Binocular vision

Binocular vision, i.e. vision with two eyes, when an object is perceived as a single image, is possible only with clear, concomitant movements of the eyeballs. The eye muscles ensure that both eyes are positioned on the object of fixation so that its image falls on identical points of the retinas of both eyes. Only in this case does single perception of the object of fixation occur.

Identical, or corresponding, are the central pits and retinal points located at the same distance from the central pits in the same meridian. Retinal points located at different distances from the central pits are called disparate, non-corresponding (non-identical). They do not have the innate property of single perception. When the image of the object of fixation falls on non-identical points of the retina, double vision, or diplopia (Greek diplos - double, opos - eye), occurs - a very painful condition. This occurs, for example, with strabismus, when one of the visual axes is shifted to one side or another from the common fixation point.

The two eyes are located in the same frontal plane at some distance from each other, so each of them forms not quite identical images of objects located in front and behind the object of fixation. As a result, doubling inevitably occurs, called physiological. It is neutralized in the central section of the visual analyzer, but serves as a conditional signal for the perception of the third spatial dimension, i.e. depth.

This displacement of images of objects (closer and further from the fixation point) to the right and left of the macula lutea on the retinas of both eyes creates the so-called transverse disparity (displacement) of images and their entry (projection) onto disparate areas (non-identical points), which causes double vision, including physiological.

Transverse disparity is the primary factor of depth perception. There are secondary, auxiliary factors that help in assessing the third spatial dimension. These are linear perspective, the size of objects, the arrangement of light and shadow, which helps in depth perception, especially in the presence of one eye, when transverse disparity is excluded.

The concept of binocular vision is associated with such terms as fusion (the psychophysiological act of merging monocular images), fusion reserves, which provide binocular fusion at a certain degree of reduction (convergence) and separation (divergence) of the visual axes.

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Features of binocular vision

Binocular vision is the ability to see volume and perceive depth using two eyes located on a person's face. This property of vision is provided by the following features:

1. Joint perception: Each eye sees an object from a slightly different angle, and the brain combines the two images into one. This fusion of images allows a person to judge depth, distance, and the three-dimensional structure of objects.
2. Stereovision: The effect of each eye seeing an image with a slight shift is called stereovision. It allows a person to estimate the proximity and distance of objects and accurately determine their position in space.
3. Overlapping images: During binocular vision, parts of the images in each eye overlap, and the brain merges these overlapped areas. This creates a sense of depth and volume.
4. Fixation: The eyes usually fixate on the same point in space. This ensures stability of vision and allows a person to follow moving objects.
5. Convergence: When a person looks at a close object, the eyes come together to focus on that object. This is called convergence. When a person looks at a distant object, the eyes diverge.
6. Stereopsis: Stereopsis is the ability to discern small differences in the position of objects in space. It allows a person to see the smallest details and assess depth perception.

Binocular vision is an important part of normal human vision and allows us to evaluate the world around us in three dimensions. Binocular vision disorders can lead to problems with depth perception and eye movement coordination, which can cause problems with visual function and perception of the world around us.

What ancestral features led to binocular vision?

Binocular vision developed during the evolution of mammals, including humans, as an adaptation to the characteristics of their environment and lifestyle. This feature has its advantages and is associated with a number of evolutionary changes:

1. Transition to Arboreal Life: Early primates shifted their life from the ground to the trees, where they began to actively move, search for food, and avoid danger. Binocular vision was an adaptive advantage, allowing them to judge distances and depths while moving through the branches of trees.
2. Hunting and foraging: Binocular vision became important for hunting insects and other small animals, as well as for finding edible fruits and plants in the forest. Deep stereo vision allowed primates to accurately aim and capture prey.
3. Social life: Primates with binocular vision exhibit complex social behavior, including various forms of communication, interaction, and recognition of group members. Binocular vision allows for more accurate detection of facial expressions and gestures of others.
4. Predator Defense: Binocular vision can also help in early detection of predators, which can increase the chances of survival.
5. Brain development: Binocular vision requires more complex information processing in the brain, which contributed to the development of the primate brain and its capacity for highly organized behavior.

As a result of these evolutionary adaptations and advantages, binocular vision has become one of the characteristic features of primates, including humans. This feature allows us to interact more effectively with the world around us and successfully adapt to various aspects of our lives.

Definition of binocular vision

A synoptophore is an instrument for assessing strabismus and quantifying binocular vision. It can detect suppression and ACS. The instrument consists of two cylindrical tubes with a mirror positioned at a right angle and a +6.50 D lens for each eye. This allows optical conditions to be created at a distance of 6 m. Pictures are inserted into a slide carrier on the outside of each tube. The two tubes are supported on columns that allow the pictures to move relative to each other, and these movements are marked on a scale. The synoptophore measures horizontal, vertical and torsional deviations.

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Identification of ACS

ACS is detected using a synoptophore as follows.

1. The examiner determines the objective angle of strabismus by projecting an image onto the fovea of one eye and then the other until the adjustment movements stop.
2. If the objective angle is equal to the subjective angle of strabismus, i.e. the images are assessed as superimposed on each other with the same position of the synoptophore handles, then the retinal correspondence is normal,
3. If the objective angle is not equal to the subjective angle, then there is an AKS. The difference between the angles is the angle of the anomaly. The AKS is harmonious if the objective angle is equal to the angle of the anomaly, and inharmonious if the objective angle exceeds the angle of the anomaly. With a harmonious AKS, the subjective angle is equal to zero (i.e., theoretically, there will be no installation movement during the cover test).

Measuring the angle of deviation

Hirschberg test

This is an approximate method for assessing the angle of manifest strabismus in poorly cooperating patients with poor fixation. At arm's length, a flashlight is placed in both eyes of the patient and the patient is asked to fixate on an object. The corneal reflex is located more or less in the center of the pupil of the fixating eye and is decentered in the squinting eye in the direction opposite to the deviation. The distance between the center of the cornea and the reflex is estimated. Presumably, each millimeter of deviation is equal to 7 (15 D). For example, if the reflex is located along the temporal edge of the pupil (with its diameter of 4 mm), the angle is 30 D, if along the edge of the limbus, the angle is about 90 D. This test is informative for detecting pseudostrabismus, which is classified as follows.

Pseudoesotropia

• epicanthus;
• small interpupillary distance with close-set eyes;
• negative angle kappa. Angle kappa is the angle between the visual and anatomical axes of the eye. Typically, the foveola is located temporally from the posterior pole. Thus, the eyes are in a state of slight abduction to achieve bifoveal fixation, which leads to a nasally shift of the reflex from the center of the cornea in both eyes. This condition is called a positive angle kappa. If it is large enough, it can simulate exotropia. A negative angle kappa occurs when the foveola is located nasally relative to the posterior pole (high myopia and ectopia of the fovea). In this situation, the corneal reflex is located temporally from the center of the cornea and can simulate esotropia.

Pseudoexotropia

• large interpupillary distance;
• positive kappa angle, described earlier.

Krimsky test

In this test, a prism is placed in front of the fixating eye until the corneal light reflexes become symmetrical. Importantly, the Krimsky test does not dissociate and only assesses the manifest deviation, but since the latent component is not taken into account, the true magnitude of the deviation is underestimated.

Cover test

The most accurate way to assess deviation is with the cover test. The cover test differentiates tropias and phorias, assesses the degree of control of deviation, and determines the fixation preference and fixation strength of each eye. This test is based on the patient's ability to fixate an object, and requires attention and interaction.

The cover-uncover test consists of two parts.

Cover test for heterotropia. Should be performed with fixation of near (using accommodative fixation cue) and distant objects as follows;

• The patient fixes an object located directly in front of him.
• If deviation of the right eye is suspected, the examiner covers the left eye and notes the movements of the right eye.
• The absence of installation movements indicates orthophoria or heterotropia on the left.
• Adduction of the right eye to restore fixation indicates exotropia, and abduction indicates esophoria.
• Downward movement indicates hypertropia, and upward movement indicates hypotropia.
• The test is repeated on the fellow eye.

The opening test reveals heterophoria. It should be performed with fixation of a near (using an accommodative stimulus) and a distant object as follows:

• The patient fixates on a distant object located directly in front of him.
• The examiner covers his right eye and opens it after a few seconds.
• Lack of movement indicates orthophoria, although an observant examiner will often detect a slight latent deviation in most healthy individuals, since true orthophoria is rare.
• If the right eye behind the shutter has deviated, then when opening, a refixation movement will appear.
• Adduction of the right eye indicates exophoria, and abduction indicates esophoria.
• An upward or downward adjustment movement indicates a vertical phoria. In latent strabismus, unlike manifest strabismus, it is never clear whether it is a hypotropia of one eye or a hypertropia of the other.
• The test is repeated on the fellow eye.

The examination usually combines the cover test and the uncover test, hence the name "cover-uncover test".

The alternating cover test disrupts the mechanisms of binocular fusion and reveals the true deviation (phoria and tropia). It should be performed after the cover-uncover test, since if it is performed earlier, it will not differentiate phoria from tropia.

• the right eye is covered for 2 seconds;
• the shutter is moved to the fellow eye and quickly shifted to the other eye for 2 seconds, then back and forth several times;
• after opening the shutter, the examiner notes the speed and smoothness of the eye's return to its original position;
• In a patient with heterophoria, the correct position of the eyes is noted before and after the test, whereas in heterotropia, a manifest deviation is noted.

The prism cover test allows you to accurately measure the angle of strabismus. It is performed as follows:

• First, an alternating cover test is performed;
• prisms of increasing power are placed in front of one eye with the base facing the direction opposite to the deviation (i.e. the apex of the prism is directed towards the direction of the deviation). For example, in the case of convergent strabismus, the prisms are placed with the base facing outward;
• The alternating cover test continues throughout this time. As the prisms become stronger, the amplitude of the refixation eye movements gradually decreases;
• The study is carried out until the moment of neutralization of eye movements. The angle of deviation is equal to the power of the prism.

Tests with different images

The Maddox Wing test separates the eyes while fixating a close object (0.33 m) and measures heterophoria. The instrument is designed so that the right eye sees only a white vertical and red horizontal arrow, and the left eye sees only a horizontal and vertical row of numbers. The measurements are taken as follows:

• Horizontal deflection: The patient is asked what number the white arrow points to.
• Vertical deviation: The patient is asked what number the red arrow points to.
• Assessment of the degree of cyclophoria: the patient is asked to move the red arrow so that it is parallel to the horizontal row of numbers.

The Maddox stick test consists of several cylindrical red glass sticks fused together, through which the image of a white spot is perceived as a red stripe. The optical properties of the sticks refract the light beam at an angle of 90: if the sticks are horizontal, the line will be vertical, and vice versa. The test is carried out as follows:

• The Maddox rod is placed in front of the right eye. This separates the two eyes because the red line in front of the right eye cannot merge with the white point source in front of the left eye.
• The degree of dissociation is measured by merging the two images using prisms. The base of the prism is directed in the direction opposite to the deviation of the eye.
• Vertical and horizontal deviation can be measured, but it is not possible to differentiate phoria from tropia.

Gradations of binocular vision

Binocular vision is classified, according to the data of the synoptophore, as follows.

1. The first degree (simultaneous perception) is tested by presenting two different but not absolutely antagonistic pictures, for example, "bird in a cage". The subject is asked to place the bird in the cage by moving the handles of the synoptophore. If the two pictures are not seen simultaneously, then there is either suppression or a significant degree of amblyopia. The term "simultaneous perception" is misleading, since two different objects cannot be localized in the same place in space. Retinal "rivalry" means that the image of one eye dominates the other. One of the pictures is smaller than the other, so its image is projected on the fovea, and the larger one on the parafovea (and thus is projected on the squinting eye).
2. The second degree (fusion) is the ability to merge two similar images that differ in minor detail into one. A classic example is two rabbits, one of which has no tail and the other has a bouquet of flowers. If a child sees a rabbit with a tail and a bouquet of flowers, this indicates the presence of fusion. Fusion reserves are assessed by shifting the synoptophore handles, and the eyes synergize or diverge to maintain fusion. Obviously, fusion with small fusion reserves is of little value in everyday life.
3. Third degree (stereopsis) is the ability to maintain depth perception when superimposing two images of the same object projected at different angles. A classic example is a bucket, which is perceived as a three-dimensional image.