Binocular vision

In biology, binocular vision is a type of vision in which an animal having two eyes is able to perceive a single three-dimensional image of its surroundings. Neurological researcher Manfred Fahle has stated six specific advantages of having two eyes rather than just one:[1]

  1. It gives a creature a spare eye in case one is damaged.
  2. It gives a wider field of view. For example, humans have a maximum horizontal field of view of approximately 190 degrees with two eyes, approximately 120 degrees of which makes up the binocular field of view (seen by both eyes) flanked by two uniocular fields (seen by only one eye) of approximately 40 degrees.[2]
  3. It can give stereopsis in which binocular disparity (or parallax) provided by the two eyes' different positions on the head gives precise depth perception. This also allows a creature to break the camouflage of another creature.
  4. It allows the angles of the eyes' lines of sight, relative to each other (vergence), and those lines relative to a particular object (gaze angle) to be determined from the images in the two eyes.[3] These properties are necessary for the third advantage.
  5. It allows a creature to see more of, or all of, an object behind an obstacle. This advantage was pointed out by Leonardo da Vinci, who noted that a vertical column closer to the eyes than an object at which a creature is looking might block some of the object from the left eye but that part of the object might be visible to the right eye.
  6. It gives binocular summation in which the ability to detect faint objects is enhanced.[4]

Other phenomena of binocular vision include utrocular discrimination (the ability to tell which of two eyes has been stimulated by light),[5] eye dominance (the habit of using one eye when aiming something, even if both eyes are open),[6] allelotropia (the averaging of the visual direction of objects viewed by each eye when both eyes are open),[7] binocular fusion or singleness of vision (seeing one object with both eyes despite each eye's having its own image of the object),[8] and binocular rivalry (seeing one eye's image alternating randomly with the other when each eye views images that are so different they cannot be fused).[9]

Binocular vision helps with performance skills such as catching, grasping, and locomotion.[10] It also allows humans to walk over and around obstacles at greater speed and with more assurance.[11] Optometrists and/or orthoptists are eyecare professionals who fix binocular vision problems.

Binocular vision
Principle of binocular vision with horopter shown


The term binocular comes from two Latin roots, bini for double, and oculus for eye.[12]

Field of view and eye movements

The field of view of a pigeon compared to that of an owl.

Some animals, usually, but not always, prey animals, have their two eyes positioned on opposite sides of their heads to give the widest possible field of view. Examples include rabbits, buffaloes, and antelopes. In such animals, the eyes often move independently to increase the field of view. Even without moving their eyes, some birds have a 360-degree field of view.

Some other animals, usually, but not always, predatory animals, have their two eyes positioned on the front of their heads, thereby allowing for binocular vision and reducing their field of view in favor of stereopsis. However, eyes on the front is a highly evolved trait in vertebrates, and there are only three extant groups of vertebrates with truly forward-facing eyes: primates, carnivorous mammals, and birds of prey.

Some predator animals, particularly large ones such as sperm whales and killer whales, have their two eyes positioned on opposite sides of their heads, although it is possible they have some binocular visual field. [13] Other animals that are not necessarily predators, such as fruit bats and a number of primates also have forward-facing eyes. These are usually animals that need fine depth discrimination/perception; for instance, binocular vision improves the ability to pick a chosen fruit or to find and grasp a particular branch.

The direction of a point relative to the head (the angle between the straight ahead position and the apparent position of the point, from the egocenter) is called visual direction, or version. The angle between the line of sight of the two eyes when fixating a point is called the absolute disparity, binocular parallax, or vergence demand (usually just vergence). The relation between the position of the two eyes, version and vergence is described by Hering's law of visual direction.

In animals with forward-facing eyes, the eyes usually move together.

Grey crowned crane portrait front and side view
The grey-crowned crane, an animal that has laterally-placed eyes which can also face forward.

Eye movements are either conjunctive (in the same direction), version eye movements, usually described by their type: saccades or smooth pursuit (also nystagmus and vestibulo-ocular reflex). Or they are disjunctive (in opposite direction), vergence eye movements. The relation between version and vergence eye movements in humans (and most animals) is described by Hering's law of equal innervation.

Some animals use both of the above strategies. A starling, for example, has laterally placed eyes to cover a wide field of view, but can also move them together to point to the front so their fields overlap giving stereopsis. A remarkable example is the chameleon, whose eyes appear as if mounted on turrets, each moving independently of the other, up or down, left or right. Nevertheless, the chameleon can bring both of its eyes to bear on a single object when it is hunting, showing vergence and stereopsis.

Binocular summation

Binocular summation is the process by which the detection threshold for a stimulus is lower with two eyes than with one.[14] There are various types of possibilities when comparing binocular performance to monocular.[14] Neural binocular summation occurs when the binocular response is greater than the probability summation. Probability summation assumes complete independence between the eyes and predicts a ratio ranging between 9-25%. Binocular inhibition occurs when binocular performance is less than monocular performance. This suggests that a weak eye affects a good eye and causes overall combined vision.[14] Maximum binocular summation occurs when monocular sensitivities are equal. Unequal monocular sensitivities decrease binocular summation. There are unequal sensitivities of vision disorders such as unilateral cataract and amblyopia.[14] Other factors that can affect binocular summation include are, spatial frequency, stimulated retinal points, and temporal separation.[14]

Binocular interaction

Apart from binocular summation, the two eyes can influence each other in at least three ways.

  • Pupillary diameter. Light falling in one eye affects the diameter of the pupils in both eyes. One can easily see this by looking at a friend's eye while he or she closes the other: when the other eye is open, the pupil of the first eye is small; when the other eye is closed, the pupil of the first eye is large.
  • Accommodation and vergence. Accommodation is the state of focus of the eye. If one eye is open and the other closed, and one focuses on something close, the accommodation of the closed eye will become the same as that of the open eye. Moreover, the closed eye will tend to converge to point at the object. Accommodation and convergence are linked by a reflex, so that one evokes the other.
  • Interocular transfer. The state of adaptation of one eye can have a small effect on the state of light adaptation of the other. Aftereffects induced through one eye can be measured through the other.

Singleness of vision

Once the fields of view overlap, there is a potential for confusion between the left and right eye's image of the same object. This can be dealt with in two ways: one image can be suppressed, so that only the other is seen, or the two images can be fused. If two images of a single object are seen, this is known as double vision or diplopia.

Fusion of images (commonly referred to as 'binocular fusion') occurs only in a small volume of visual space around where the eyes are fixating. Running through the fixation point in the horizontal plane is a curved line for which objects there fall on corresponding retinal points in the two eyes. This line is called the empirical horizontal horopter. There is also an empirical vertical horopter, which is effectively tilted away from the eyes above the fixation point and towards the eyes below the fixation point. The horizontal and vertical horopters mark the centre of the volume of singleness of vision. Within this thin, curved volume, objects nearer and farther than the horopters are seen as single. The volume is known as Panum's fusional area (it's presumably called an area because it was measured by Panum only in the horizontal plane). Outside of Panum's fusional area (volume), double vision occurs.

Eye dominance

When each eye has its own image of objects, it becomes impossible to align images outside of Panum's fusional area with an image inside the area.[15] This happens when one has to point to a distant object with one's finger. When one looks at one's fingertip, it is single but there are two images of the distant object. When one looks at the distant object it is single but there are two images of one's fingertip. To point successfully, one of the double images has to take precedence and one be ignored or suppressed (termed "eye dominance"). The eye that can both move faster to the object and stay fixated on it is more likely to be termed as the dominant eye.[15]


The overlapping of vision occurs due to the position of the eyes on the head (eyes are located on the front of the head, not on the sides). This overlap allows each eye to view objects with a slightly different viewpoint. As a result of this overlap of vision, binocular vision provides depth.[16] Stereopsis (from stereo- meaning "solid" or "three-dimensional", and opsis meaning “appearance” or “sight”) is the impression of depth that is perceived when a scene is viewed with both eyes by someone with normal binocular vision.[16] Binocular viewing of a scene creates two slightly different images of the scene in the two eyes due to the eyes' different positions on the head. These differences, referred to as binocular disparity, provide information that the brain can use to calculate depth in the visual scene, providing a major means of depth perception.[16] There are two aspects of stereopsis: the nature of the stimulus information specifying stereopsis, and the nature of the brain processes responsible for registering that information.[16] The distance between the two eyes on an adult is almost always 6.5 cm and that is the same distance in shift of an image when viewing with only one eye.[16] Retinal disparity is the separation between objects as seen by the left eye and the right eye and helps to provide depth perception.[16] Retinal disparity provides relative depth between two objects, but not exact or absolute depth. The closer objects are to each other, the retinal disparity will be small. If the objects are farther away from each other, then the retinal disparity will be larger. When objects are at equal distances, the two eyes view the objects as the same and there is zero disparity.[16]


Because the eyes are in different positions on the head, any object away from fixation and off the plane of the horopter has a different visual direction in each eye. Yet when the two monocular images of the object are fused, creating a Cyclopean image, the object has a new visual direction, essentially the average of the two monocular visual directions. This is called allelotropia.[7] The origin of the new visual direction is a point approximately between the two eyes, the so-called cyclopean eye. The position of the cyclopean eye is not usually exactly centered between the eyes, but tends to be closer to the dominant eye.

Binocular rivalry

When very different images are shown to the same retinal regions of the two eyes, perception settles on one for a few moments, then the other, then the first, and so on, for as long as one cares to look. This alternation of perception between the images of the two eyes is called binocular rivalry.[17] Humans have limited capacity to process an image fully at one time. That is why the binocular rivalry occurs. Several factors can influence the duration of gaze on one of the two images. These factors include context, increasing of contrast, motion, spatial frequency, and inverted images.[17] Recent studies have even shown that facial expressions can cause longer attention to a particular image.[17] When an emotional facial expression is presented to one eye, and a neutral expression is presented to the other eye, the emotional face dominates the neutral face and even causes the neutral face to not been seen.[17]


To maintain stereopsis and singleness of vision, the eyes need to be pointed accurately. The position of each eye in its orbit is controlled by six extraocular muscles. Slight differences in the length or insertion position or strength of the same muscles in the two eyes can lead to a tendency for one eye to drift to a different position in its orbit from the other, especially when one is tired. This is known as phoria. One way to reveal it is with the cover-uncover test. To do this test, look at a cooperative person's eyes. Cover one eye of that person with a card. Have the person look at your finger tip. Move the finger around; this is to break the reflex that normally holds a covered eye in the correct vergence position. Hold your finger steady and then uncover the person's eye. Look at the uncovered eye. You may see it flick quickly from being wall-eyed or cross-eyed to its correct position. If the uncovered eye moved from out to in, the person has exophoria. If it moved from in to out, the person has esophoria. If the eye did not move at all, the person has orthophoria. Most people have some amount of exophoria or esophoria; it is quite normal. If the uncovered eye also moved vertically, the person has hyperphoria (if the eye moved from up to down) or hypophoria (if the eye moved from down to up). Such vertical phorias are quite rare. It is also possible for the covered eye to rotate in its orbit. Such cyclophorias cannot be seen with the cover-uncover test; they are rarer than vertical phorias.

The cover-uncover test can also be used for more problematic disorders of binocular vision, the tropias. In the cover part of the test, the examiner looks at the first eye as he or she covers the second. If the eye moves from out to in, the person has exotropia. If it moved from in to out, the person has esotropia. People with exotropia or esotropia are wall-eyed or cross-eyed respectively. These are forms of strabismus that can be accompanied by amblyopia. There are numerous definitions of amblyopia.[14] A definition that incorporates all of these defines amblyopia as a unilateral condition in which vision in worse than 20/20 in the absence of any obvious structural or pathologic anomalies, but with one or more of the following conditions occurring before the age of six: amblyogenic anisometropia, constant unilateral esotropia or exotropia, amblyogenic bilateral isometropia, amblyogenic unilateral or bilateral astigmatism, image degradation.[14] When the covered eye is the non-amblyopic eye, the amblyopic eye suddenly becomes the person's only means of seeing. The strabismus is revealed by the movement of that eye to fixate on the examiner's finger. There are also vertical tropias (hypertropia and hypotropia) and cyclotropias.

Binocular vision anomalies include: diplopia (double vision), visual confusion (the perception of two different images superimposed onto the same space), suppression (where the brain ignores all or part of one eye's visual field), horror fusionis (an active avoidance of fusion by eye misalignment), and anomalous retinal correspondence (where the brain associates of the fovea of one eye with an extrafoveal area of the other eye).

Binocular vision anomalies are among the most common visual disorders. They are usually associated with symptoms such as headaches, asthenopia, eye pain, blurred vision, and occasional diplopia.[18] About 20% of patients who come to optometry clinics will have binocular vision anomalies.[18] The most effective way to diagnosis vision anomalies is with the near point of convergence test.[18] During the NPC test, a target, such as a finger, is brought towards the face until the examiner notices that one eye has turned outward and/or the person has experienced diplopia or doubled vision.[18]

Up to a certain extent, binocular disparities can be compensated for by adjustments of the visual system. If, however, defects of binocular vision are too great – for example if they would require the visual system to adapt to overly large horizontal, vertical, torsional or aniseikonic deviations – the eyes tend to avoid binocular vision, ultimately causing or worsening a condition of strabismus.

See also


  1. ^ Fahle, M (1987). "Wozu zwei Augen? [Why two eyes?]". Naturwissenschaften. 74 (8): 383–385. Bibcode:1987NW.....74..383F. doi:10.1007/BF00405466.
  2. ^ Henson, D.B. (1993). Visual Fields. Oxford: Oxford University Press.
  3. ^ Longuet-Higgins, H. C. (1982). "The role of the vertical dimension in stereoscopic vision". Perception. 11 (4): 377–386. doi:10.1068/p110377. PMID 7182797.
  4. ^ Blake, Randolph; Fox, Robert (August 1973). "The psychophysical inquiry into binocular summation". Perception & Psychophysics. 14 (1): 161–85. doi:10.3758/BF03198631.
  5. ^ Blake, R.; Cormack, R. H. (1979). "On utrocular discrimination". Perception & Psychophysics. 26: 53–68. doi:10.3758/BF03199861.
  6. ^ Miles, W. R. (1930). "Ocular dominance in human adults". Journal of General Psychology. 3 (3): 412–430. doi:10.1080/00221309.1930.9918218.
  7. ^ a b Hariharan-Vilupuru, S.; Bedell, H. E. (2009). "The perceived visual direction of monocular objects in random-dot stereograms is influenced by perceived depth and allelotropia". Vision Research. 49 (2): 190–201. doi:10.1016/j.visres.2008.10.009. PMID 18992271.
  8. ^ Panum, P. L. (1858). Über die einheitliche Verschmelzung verschiedenartiger Netzhauteindrucke beim Sehen mit zwei Augen. Kiel.
  9. ^ Wheatstone, C (1838). "Contributions to the physiology of vision.—Part the First. On some remarkable, and hitherto unobserved, phænomena of binocular vision". Philosophical Transactions of the Royal Society of London. 128: 371–394. doi:10.1098/rstl.1838.0019.
  10. ^ Heinen, T., & Vinken, P. M. (2011). Monocular and binocular vision in the performance of a complex skill. Journal of Sports Science & Medicine 10(3), 520-527. Retrieved from:
  11. ^ Hayhoe M, Gillam B, Chajka K, Vecellio E (2009). "The role of binocular vision in walking". Visual Neuroscience. 26 (1): 73–80. doi:10.1017/S0952523808080838. PMC 2857785. PMID 19152718.
  12. ^ Harper, D. (2001). Online etymological dictionary. Retrieved April 2, 2008, from
  13. ^ Fristrup, K. M.; Harbison, G. R. (2002). "How do sperm whales catch squids?". Marine Mammal Science. 18: 42–54. doi:10.1111/j.1748-7692.2002.tb01017.x.
  14. ^ a b c d e f g Pardhan, S.; Whitaker, A. (2000). "Binocular summation in the fovea and peripheral field of anisometropicamblyopes". Current Eye Research. 20 (1): 35–44. doi:10.1076/0271-3683(200001)20:1;1-h;ft035. PMID 10611713.
  15. ^ a b Bingushi, K.; Yukumatsu, S. (2005). "Disappearance of a monocular image in Panum's limiting case". Japanese Psychological Research. 47 (3): 223–229. doi:10.1111/j.1468-5884.2005.00291.x.
  16. ^ a b c d e f g Blake, R., & Sekuler, R. (2006) Perception (5th ed.). New York, NY: McGraw-Hill.
  17. ^ a b c d Bannerman, R. L.; Milders, M.; De Gelder, B.; Sahraie, A. (2008). "Influence of emotional facial expressions on binocular rivalry". Ophthalmic & Physiological Optics. 28 (4): 317–326. doi:10.1111/j.1475-1313.2008.00568.x. PMID 18565087.
  18. ^ a b c d Hamed, M.; Goss, D. A.; Marzieh, E. (2013). "The relationship between binocular vision symptoms and near point of convergence. Indian". Journal of Ophthalmology. 61 (7): 325–328. doi:10.4103/0301-4738.97553. PMC 3759101. PMID 23552348.

Further reading

External links


Anisometropia is the condition in which the two eyes have unequal refractive power. Each eye can be nearsighted (myopia), farsighted (hyperopia) or a combination of both, which is called antimetropia. Generally a difference in power of two diopters or more is the accepted threshold to label the condition anisometropia.

In certain types of anisometropia, the visual cortex of the brain will not use both eyes together (binocular vision), and will instead suppress the central vision of one of the eyes. If this occurs often enough during the first 10 years of life while the visual cortex is developing, it can result in amblyopia, a condition where even when correcting the refractive error properly, the person's vision in the affected eye is still not correctable to 20/20.

The name is from four Greek components: an- "not," iso- "same," metr- "measure," ops "eye."

An estimated 6% of subjects aged 6 to 18 have anisometropia.


Binocular may refer to:

Binocular vision, seeing with two eyes

Binoculars, a telescopic tool

Binocular microscope, see stereo microscope

Binocular (horse), a thoroughbred race horse

Binocular (band), Kevin Rudolf's band that released a self-titled album in 2001

Binocular (album)

Binocular summation

Binocular summation refers to the improved visual performance of binocular vision compared to that of monocular vision. The most vital benefit of binocular vision is stereopsis or depth perception, however binocular summation does afford some subtle advantages as well. By combining the information received in each eye, binocular summation can improve visual acuity, contrast sensitivity, flicker perception, and brightness perception. Though binocular summation generally enhances binocular vision, it can worsen binocular vision relative to monocular vision under certain conditions. Binocular summation decreases with age and when large interocular differences are present.

Botulinum toxin therapy of strabismus

Botulinum toxin therapy of strabismus is a medical technique used sometimes in the management of strabismus, in which botulinum toxin is injected into selected extraocular muscles in order to reduce the misalignment of the eyes. The injection of the toxin to treat strabismus, reported upon in 1981, is considered to be the first ever use of botulinum toxin for therapeutic purposes. Today, the injection of botulinum toxin into the muscles that surround the eyes is one of the available options in the management of strabismus. Other options for strabismus management are vision therapy and occlusion therapy, corrective glasses (or contact lenses) and prism glasses, and strabismus surgery.

The effects that are due only to the toxin itself (including the side effects) generally wear off within 3 to 4 months. In contrast, improvements in alignment may be long-lasting, particularly in two circumstances. First, if the "antagonist" muscle (the muscle pulling in the opposite direction) is active, the injected muscle will be stretched, and may permanently lengthen by adding tissue during the period of toxin paresis. Second, if binocular vision has been achieved and stabilized, alignment may "lock in". There are indications that botulinum toxin therapy is as successful as strabismus surgery for patients with binocular vision and that it is less successful than surgery for those who have no binocular vision.

Convergence insufficiency

Convergence insufficiency or convergence disorder is a sensory and neuromuscular anomaly of the binocular vision system, characterized by a reduced ability of the eyes to turn towards each other, or sustain convergence.

Depth perception

Depth perception is the visual ability to perceive the world in three dimensions (3D) and the distance of an object. Depth sensation is the corresponding term for animals, since although it is known that animals can sense the distance of an object (because of their ability to move accurately, or to respond consistently, according to that distance), it is not known whether they "perceive" it in the same subjective way that humans do.Depth perception arises from a variety of depth cues. These are typically classified into binocular cues that are based on the receipt of sensory information in three dimensions from both eyes and monocular cues that can be represented in just two dimensions and observed with just one eye. Binocular cues include stereopsis, eye convergence, disparity, and yielding depth from binocular vision through exploitation of parallax. Monocular cues include size: distant objects subtend smaller visual angles than near objects, grain, size, and motion parallax.

Dinosaur vision

Dinosaur vision was, in general, better than the vision of most other reptiles, although vision varied between dinosaur species. Coelurosaurs, for example, had good stereoscopic or binocular vision, whereas large carnosaurs had poor binocular vision, comparable to that of modern alligators.


Exotropia is a form of strabismus where the eyes are deviated outward. It is the opposite of esotropia and usually involves more severe axis deviation than exophoria. People with exotropia often experience crossed diplopia. Intermittent exotropia is a fairly common condition. "Sensory exotropia" occurs in the presence of poor vision. Infantile exotropia (sometimes called "congenital exotropia") is seen during the first year of life, and is less common than "essential exotropia" which usually becomes apparent several years later.

The brain's ability to see three-dimensional objects depends on proper alignment of the eyes. When both eyes are properly aligned and aimed at the same target, the visual portion of the brain fuses the forms into a single image. When one eye turns inward, outward, upward, or downward, two different pictures are sent to the brain. This causes loss of depth perception and binocular vision. There have also been some reports of people that can "control" their afflicted eye. The term is from Greek exo meaning "outward" and trope meaning "a turning".

Field of view

The field of view (FoV) is the extent of the observable world that is seen at any given moment. In the case of optical instruments or sensors it is a solid angle through which a detector is sensitive to electromagnetic radiation.

Fixation disparity

Fixation disparity exists when there is a small misalignment of the eyes when viewing with binocular vision. The misalignment may be vertical, horizontal or both. The misalignment (a few minutes of arc) is much smaller than that of strabismus, which prevents binocular vision, although it may reduce a patient's level of stereopsis. A patient may or may not have fixation disparity and a patient may have a different fixation disparity at distance than near.


Gannets are seabirds comprising the genus Morus, in the family Sulidae, closely related to boobies. "Gannet" is derived from Old English ganot "strong or masculine", ultimately from the same Old Germanic root as "gander". Morus is derived from Ancient Greek moros, "foolish", due to the lack of fear shown by breeding gannets and boobies allowing them to be easily killed.The gannets are large white birds with yellowish heads; black-tipped wings; and long bills. Northern gannets are the largest seabirds in the North Atlantic, having a wingspan of up to 2 metres (6.6 ft). The other two species occur in the temperate seas around southern Africa, southern Australia and New Zealand.

Gannets hunt fish by diving into the sea from a height and pursuing their prey underwater. Gannets have a number of adaptations which enable them to do this:

no external nostrils, they are located inside the mouth instead;

air sacs in the face and chest under the skin which act like bubble wrapping, cushioning the impact with the water;

positioning of the eyes far enough forward on the face for binocular vision, allowing them to judge distances accurately.Gannets can dive from a height of 30 metres (98 ft), achieving speeds of 100 kilometres per hour (62 mph) as they strike the water, enabling them to catch fish much deeper than most airborne birds.

The gannet's supposed capacity for eating large quantities of fish has led to "gannet" becoming a description of somebody with a voracious appetite.

Hans-Joachim Haase (optician)

Hans-Joachim Haase (1915 – December 20, 2001) was a German clockmaker, optician and inventor, who became known for an apparatus for testing binocular vision and for the MKH method, an alternative method intended to improve binocular vision using corrective lenses. This method, which is controversial, has mainly found application in German-speaking countries.


A pseudoscope is a binocular optical instrument that reverses depth perception. It is used to study human stereoscopic perception. Objects viewed through it appear inside out, for example: a box on a floor would appear as a box shaped hole in the floor.

It typically uses sets of optical prisms, or periscopically arranged mirrors to swap the view of the left eye with that of the right eye.


Stereoblindness (also stereo blindness) is the inability to see in 3D using stereopsis, or stereo vision, resulting in an inability to perceive stereoscopic depth by combining and comparing images from the two eyes.

Individuals with only one functioning eye always have this condition; the condition also results when two eyes do not function together properly.

Most stereoblind persons with two healthy eyes do employ binocular vision to some extent, albeit less than persons with normally developed eyesight. This was shown in a study in which stereoblind subjects were posed with the task of judging the direction of rotation of a simulated transparent cylinder: the subjects performed better when using two eyes than when using their preferred eye. They appeared to judge the direction of rotation from the images in each eye separately and then to combine these judgments, rather than relying on differences between the images in the two eyes. Also, purely binocular motion stimuli appear to influence stereoblind persons' sensation of self-motion. Furthermore, in some cases each eye can contribute to peripheral vision for one side of the field of view (see also monofixation syndrome).

However, there is an exception to this: those with a true congenital alternating squint. Those with true congenital alternating squints have two healthy eyes, and the ability to switch (by choice) between seeing with either eye. However, stereoscopic and three dimensional vision can never be achieved in this condition (attempts to train those with true congenital alternating squints into binocular vision results in double vision, which can be irreversible).


Stereopsis (from the Greek στερεο- stereo- meaning "solid", and ὄψις opsis, "appearance, sight") is a term that is most often used to refer to the perception of depth and 3-dimensional structure obtained on the basis of visual information deriving from two eyes by individuals with normally developed binocular vision. Because the eyes of humans, and many animals, are located at different lateral positions on the head, binocular vision results in two slightly different images projected to the retinas of the eyes. The differences are mainly in the relative horizontal position of objects in the two images. These positional differences are referred to as horizontal disparities or, more generally, binocular disparities. Disparities are processed in the visual cortex of the brain to yield depth perception. While binocular disparities are naturally present when viewing a real 3-dimensional scene with two eyes, they can also be simulated by artificially presenting two different images separately to each eye using a method called stereoscopy. The perception of depth in such cases is also referred to as "stereoscopic depth".The perception of depth and 3-dimensional structure is, however, possible with information visible from one eye alone, such as differences in object size and motion parallax (differences in the image of an object over time with observer movement), though the impression of depth in these cases is often not as vivid as that obtained from binocular disparities.

Therefore, the term stereopsis (or stereoscopic depth) can also refer specifically to the unique impression of depth associated with binocular vision; what is colloquially referred to as seeing "in 3D".

It has been suggested that the impression of "real" separation in depth is linked to the precision with which depth is derived, and that a conscious awareness of this precision – perceived as an impression of interactability and realness – may help guide the planning of motor action.

Stereoscopic acuity

Stereoscopic acuity, also stereoacuity, is the smallest detectable depth difference that can be seen in binocular vision.

Suppression (eye)

Suppression of an eye is a subconscious adaptation by a person's brain to eliminate the symptoms of disorders of binocular vision such as strabismus, convergence insufficiency and aniseikonia. The brain can eliminate double vision by ignoring all or part of the image of one of the eyes. The area of a person's visual field that is suppressed is called the suppression scotoma (with a scotoma meaning, more generally, an area of partial alteration in the visual field). Suppression can lead to amblyopia.


A vergence is the simultaneous movement of both eyes in opposite directions to obtain or maintain single binocular vision.When a creature with binocular vision looks at an object, the eyes must rotate around a horizontal axis so that the projection of the image is in the centre of the retina in both eyes. To look at an object closer by, the eyes rotate towards each other (convergence), while for an object farther away they rotate away from each other (divergence). Exaggerated convergence is called cross eyed viewing (focusing on the nose for example). When looking into the distance, the eyes diverge until parallel, effectively fixating the same point at infinity (or very far away).

Vergence movements are closely connected to accommodation of the eye. Under normal conditions, changing the focus of the eyes to look at an object at a different distance will automatically cause vergence and accommodation, sometimes known as the accommodation-convergence reflex.

As opposed to the 500°/s velocity of saccade movements, vergence movements are far slower, around 25°/s. The extraocular muscles may have two types of fiber each with its own nerve supply, hence a dual mechanism.

Worth 4 dot test

The Worth Four Light Test, also known as the Worth's Four Dot test or W4LT, is a clinical test mainly used for assessing a patient's degree of binocular vision and binocular single vision. Binocular vision involves an image being projected by each eye simultaneously into an area in space and being fused into a single image. The Worth Four Light Test is also used in detection of suppression of either the right or left eye. Suppression occurs during binocular vision when the brain does not process the information received from either of the eyes. This is a common adaptation to strabismus, amblyopia and aniseikonia.

The W4LT can be performed by the examiner at two distances, at near (at 33 cm from the patient) and at far (at 6m from the patient). At both testing distances the patient is required to wear red-green goggles (with one red lens over one eye, usually the right, and one green lens over the left) When performing the test at far (distance) the W4LT instrument is composed of a silver box (mounted on the wall in front of the patient), which has 4 lights inside it. The 4 lights are arranged in a diamond formation, with a red light at the top, two green lights at either side (left and right) and a white light at the bottom. When performing the test at near (at 33 cm ) the 4 lights are arranged in exactly the same manner (diamond formation), with the difference being that at near, the 4 lights are located in a hand held instrument which is similar to a light torch.

Because the red filter blocks the green light and the green filter blocks the red light, it is possible to determine if the patient is using both eyes simultaneously and in a coordinated manner. With both eyes open, a patient with normal binocular vision will appreciate four lights. If the patient either closes or suppresses an eye they will see either two or three lights. If the patient does not fuse the images of the two eyes, they will see five lights (diplopia).


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