The Use of Illusory Visual Information in Perception and Action

Abstract

Humans constantly interact with objects in the environment (e.g. grasp a pencil for writing or pick up a cup of tea) without making many mistakes in these performed actions. To guide these actions, visual information is used. In order to accurately grasp and pick up the pencil, vision provides information such as the size of the pencil and its position relative to the body and other objects in the environment. But what happens if the interpretation of the visual information differs from physical reality? When this happens, an illusion is experienced (Gillam, 1998; Gregory, 1998). In our everyday experiences, we often perceive the world inaccurately. For example, when a car drives by, at a certain speed it looks like the wheels are turning backwards. Furthermore, distances appear shorter and hills appear steeper than they really are (Proffitt et al., 1995). And estimating a distance from a map is inaccurate when Muller-Lyer elements (figure 1.2b) are present in the map (Gillan et al., 1999). Are these visual illusions also reflected in our actions? Sometimes they are and the performed action does not have the intended goal. When flying an airplane, an illusion of motion direction can make the pilot incorrectly adjust the flying direction, which can cause an accident (Shebilske, 1981). Visual illusions are a tool to fmd out how (illusory) visual information is processed and how this information influences visually guided actions. Most theories of visual perception try to explain how the visual system acb,ieves normal veridical perception. However, veridical perception carmot easily discriminate among such theories. They can better be tested with stimuli that will result in an unrealistic percept in one theory and not in others. In this thesis, geometrical-optical illusions are used to test different theories that try to explain how the visual system processes (illusory) visual information. Gillam (1998) defmed these illusions as simple line drawings in which one or another metric property is wrongly perceived. In the next part, theories regarding the processing of (illusory) visual information are described.