Binocular vision is vision with both eyes, on condition that the image, falling on the macular field in cerebral cortex, fuses into a single cortical image. Due to binocular single vision we determine the distance from one object to another, volume and relative position of objects.
Binocular vision is a very important visual function. Its absence makes it impossible to do well the work of a pilot, adjuster, surgeon etc.
Newborn babies have no associated eye movements, that occur only in 2-3 weeks, binocular vision is absent. A six-eight-week-old baby shows a capability to fix an object by both eyes and keeps watch over it, and a 3-4-month-old baby has a rather sustained binocular fixation. By 5 - 6 months of age the general reflex mechanism of binocular single vision is formed – a fusion reflex, the ability to converge in cerebral cortex two images from both retinas into a single three-dimensional picture. If a 3-4-month-old baby still has dissociated eye movements, you should consult an eye specialist.
Binocular vision is considered to be formed by 3-4 years of age and is finally established in six- to seven-year-old infants. Therefore, preschool age is the most dangerous for the development of binocular vision disorder (formation of strabismus).
Binocular vision can be regarded as a closed-loop dynamic system of relationships between sensitive elements of the retina, subcortical centers and cerebral cortex (sensory), as well as 12 oculomotor muscles (motility). For binocular vision the following conditions are required: visual acuity of each eye (not below 0,3—0,4), parallel position of eye bulbs in case of looking into the distance, respective convergence in case of looking near-hand, correct associated eye movements towards the viewed object, ability to the bifoveal fusion.
Binocular vision is normally formed upon availability of:
- good optical device (transparent medium, rays of light should gather on the retina).
- good photoreceptive device
- good muscular system
In case of looking into the distance, divergence occurs (divergence of visual lines), whereas in case of looking near-hand it will be convergence (convergence of visual lines). Cerebral cortex suppresses physiological double vision if you turn your gaze to the near objects and vice versa.
Any disorder of binocular vision leads to concomitant strabismus. Presence or absence of binocular vision offers the opportunity to distinguish real strabismus from false, spurious, and from latent deviation — heterophoria.
Binocular vision can be determined through a variety of means. One of the most successful and generally accepted is investigation with the use of a color device of Е. М. Belostotsky and S. Ya. Friedman. If you want to receive a visualization of your own binocular vision, you can do the Sokolov’s test with «a hole in a palm», as well as experiments with knitting needles and reading with a pencil.
The Sokolov’s experiment is that the examined person looks with one eye through the tube (for instance, a rolled exercise-book), to the end of which on the side of the other, open eye, the palm is put. In case of binocular vision one gets the impression that the palm has «a hole» through which the picture viewed through the tube is perceived. This phenomenon can be explained in such a manner: the picture viewed through the tube is overlayed on the image of the palm in the other eye. In case of simultaneous vision, as distinct from binocular, the «hole» does not coincide with the centre of the palm, and in case of monocular vision the phenomenon of a «hole» in the palm does not occur.
The experiment with knitting needles (they can be replaced by refills of a ball-point pen and the like) is conducted in such a way: a needle should be fastened upright or should be held by the examiner. The task of the examined person, having the second needle in his/her hand, is to coincide it with the first needle along the axis. In the presence of binocular vision the task can be done easily. In the absence thereof, past-pointing occurs, in which one may be convinced, making an experiment with two and one open eyes.
The test to read with a pencil (or a pen) is as follows: a pencil should be placed several centimeters before the nose of a reading person and 10—15 cm above the text, the pencil naturally covers a part of the text letters. One can read with such an obstacle without moving his/her head only in case of having the binocular vision, as the letters, covered by the pencil for one eye, are seen with the other eye and vice versa.