essentially blind to visual inputs outside their window of attention . Therefore, attention gates visual perception and awareness , and theories of visual attention are cornerstones of theories of visual functions in the brain. A saliency map is by definition computed from, or caused by,

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