Because our visual system assumes the light comes from above, a totally different perception is obtained if the image is viewed upside down. Binocular Cues Stereopsis is an important binocular cue to depth perception. Stereopsis cannot occur monocularly and is due to binocular retinal disparity within Panum's fusional space. Stereopsis is the perception of depth produced by binocular retinal disparity. Fusion describes the neural process that brings the retinal images in the two eyes to form one single image.
Fusion occurs to allow single binocular vision.
Fusion takes place when the objects are the same. Suppression occurs to eliminate one image to prevent confusion. Superimposition results in one image presented on top of the other image. Binocular rivalry describes alternating suppression of the two eyes resulting in alternating perception of the two images. This usually occurs when lines are presented to the two eyes differ in orientation, length or thickness.