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Diplopia is the simultaneous perception of two images of a single object that may be displaced in relation to each other. [1] Also called double vision, it is a loss of visual focus under regular conditions, and is often voluntary.
Thus, each eye has a slightly different view of the world around. This can be easily seen when alternately closing one eye while looking at a vertical edge. The binocular disparity can be observed from apparent horizontal shift of the vertical edge between both views. At any given moment, the line of sight of the two eyes meet at a point in space.
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 is presumably called an area because it was measured by Panum only in the horizontal plane).
Larger cyclodisparity normally results in double vision. [3] It has been shown that the tolerance of human stereopsis to cyclodisparity of lines (orientation disparity) is greater for vertical lines than for horizontal lines. [4] The visually evoked cyclovergence relaxes once the cyclodisparity is reduced to zero.
Method for measuring vertical deviations: 1. The Maddox Rod is held in front of the patient's right eye with the cylinders vertical, making the red line horizontal. 2. The patient is then asked whether the white light is superimposed on the red line or if it appears above or below the red line.
The misalignment may be vertical, horizontal or both. The misalignment (a few minutes of arc) is much smaller than that of strabismus. While strabismus prevents binocular vision, fixation disparity keeps binocular vision, however it may reduce a patient's level of stereopsis. A patient may have a different fixation disparity at distance than near.
Diplopia, or double vision, occurs commonly after strabismus surgery. Although the surgery can be used to treat some types of double vision, it can instead end up making existing symptoms worse or create a new type of double vision. [12] The type of double vision can be horizontal, vertical, torsional, or a combination.
Each fixation involves binocularly coordinated movements of the eyes to acquire the new target in three dimensions: horizontal and vertical, but also in-depth. In literature it has been shown how an upward or a vertical saccade is generally accompanied by a divergence of the eyes, while a downward saccade is accompanied by a convergence. [22]