Binocular vision
[10] Binocular vision helps with performance skills such as catching, grasping, and locomotion.
[11] It also allows humans to walk over and around obstacles at greater speed and with more assurance.
[12] Optometrists and orthoptists are eyecare professionals who fix binocular vision problems.
][14] 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.
The relation between the position of the two eyes, version and vergence is described by Hering's law of visual direction.
The relation between version and vergence eye movements in humans (and most animals) is described by Hering's law of equal innervation.
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.
Probability summation assumes complete independence between the eyes and predicts a ratio ranging between 9-25%.
[15] Other factors that can affect binocular summation include, spatial frequency, stimulated retinal points, and temporal separation.
Once the fields of view overlap, there is a potential for confusion between the left and right eye's image of the same object.
The horizontal and vertical horopters mark the centre of the volume of singleness of vision.
Within this thin, curved volume, objects nearer and farther than the horopters are seen as single.
To point successfully, one of the double images has to take precedence and one be ignored or suppressed (termed "eye dominance").
When very different images are shown to the same retinal regions of the two eyes, perception settles on one for a few moments, then the other, then the first, and so on, for as long as one cares to look.
These factors include context, increasing of contrast, motion, spatial frequency, and inverted images.
[18] Recent studies have even shown that facial expressions can cause longer attention to a particular image.
Move the finger around; this is to break the reflex that normally holds a covered eye in the correct vergence position.
[15] A definition that incorporates all of these defines amblyopia as a unilateral condition in which vision in 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.
They are usually associated with symptoms such as headaches, asthenopia, eye pain, blurred vision, and occasional diplopia.
[21] 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.
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.
