The perception of an illusory figure

Visual illusions are an example where the visual system makes mistakes. However, as the ability to perceive illusions is spread among various species and that ability emerges very early in human development as well, the perception of illusory figures supposedly tells something fundamental about how the visual system works. The Kanizsa triangle (see the figure beside) is a fragmented, inducer-based presentation of a real complete triangle. Thus, studying the formation of the perception of an illusory figure gives information on how the visual system completes objects from fragmentary information, in accordance with the real-life situation when objects are seen only partially. The presentation will give some examples on the stimulus conditions that elicit the perception of an illusory triangle. The results are obtained with psychophysical methods on human subjects and they confirm the assumed importance of the perception of illusory figures in everyday viewing. This is indicated by the variety of inducer modifications that can be made without destroying the percept: temporal and spatial separation, adding volume and colour as well as using motion.