Binocular vision

Binocular vision, that is, seeing with two eyes, when an object is perceived as a single image, is possible only with the clear and friendly movements of the eyeballs. The eye muscles provide the installation of two eyes on the fixation object so that its image falls on identical points of the retina of both eyes. Only in this case there is a single perception of the object of fixation.

Identical, or corresponding, are the central fossa and retina points that are separated by the same distance from the central pits in the same meridian. The points of the retina, located at different distances from the central pits, are called disparate, non-conforming (non-identical). They do not possess the innate property of solitary perception. When the image of the fixation object hits the non-identical retinal points, doubling occurs, or diplopia (Greek diplos - double, opos - eye) - a very painful condition. This happens, for example, with strabismus, when one of the visual axes is shifted to either side of the common fixation point.

Two eyes are located in the same frontal plane at a certain distance from each other, therefore in each of them are formed not completely identical images of objects located in front and behind the fixation object. As a result of this, double-vision, called the physiological one, inevitably arises. It is neutralized in the central part of the visual analyzer, but serves as a conditional signal for the perception of the third spatial dimension, i.e., depth.

Such a displacement of the images of objects (closer and further located fixation points) to the right and left of the yellow spot on the retinas of both eyes creates the so-called transverse disparation (displacement) of the images and the projection (projection) of them onto disparate areas (non-identical points) including physiological.

Transverse dysparation is the primary factor of deep perception. There are secondary, auxiliary factors that help in assessing the third spatial dimension. This is a linear perspective, the size of the objects, the location of the chiaroscuro, which helps the perception of depth, especially in the presence of one eye, when transverse dysparation is excluded.

With the concept of binocular vision, terms such as fusion (the psychophysiological act of merging monocular images), fusional reserves, which ensure binocular fusion with a certain degree of convergence and divergence of the visual axes, are associated.

[1], [2], [3]

Definition of binocular vision

Synoptophor is a tool for assessing strabismus and quantifying binocular vision. With its help, it is possible to detect suppression and ACS. The tool consists of two cylindrical tubes with a mirror located at a right angle, and a lens of + 6.50 D for each eye. This allows you to create optical conditions at a distance of 6 m. The pictures are inserted into the slide carrier from the outside of each tube. Two tubes are supported on the columns, which allow the pictures to move relative to each other, and these movements are marked on the scale. Synoptophor measures horizontal, vertical and torsional deflections.

[4], [5], [6]

Identifying AKS

AKS is detected using a synoptophore as follows.

1. The examiner determines the objective angle of strabismus, projecting on the fovea of one eye, then another eye, until the setting movements stop.
2. If the objective angle is equal to the subjective angle of strabismus, i.e. Images are evaluated as superimposed on each other with the same position of synoptophora handles, then retinal correspondence is normal,
3. If the objective angle is not equal to the subjective angle, then the ACS takes place. The difference between the angles and is the angle of the anomaly. AKS is harmonic if the objective angle is equal to the angle of the anomaly, and is non-harmonic if the objective angle exceeds the angle of the anomaly. With a harmonious ACS, the subjective angle is equal to zero (i.e., theoretically, the set-up motion for the cover test will not be).

Measuring the deflection angle

Test Hirschberg

This is an indicative way to assess the angle of manifest strabismus in poorly cooperating patients with poor fixation. At the distance of the outstretched hand, a flashlight illuminates both eyes of the patient and requires fixation of the sight on the object. The corneal reflex is located more or less at the center of the pupil of the fixing eye and is centered in the mowing eye in the direction opposite to the deflection. Estimates the distance between the center of the cornea and the reflex. Presumably, every millimeter of deviation is 7 (15 D). For example, if the reflex is located at the temporal edge of the pupil (with its diameter of 4 mm), the angle is 30 D if the edge of the limb is about 90 degrees. This test is informative for detecting pseudostrabism, which is classified as follows.

Pseudo-isotropy

• epicanthus;
• small interpupillary distance with closely set eyes;
• negative angle of the kappa. The angle of the kappa is the angle between the visual and anatomical axes of the eye. As a rule, the foveola is located on the temporal side of the posterior pole. Thus, the eyes are in a state of slight abduction to achieve bifovel fixation, which causes the reflex to shift nasally from the center of the cornea in both eyes. This state is called the positive angle of the kappa. If it is large enough, it can simulate exotrophy. A negative angle of the kappa occurs when the foveola is located nasally relative to the posterior pole (high degree myopia and fovea ectopia). In this situation, the corneal reflex is located to the temple from the center of the cornea and can simulate esotropia.

Pseudoexotropy

• large interpupillary distance;
• the positive angle of the kappa described previously.

The Krimsky test

In this test, the prism is placed in front of the fixing eye until the light corneal reflexes become symmetrical. It is important that in the Krimsky test there is no separation and only a manifest deviation is evaluated, but since the latent component is not taken into account, the true deviation value is underestimated.

Test with cover

The deviation can be most accurately estimated using a cover test. Om allows you to differentiate the tropes and fores, assess the degree of control of the deviation and determine the preference for fixation and the strength of fixing each eye. This test is based on the ability of the patient to fix the object, while it requires attention and interaction.

The test with a cover-uncover test consists of two parts.

A test with a cover for detecting heterotrophy. Should be carried out while fixing the close (using the accomodative fixation mark) and distant objects as follows;

• The patient locks the object directly in front of him.
• If the right eye is being rejected, the examiner covers the left eye and marks the movements of the right eye.
• The absence of locational motions means orthotropy or heterotropy on the left.
• Adduction of the right eye to restore fixation indicates exotrophy, and abduction - on esophoria.
• Movement down indicates hypertropy, and upward - to hypotrophy.
• The test is repeated on the pair eye.

The open test reveals heterophoria. It should be carried out by fixing a close (using the accommodative stimulus) and distant objects as follows:

• The patient locks the distant object located directly in front of him.
• The examiner covers his right eye and opens it after a few seconds.
• Lack of movement means orthophoresis, although the observant examiner will often reveal a slight latent deviation in most healthy people, since true orthophyroidism is rare.
• If the right eye behind the flap is deflected, then the reflex movement will appear when opening.
• Adduction of the right eye indicates exophoria, and abduction - on esophoria.
• The setting movement up or down points to the vertical forehead. With latent strabismus, unlike the manifest, it is never clear whether this is a one-eye hypotropy or another's hypertropy.
• The test is repeated on the pair eye.

In the survey, the test is normally combined with a cover and an open test, hence the name "cover-open test".

Alternating test with cover violates the mechanisms of binocular fusions and reveals the true deviation (the background and the tropia). It must be carried out after the test with a cover-opening, because if you hold it earlier, it will not allow you to differentiate the background from the path.

• the right eye is covered for 2 seconds;
• The flap is moved to the pair eye and quickly shifted to the other eye for 2 seconds, then back and forth several times;
• after opening the shutter, the examiner marks the speed and smoothness of returning the eye to its original position;
• in a patient with heterophoria, the correct position of the eyes is noted before and after the test, whereas in heterotrophy, a manifest deviation is noted.

A test with a cover with prisms allows you to accurately measure the angle of strabismus. It is carried out as follows:

• Initially, an alternating test is performed with a cover;
• The prisms of the increasing force are placed in front of one eye by a base in the direction opposite to the deflection (ie, the top of the prism is directed toward the deviation). For example, with convergent strabism, prisms are placed with the base outward;
• alternating test with cover continue all this time. As the prism increases, the amplitude of the reflex movements of the eyes gradually decreases;
• The study is carried out until the moment of neutralization of eye movements. The angle of deflection is equal to the strength of the prism.

Tests with different images

The "Wing" test Maddox separates the eyes when fixing a nearby object (0.33 m) and measures the heterophore. The instrument is designed in such a way that the right eye sees only the white vertical and red horizontal arrows, and the left eye - only a horizontal and vertical series of digits. The measurements are carried out as follows:

• Horizontal deviation: the patient is asked what the white arrow indicates.
• Vertical deviation: the patient is asked what number the red arrow points to.
• Evaluation of the degree of cyclophore: the patient is asked to move the red arrow to stand parallel to the horizontal row of numbers.

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

• The Maddox wand is placed in front of the right eye. This separates the two eyes, since the red line in front of the right eye can not merge with the white point source in front of the left eye.
• The degree of dissociation is measured by the fusion of two images with the help of prisms. The base of the prism is directed in the direction opposite to the deflection of the eye.
• Vertical and horizontal deviation can be measured, but it is impossible to differentiate the view from the path.

Grades of binocular vision

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

1. The first degree (simultaneous perception) is tested by the presentation of two different, but not absolutely antagonistic images, for example "birds in a cage". The subject is offered to put the bird in the cage, moving the handles of the synoptophore. If two pictures are not visible at the same time, then this is either a suppression or a significant degree of amblyopia. The term "simultaneous perception" is disorienting, since two different objects can not be localized in one place of 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 projected onto the mowing eye).
2. The second degree (fusia) is the ability to merge two similar images, differing by an insignificant detail, into one. A classic example are two rabbits, one of which has no tail, and the other has a bouquet of flowers. If the child sees a rabbit with a tail and a bouquet of flowers, this indicates the presence of fusi. Fusional reserves are evaluated by shifting the synoptophor handles, and the eyes are synergized or diverged to support fusions. Obviously, fusion with small fusional reserves is of low value in everyday life.
3. The third degree (stereopsis) is the ability to maintain the perception of depth when two images of the same object are projected at different angles. A classic example is a bucket, which is perceived as a three-dimensional image.