The phenomenon where, if different images are presented to each eye, the perception is not fusion of the two, but alternation between monocular cues resulting in a single coherent perception. The latter is a challenge to the classical view of binocular rivalry (viz., that the two monocular cues are somehow united). The underlying mechanisms, however, remain controversial (e.g., does it involve competition between monocular or binocular cells? What determines if monocular cues undergo rivalry or fusion?). An emerging view is that it is not the physical similarity between images, but the similarity in their perceptual or empirical meaning (at least in adults).
See Binocular disparity, Binocular vision, Depth perception