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Even without moving their eyes, some birds have a degree field of view. Some other animals - usually, but not always, predatory animals - have their two eyes positioned on the front of their heads, thereby allowing for binocular vision and reducing their field of view in favor of stereopsis. However, eyes on the front is a highly evolved trait in vertebrates, and there are only three extant groups of vertebrates with truly forward-facing eyes: primates , carnivorous mammals , and birds of prey. Some predator animals, particularly large ones such as sperm whales and killer whales , have their two eyes positioned on opposite sides of their heads, although it is possible they have some binocular visual field.

The direction of a point relative to the head the angle between the straight ahead position and the apparent position of the point, from the egocenter is called visual direction, or version. The angle between the line of sight of the two eyes when fixating a point is called the absolute disparity, binocular parallax, or vergence demand usually just vergence. The relation between the position of the two eyes, version and vergence is described by Hering's law of visual direction.

Eye movements are either conjunctive in the same direction , version eye movements, usually described by their type: saccades or smooth pursuit also nystagmus and vestibulo-ocular reflex. Or they are disjunctive in opposite direction , vergence eye movements. The relation between version and vergence eye movements in humans and most animals is described by Hering's law of equal innervation.

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Some animals use both of the above strategies. A starling , for example, has laterally placed eyes to cover a wide field of view, but can also move them together to point to the front so their fields overlap giving stereopsis. 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.

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Binocular summation is the process by which the detection threshold for a stimulus is lower with two eyes than with one. Binocular inhibition occurs when binocular performance is less than monocular performance. This suggests that a weak eye affects a good eye and causes overall combined vision. Unequal monocular sensitivities decrease binocular summation.

How Does Binocular Vision Work?

There are unequal sensitivities of vision disorders such as unilateral cataract and amblyopia. Once the fields of view overlap, there is a potential for confusion between the left and right eye's image of the same object. This can be dealt with in two ways: one image can be suppressed , so that only the other is seen, or the two images can be fused.

If two images of a single object are seen, this is known as double vision or diplopia. Fusion of images commonly referred to as 'binocular fusion' occurs only in a small volume of visual space around where the eyes are fixating. Running through the fixation point in the horizontal plane is a curved line for which objects there fall on corresponding retinal points in the two eyes. This line is called the empirical horizontal horopter.

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There is also an empirical vertical horopter , which is effectively tilted away from the eyes above the fixation point and towards the eyes below the fixation point. The horizontal and vertical horopters mark the centre of the volume of singleness of vision.

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  4. Within this thin, curved volume, objects nearer and farther than the horopters are seen as single. The volume is known as Panum's fusional area it's presumably called an area because it was measured by Panum only in the horizontal plane. Outside of Panum's fusional area volume , double vision occurs. When each eye has its own image of objects, it becomes impossible to align images outside of Panum's fusional area with an image inside the area.

    When one looks at one's fingertip, it is single but there are two images of the distant object.

    Binocular Vision

    When one looks at the distant object it is single but there are two images of one's fingertip. To point successfully, one of the double images has to take precedence and one be ignored or suppressed termed "eye dominance". The eye that can both move faster to the object and stay fixated on it is more likely to be termed as the dominant eye. The overlapping of vision occurs due to the position of the eyes on the head eyes are located on the front of the head, not on the sides. This overlap allows each eye to view objects with a slightly different viewpoint. As a result of this overlap of vision, binocular vision provides depth.

    These differences, referred to as binocular disparity, provide information that the brain can use to calculate depth in the visual scene, providing a major means of depth perception. The closer objects are to each other, the retinal disparity will be small.

    Depth perception

    If the objects are farther away from each other, then the retinal disparity will be larger. When objects are at equal distances, the two eyes view the objects as the same and there is zero disparity. Because the eyes are in different positions on the head, any object away from fixation and off the plane of the horopter has a different visual direction in each eye. Yet when the two monocular images of the object are fused, creating a Cyclopean image , the object has a new visual direction, essentially the average of the two monocular visual directions.

    This is called allelotropia. The position of the cyclopean eye is not usually exactly centered between the eyes, but tends to be closer to the dominant eye. 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.

    This alternation of perception between the images of the two eyes is called binocular rivalry. That is why the binocular rivalry occurs. Several factors can influence the duration of gaze on one of the two images. These factors include context, increasing of contrast, motion, spatial frequency, and inverted images. To maintain stereopsis and singleness of vision, the eyes need to be pointed accurately. The position of each eye in its orbit is controlled by six extraocular muscles. Slight differences in the length or insertion position or strength of the same muscles in the two eyes can lead to a tendency for one eye to drift to a different position in its orbit from the other, especially when one is tired.

    This is known as phoria. One way to reveal it is with the cover-uncover test. To do this test, look at a cooperative person's eyes. Cover one eye of that person with a card.

    Have the person look at your finger tip. Move the finger around; this is to break the reflex that normally holds a covered eye in the correct vergence position. The reading of both the arms is noted at this moment and the sum total of the reading of both the arms gives the objective angle of anomaly.

    The subjective angle of anomaly is the angle at which the visual targets are superimposed. Worth Four Dot Test: This is a simple test utilizing red-green color dissociation. It is more dissociating than the bagolini glasses and so less physiological.


    The apparatus for this test consists of a box containing four panes of glass, arranged in diamond formation, which are illuminated internally. The two internal panes are green, the upper one is red and lower one is white. The patient wears red and green goggles as a convention red in front of right and green in front of left.

    The test can be performed separately for distance and near vision. Hering Bielschowsky After-Image Test: This is a highly dissociating orthoptic test in which battery- powered camera flash is used to produce a vertical after image in one eye and a horizontal after image in the other eye.

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    The center of flash is covered with a black mark serves as a point of fixation and protects the fovea. Once an afterimage is created in each eye, the position of the images in relation to each other no longer depends on whether the eyes are open, closed, straight or crossed.

    Depth Perception - American Academy of Ophthalmology

    The interpretation of this test depends on the fixation behaviour. Each eye fixates on the center black mark of a glowing filament, first presented horizontally to the eye with a better visual acuity and then vertically to the poorer eye for 20 sec in a darkened room while the fellow eye is occluded. The patient indicates the relative position of the two gaps in the center of each afterimage. The gaps correspond to the visual direction of each fovea if central fixation is present.