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. An example of binocular rivalry occurs when one eye is presented with a horizontal line and the other eye is presented with a vertical line. Keep you eye on the tip of the pen and notice the two bars merge.