In this essay, the four principal kinds of visual illusions: ambiguities, distortions, paradoxes and fictions are precisely discussed (Gregory, 1997). .
Often, the visual cues one receives are ambiguous yet his/her visual system always is able to make a choice about how to interpret what is being seen. Human's conscious mind must choose one and only one interpretation; otherwise, the mental state would be complete chaos (Wade, 1982). When ambiguities exist, the brain must solve the ambiguities in shape, orientation, light source, reflectance, shadow, size and distance. Thus, interpretations that are most consistent with the model of the world are mainly chose. The model is based on what our experiences tell us is most probable (Wade, 1982). The perception of ambiguities is never certain, and probably always has some errors (Coren, & Girgus, 1978). .
The most well known illusion of ambiguity is the Necker cube (Gregory, 1987, p339-400). The effect works because the drawing of the cube (an orthographic projection) carefully eliminates all depth cues. In attempting to fit the expected model of a cube to the picture, the human brain must resolve the ambiguity as to which corner of the cube is closer (Robinson, 1972). Different people resolve this ambiguity in different ways, and individuals resolve it differently at different times (Robinson, 1972). This Necker cube illusion is one of several in this collection in which there is an ambiguity to be resolved by the visual system (Coren, & Girgus, 1978). In this case, there are no depth cues on which to base a choice of orientation for the cube. Our visual system must at some level choose which of the two interior vertices appears closer (Coren, & Girgus, 1978). It is interesting to consider that the image of Necker cube shown is not really three dimensional but is nevertheless interpreted as a two dimensional projection of a three dimensional object.
