<P> 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 . 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 . 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 . </P> <P> 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 . This happens when one has to point to a distant object with one's finger . When one looks at one's fingertip, it is single but there are two images of the distant object . 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 . </P> <P> 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 . Stereopsis (from stereo - meaning "solid" or "three - dimensional", and opsis meaning "appearance" or "sight") is the impression of depth that is perceived when a scene is viewed with both eyes by someone with normal binocular vision . Binocular viewing of a scene creates two slightly different images of the scene in the two eyes due to the eyes' different positions on the head . 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 . There are two aspects of stereopsis: the nature of the stimulus information specifying stereopsis, and the nature of the brain processes responsible for registering that information . The distance between the two eyes on an adult is almost always 6.5 cm and that is the same distance in shift of an image when viewing with only one eye . Retinal disparity is the separation between objects as seen by the left eye and the right eye and helps to provide depth perception . Retinal disparity provides relative depth between two objects, but not exact or absolute depth . The closer objects are to each other, the retinal disparity will be small . 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 . </P> <P> 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 origin of the new visual direction is a point approximately between the two eyes, the so - called cyclopean eye . The position of the cyclopean eye is not usually exactly centered between the eyes, but tends to be closer to the dominant eye . </P>

Difference between binocular vision and binocular single vision