... were based solely on the information coming into the eye2. Constructivists, however, argued that there must be intermediate processes at work, which use previous knowledge and experience to influence our perception. This is known as Top down processing, which emphasises the use of cognition within perception. Illusions such as the Necker cube (a cube that can be perceived in two different ways) rely on our stored knowledge of depth and therefore uses top down processing. Other illusions such as the Hermann Grid (see later on) occur due to physiological processes and therefore can be seen as direct processing. The study of visual illusions has led us to believe that it is a combination of top down and bottom-up processing which constructs our perceptual processes. Such is the case with the Kanizsa triangle, where physiological processes allow us to perceive lines which aren't really there and our knowledge of depth perception ...

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