The way you look at an object can affect how you see it. Sometimes there are two images in the same picture, but you can only see one at a time so your brain chooses one (when it deals with too much information). Cognitive “illusions” rely on stored knowledge about the world (depth, rabbits, women) and are also under some degree of conscious control (we can generally reverse the perception at will).
Instead of demonstrating a physiological base they interact with different levels of perceptual processing, in-built assumptions or ‘knowledge’ are misdirected. Cognitive illusions are commonly divided into ambiguous illusions, distorting illusions, paradox illusions, or fiction illusions. They often exploit the predictive hypotheses of early visual processing. Stereograms are based on a cognitive visual illusion.