The amount of disparity depends on the depth (i.e., the difference in distance to the two object and the distance to the point of fixation), and hence it is a cue that the visual system uses to infer depth. The muscles relax when the object is distant and contract when the object is near. Binocular disparity is defined as the difference in the location of a feature between the right eyes and left eyes image. The degree of contraction of the muscles provides a cue to distance. These muscles change the thickness of the eye lens. (iii) Accommodation: It is the process through which the image is focussed on the retina with the help of a ciliary muscle. The degree of convergence decreases with an increase in distance of the object. (ii) Convergence: When the eyes converge inward to bring the image on the fovea of each eye, a group of muscles sends messages to the brain which are interpreted as cues to the perception of depth. Large retinal disparity means a close object and small retinal disparity means a distant object. The difference in the images formed by the eyes is due to the retinal disparity. Illusions provide good examples in understanding how perception is organized. (i) Retinal or Binocular disparity: It occurs when the two eyes have different locations in the head and are separated horizontally with a difference of 6.5 centimetres. Monocular cues consist of static information including relative size, perspective, interposition. Their role in the perception of depth is as follows: Binocular cues include binocular disparity and vergence. The binocular cues of depth perception are provided by both the eyes in three-dimensional spaces. Important monocular cues are relative size and height, interposition, linear and aerial perspective, light and shade, texture gradient, and motion parallax. They are also known as pictorial cues as they are used by artists to induce depth in two-dimensional paintings.
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The monocular cues of depth perception induce depth in objects when viewed through a single eye.