Binocular vision
Among binocular vision (binocular = binocular ) refers to all sensory and motor aspects of the common vision of right and left eye . In contrast, seeing with only one eye or using several eyes independently of one another is called monocular vision .
In humans , as in primates in general, the eyes are directed forward. As a result, the field of vision is smaller, but the associated larger overlap of the fields of vision of the two eyes benefits spatial vision . However, this requires additional sensory and motor skills, which are part of the binocular vision and which can sometimes be disturbed.
Sensors
The sensory aspects of binocular vision can be divided into individual levels.
The first stage is simultaneous viewing , the simultaneous perception of the visual impressions of both eyes. The next level is called fusion and is the ability to fuse the images of both eyes into a single image. This is the basis for the third and highest form of binocular vision, spatial vision ( stereopsis ). These forms are also summarized as binocular functions .
Basics
The requirement for binocular vision is the correspondence between certain retinal areas of both eyes. As a rule, the image of a fixed object takes place on physiologically identically localized retinal points, which perceive it at the same point in space and thus superimpose it. One speaks here of a normal vision community or normal retinal correspondence (NRK). Since this condition is normally linked to the foveae of both eyes, one also speaks of a bifoveal vision community. Depending on the prevailing correspondence relationships, the quality of binocular vision will also appear.
The entirety of the points that fall on corresponding retinal points when an object is fixed is called a horopter . This horopter is a line that, like a parabola , is slightly curved. Objects that are close in front of or behind this horopter are usually not seen twice, although they fall on non-corresponding retinal locations. This area is called the Panumareal or Panumraum. In this zone, the slightly offset images (lateral disparity) create spatial vision. So-called physiological diplopia occurs when the limits of the panum area are exceeded .
to form
There are basically three types of binocular functions.
Simultaneous viewing
- Simultaneous viewing is given when the visual impressions of both eyes are perceived at the same time. This is also the case when there is diplopia or confusion. A quality of simultaneous viewing can at best be expressed by how stable or how often the visual impressions are perceived at the same time. In the case of congenital strabismus, for example, the visual impression of the cross-eyed eye is usually suppressed. One speaks here of the suppression or exclusion of visual impressions. This is to avoid the perception of annoying double images. In this situation there is therefore no simultaneous viewing.
fusion
- The fusion merges the two separately perceived images of the right and left eyes into a single one. A distinction is made between sensory fusion as a central image merging and motor fusion, which takes place via the outer eye muscles. The ability to fuse can be exposed to various stresses, for example poor visual acuity , latent squint , unfavorable lighting conditions or a poor general condition. The limits of this resilience, which are different in the different directions of view, can be measured with certain methods and thus conclusions can be drawn that can play a role in the assessment of strabismus diseases or sensory disorders. On the one hand, one speaks here of the so-called fusion width , which represents the maximum load of the fusional vergence (opposing eye movement) in one viewing direction and is determined in horizontal, vertical and rotational alignment. This can be used in test situations e.g. B. induced by prism glasses. On the other hand, there is the so-called fusion field of vision, which documents the basic ability to binocular single vision in different and differently pronounced directions of view. If a person is no longer able to merge the images of both eyes, the perception of double images will either set in with intact simultaneous viewing or if there is no simultaneous viewing, the visual impression of the deviating eye will be "switched off".
Stereopsis
- The quality of spatial vision is expressed in arc seconds. This means the transverse disparity of the displayed test objects or images. The smaller the lateral disparity, the higher the quality of spatial vision. The value of a person with normal vision is around 20 arc seconds. Differences in the examination result can arise from different measurement methods. An intact fusion is a prerequisite for the perception of a spatial visual impression. If the images of both eyes are not fused, spatial vision is also not possible. In the English-speaking world in particular, the complete lack of spatial vision is referred to as "stereoblindness", a term that is not used in German-speaking specialist terminology in its translation " stereoblindness ".
Investigation methods
| Degree of dissociation | method |
|---|---|
| no | Attempt to hit , horopter apparatus |
| very low | Prisms and diplopia , light tail test (Bagolini), long stereo test |
| low | Polarizing filter , Phasendifferenzhaploskop , Deviometer |
| medium | negative afterimages in the room, TNO test, Schober test , Worth test |
| medium strength | Synoptometer , synoptophore |
| strong | Ophthalmoscopic correspondence check, dark red glass, Maddox cylinder, Haidinger tufts |
There are numerous and diverse examination methods and procedures for testing the various forms of binocular vision that are generally used in orthoptics and, in a simplified form, in optometry . Here, subjective information from the patient (e.g. image localization) must be taken into account as well as objective factors (e.g. squint angle). In general, one can differentiate between apparatus-based procedures (e.g. haploscopes ) and those that are used for testing in open space. It is not uncommon for processes to exist that investigate the same function, but produce results that differ significantly from one another due to different arrangements, degree of dissociation and underlying principles. In addition, some examinations take place in very unnatural test environments, which in turn must be taken into account when assessing the results. In the table, some processes are listed according to individual degrees of dissociation.
pathology
| Classification according to ICD-10 | |
|---|---|
| H53.2 | Diplopia |
| H53.3 | Other disorders of binocular vision |
| ICD-10 online (WHO version 2019) | |
Disturbances of the binocular vision can have very different reasons, characteristics and thus effects. Essential characteristics are diplopia and confusion , as well as the suppression of the visual impression of one eye in order to counteract threatening double vision . These always go hand in hand with the loss of spatial vision.
- Diplopia is triggered by irritating areas of the retina in a cross-eyed eye that have a different spatial localization to that of the non-cross-eyed eye. So you perceive identical objects in different places in space with both eyes. Diplopia means the loss of the ability to fuse and spatial vision while maintaining simultaneous viewing.
- Confusion, on the other hand, arises when different objects fall on retinal points with the same spatial localization. Both eyes see different things in the same place in the room and thus trigger a kind of competition for localization. Confusion does not occur because of the inhibition mechanisms ( scotomas ) that usually appear in daily life, but can be evoked to measure objective squint angles in examination situations (e.g. with a dark red glass filter ) .
- Suppression is the name of the mechanism that completely or partially suppresses the visual impression of the cross-eyed eye when the eyes are open on both sides. This mechanism can only affect certain areas of the retina ( central scotoma, fixation point scotoma ), or it leads to a complete elimination of the perception of the cross-eyed eye, which in turn can cause amblyopia . In contrast to otherwise caused visual field deficits, this inhibition mechanism can only be demonstrated when both eyes are open. The suppression of one eye means the loss of all binocular functions.
Another disorder arises (in contrast to the above-mentioned NRK) from an anomalous vision community or anomalous retinal correspondence (ARK). This is the phenomenon that the fixation of an object leads to the stimulus of physiologically non-identical retinal points, which can nevertheless lead to an abnormal superimposition of the images. As a rule, the fovea of one eye corresponds to a non-foveal part of the retina in the other eye. If the images overlap in this situation, a so-called harmonic anomalous retinal correspondence (HARK) is present. In contrast, one speaks of a disharmonious anomalous retinal correspondence (DHARK) if, under these circumstances, the fixed images are not superimposed. As a result of a HARK , so-called subnormal, i.e. qualitatively differently inferior, binocular vision can arise. This can express itself z. B. in a narrow range of fusion or in only roughly pronounced spatial vision.
Disturbances of the fusion and stereopsis can also be caused by pronounced differences in the size of the retinal images ( aniseiconia ), which no longer allow a congruent image overlay and merging.
The disparity of fixation is a sensorimotor disorder of binocular vision . It is expressed by a lack of accuracy of the vergence system and by the inability to bifoveal fixation under fusional loading.
Another, but much rarer disorder of binocular vision is known as horror fusionis . In this condition, the ability to suppress is lost because of a small squint angle of closely spaced double images. This can be caused, for example, by orthoptic or pleoptic exercise treatments, which in such cases have not achieved the establishment of normal binocularity, but a reduction in the double image inhibition. The result is a permanent double image perception at a short distance that cannot be eliminated.
etiology
The best-known causes of disorders of the binocular vision are all forms of strabismus and nystagmus . Optical, organic and neurological problems (for example, paralysis of the eyes ) can also affect both eyes to different degrees. Treatment belongs in the hands of orthoptists , strabologists and, if necessary, neuroophthalmologists or neurologists . The manufacture and adaptation of special glasses (e.g. prisms) is carried out by opticians .
Motor skills
The motor aspects of binocular vision deal with the physiology and pathophysiology of the musculoskeletal system of the eyes as well as their position in relation to one another. They are described, among other things, in the articles eye movement and squinting .
See also
literature
- Herbert Kaufmann (Ed.) : Strabismus . 4th fundamentally revised and expanded edition, with Heimo Steffen, Georg Thieme Verlag, Stuttgart, New York 2012, ISBN 3-13-129724-7 .
