Various visual events involve potential perceptual ambiguity, and under certain circumstances they may provide a distorted, illusionary view of the physical environment (Schiffman, 2000). These misperception of events are referred as visual illusions (Schiffman, 2000). In a sense of visual perception, everything perceived is an illusion since one can never exactly recreate objective reality through his/her senses (Rock, 1984). However, the term "illusion" is reserved for those situations in which our perceptions differ markedly from what we know corresponds to the actual physical situation (Sckuler & Blake, 1994). Visual illusions are produced when we are presented with an impoverished visual environment that eliminates the normal redundancy and overloads or deliberately misinforms a single functional system (Hubel, 1987). Generally, the causative factor is some circumstance associated with the scene or the context in which it is viewed (Sckuler & Blake, 1994). To define visual illusions, they are considered as distortions, misperceptions or false impressions of real objects (Ramon, 2002). Visual illusions can consist of seeing a person's form or shape in a chair or a coat-rack, alternatively, seeing water shimmering on in the desert. In general, visual illusions are mirages (Ramon, 2002). It is generally believed that visual illusions are simply curiosities (Robinson, 1972). Nevertheless, by studying illusions, it is possible to come to an understanding of how normal vision works - and how it can be fooled by unexpected cues, such as those provided in many ads containing semi-subliminal material. Furthermore, understanding visual illusion shows how knowledge of such rules can be put to use in constructing semi-subliminal and manipulative adverts. Vision is our richest sense, and is the most studied (Bruce, Green & Georgeson, 1996). Consequently, the study of illusions may provide trace to the more general mechanisms and principles of space perception (Schiffman, 2000).
