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Directional vision

Main article: Directional vision

Directional vision focuses on the question of how the images from the two eyes are combined in perception into a combined image and how the directions in which each eye sees the objects around us are converted into practically useful information.

The main article on directional vision describes that the direction in which the left and right eyes see an object can be combined in three different ways. This leads to seeing a single image, double images or a fused image. This perception is linked to a certain quality of depth perception: fine stereopsis or coarse stereopsis.

Depth vision

Main article: Stereopsis

Depth perception focuses on the question of how the brain uses the difference in perspective between the two eyes to recognise shapes and objects, to see through camouflage and to gather information about spatial relationships.

The main article on stereopsis discusses the qualities of depth perception, the area of ​​space they cover, and how the observer controls the input through attention and eye movements.

Binocular interaction

Binocular interaction occurs when there is an interaction between the two eyes, which causes vision with both eyes to be different than with one eye alone. Vision can be better (binocular summation) or worse (binocular inhibition).

Binocular summation

Main article: Binocular summation

In binocular summation, the signals from both eyes reinforce each other¹ so that visual acuity, contrast sensitivity, flicker sensitivity and brightness sensitivity improve.² Maximum binocular summation occurs when the sensitivities of each eye are equal. Differences in sensitivity decrease the effect of binocular summation. The effect of binocular summation decreases with age.³

Binocular inhibition

In binocular inhibition, vision with both eyes is worse than with one eye. This can occur with strabismus or a lazy eye, because the weaker eye interferes with the stronger one.⁴ Eye dominance is a form of binocular inhibition.

Eye dominance

Main article: Eye dominance

Eye dominance is the phenomenon that the image produced by one eye in the brain can suppress the image from the other eye.

Perception systems

Information processing for direction perception and depth perception takes place in two systems. One system specializes in color and fine detail and is concerned with discovering shapes and objects in a relatively static environment. The other system specializes in discovering spatial relationships in a rapidly changing environment. The first system contributes to the perception of fused images with fine depth information. The second system contributes to the perception of double images that quickly cover large distances in space and in which the coarse location is the most important information.

Disorders and tests

About eighty percent of people can see depth, but not always equally well. There are a large number of tests to determine how well someone can see depth and there are exercises to improve depth perception. If one eye does not function properly or is even blind, this can cause complete stereoblindness. There are a large number of other eye disorders that affect binocular vision. For example, sometimes the eye muscles do not work properly, so that the images from both eyes cannot be aligned properly. Or one eye is dominant, so that the signals from the other eye do not come through in the binocular image, or so that the direction in which an object is seen changes. If eye dominance is noticed in time, an attempt can be made to reduce it through exercise, for example by temporarily taping the dominant eye.

Amblyopia

Main article: Amblyopia

Lazy eye or amblyopia is a neurovisual developmental disorder. The condition is characterized by underdevelopment of several visual features and skills such as visual acuity, eye movements, eye teamwork and binocular depth perception.

Squint

Squint or strabismus is an eye condition in which the eyes do not look in the same direction, more....

It has long been known that full binocular vision, including stereopsis, is an important factor in stabilizing the postoperative outcome of strabismus corrections. Many people with a lack of stereopsis have (or had) visible strabismus, which has a potential socioeconomic impact on children and adults. Both wide-angle and narrow-angle strabismus in particular can negatively impact self-confidence because it disrupts normal eye contact, often leading to embarrassment, anger, and feelings of discomfort.⁵ See psychosocial effects of strabismus for more information about this.

Stereoblindness

Stereoblindness is the inability to perceive binocular depth.

Stereopsis tests

In stereopsis testing (abbreviated to stereotesting), stereograms are used to measure the presence and sharpness of binocular depth perception (stereopsis).

There are two types of common clinical tests: random dot stereotesting and contour stereotesting. Random-dot stereotesting uses images of stereo figures embedded in a background of random dots. Contour stereo tests use images in which the targets presented to each eye are separated horizontally.⁶

Random-dot stereo tests

Main article: Random dot stereogram § Random dot stereo tests

For example, stereopsis ability can be tested with the Lang Stereo Test, which consists of a random-dot stereogram on which a series of parallel strips cylindrical lenses are printed in certain shapes, which represent the images which each eye sees in these areas, separate from each other.⁷ similar to a hologram. Without stereopsis, the image appears as a field of random dots, but the shapes become visible with increasing stereopsis and generally consist of a cat (indicating that a stereopsis of 1200 arc seconds of retinal disparity is possible), a star (600 arc seconds), and a car (550 arc seconds).⁸ To standardize the results, the image should be viewed at a distance of 40 cm from the eye and exactly in the frontoparallel plane.⁹ While most random dot stereotests, such as the Random Dot "E" stereotest or the TNO stereotest, require special glasses (i.e., polarized or red-green lenses), the Lang stereotest works without special glasses, making it easier to use with young children.¹⁰

Contour stereotests

Examples of contour stereotests include the Titmus stereotests, of which the Titmus fly stereotest is the best-known example, in which an image of a fly is shown with deviations at the edges. The patient uses 3D glasses to look at the image and determine whether a 3D figure can be seen. The degree of deviation in the images varies, for example 400-100 arc seconds and 800-40 arc seconds.¹¹

Vision therapy

Vision therapy is a controversial treatment to improve stereopsis.

Other disorders

To maintain stereopsis and singleness of vision, the eyes need to be pointed accurately. The position of each eye 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 esophoria. If it moved from in to out, the person has exophoria. 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 down to up) or hypophoria (if the eye moved from up to down). Such vertical phorias are quite rare. It is also possible for the covered eye to rotate in its orbit, such a condition is known as cyclophoria. They are rarer than vertical phorias. Cover test may be used to determine direction of deviation in cyclophorias also.¹²

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 in to out, the person has exotropia. If it moved from out to in, 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.¹³ A definition that incorporates all of these defines amblyopia as a unilateral condition in which vision is 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 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.¹⁵ About 20% of patients who come to optometry clinics will have binocular vision anomalies.¹⁶ As digital device use has become more common, many children are using digital devices for a significant period of time. This could lead to various binocular vision anomalies (such as reduced amplitudes of accommodation, accommodative facility, and positive fusional vergence both at near and distance).¹⁷ 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.¹⁹ The most effective way to diagnosis vision anomalies is with the near point of convergence test.²⁰ 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.²¹

To some 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.

In animals

Stereopsis requires that the visual fields of both eyes overlap (eyes in front) and therefore comes at the expense of the width of the visual field (eyes at the sides). Some animals have their eyes placed on either side of their head to provide the widest possible field of vision. Examples of this are prey animals such as rabbits, buffalo and antelopes. This allows them to see their attacker coming in all directions. In such animals, the eyes often move independently of each other to further increase the field of vision. Even without moving their eyes, some birds have a 360-degree field of vision.

Some other animals have their eyes placed at the front of their head, which allows them to perceive depth and see through camouflage of their prey at the expense of a smaller field of vision. Examples include predators such as tigers and lions, but also primates such as humans and monkeys.

Advantages of two eyes

Manfred Fahle has stated six specific advantages of having two eyes rather than just one:²²

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.²³
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)²⁴ to be determined from the images in the two eyes.²⁵ 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.²⁶

Field of view and eye movements

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, buffalo, 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, front-facing eyes are 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 predatory 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.[how?]²⁷ 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. In animals with forward-facing eyes, the eyes usually move together.

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. 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.

Stereopsis in animals

Stereopsis has been found in many vertebrates²⁸ including mammals such as horses,²⁹ birds such as falcons³⁰ and owls,³¹ reptiles, amphibia including toads³² and fish. It has also been found in invertebrates³³ including cephalopods like the cuttlefish,³⁴ crustaceans, spiders, and insects such as mantis.³⁵ Stomatopods even have stereopsis with just one eye.³⁶

Applications

Applications for binocular vision are aids for binocular vision, aimed at making, recording and viewing stereo images.

The binocular microscope and binoculars can magnify images. By increasing the distance between the front lenses of the binoculars and decreasing the distance of the front lenses of the microscope, the perceived depth is in proportion to the magnification. In the course of history, various types of stereoscopes have been developed with which specially prepared stereo recordings (stereograms) can be viewed in 3D, both at home and in the cinema. The most recent development is the VR glasses.

Binocular viewers

The observable three-dimensional space can be seemingly enlarged with a binocular telescope for things that are far away and a binocular microscope for very small things. It is not self-evident that by enlarging the image, depth is also seen. This is explained below.

Binoculars

With binoculars, the world that we know up close can also be viewed from a distance. The optics in the binoculars ensure that the retinal images are enlarged. The perceived depth is reduced, the image appears flatter. In order to restore the normal aspect ratio, binocular binoculars must be used to view from two points that are further apart than the two eyes. In binocular binoculars, the front lenses are therefore placed further apart using optical means (prisms or mirrors). The enlargement of the depth dimension that can be achieved in this way is practically limited to the area in which stereopsis is possible. At greater depth (disparity), the image appears flat. For a natural depth experience, it is important that the distance between the two lenses is adjusted to the magnification factor of the binoculars.

Microscope

A binocular microscope can be used to magnify and view a microscopic world. In order to see depth in this small world that is in proportion to the size of the objects present, the distance between the front lenses of the microscope must be much smaller than the normal distance between our eyes. This is done with the same optical means as with binocular binoculars, but then in a mirrored arrangement, see figure.

Without depth

Even without seeing depth, binocular vision has advantages over seeing with one eye. If care is taken to ensure that the images from both eyes overlap well and are sharp, then the images from both eyes reinforce each other (binocular summation) and it is as if the image is brighter. This can be compared to increasing the light intensity by using larger lenses.

Stereo images

Binocular images can be captured by recording both the image seen by the left eye and the image seen by the right eye in a stereogram. The images can be recorded simultaneously (stereo photography) or one after the other (scout stereogram, moon stereogram). The advantage of recording the images of the left and right eyes simultaneously with a stereo camera is that no false disparities arise because the scene to be recorded has changed between the recordings.

Stereogram

A stereogram is a set of two images (pictures, videos or computer-generated images), one for each eye, with which a binocular three-dimensional scene can be evoked.

In a stereogram, the two pictures can be attached to each other, with the picture for the left eye on the left and the picture for the right eye on the right (L-R stereogram), or with the picture for the right eye on the left and for the left eye on the right (R-L stereogram). A stereogram can also consist of two separate pictures that are placed separately in a stereoscope, or that are specially prepared and placed on top of each other and where color filters, polaroid filters or optical ridges are used to ensure that each eye sees only one of the pictures.

Stereograms are and still are widely used in research into depth perception, for entertainment and education.

Stereograms can be made by hand, by drawing with a computer program, or by taking pictures with one or two cameras (stereo camera) or video cameras.³⁷³⁸³⁹ The geometry used to design the correct disparities for these images is described in epipolar geometry and computer stereo vision.

For natural scenes, the recordings for a stereogram are usually made from observation positions that are as far apart as the distance between the two eyes. For macro photography, this distance must be smaller to get a natural depth effect.

See also

• Binocular illusions
• Ophthalmology

Bibliography

• Julesz, B. (1971). Foundations of cyclopean perception. Chicago: University of Chicago Press
• Steinman, Scott B. & Steinman, Barbara A. & Garzia, Ralph Philip (2000). Foundations of Binocular Vision: A Clinical perspective. McGraw-Hill Medical. ISBN 0-8385-2670-5.
• Howard, I. P., & Rogers, B. J. (2012). Perceiving in depth. Volume 2, Stereoscopic vision. Oxford: Oxford University Press. ISBN 978-0-19-976415-0
• Cabani, I. (2007). Segmentation et mise en correspondance couleur – Application: étude et conception d'un système de stéréovision couleur pour l'aide à la conduite automobile. ISBN 978-613-1-52103-4

External links

• Middlebury Stereo Vision Page
• VIP Laparoscopic / Endoscopic Video Dataset (stereo medical images) Archived 2011-05-12 at the Wayback Machine
• What is Stereo Vision?
• Learn about Stereograms then make your own Magic Eye
• International Orthoptic Association

References

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