Visual Illusions

Visual illusions have provided much of the information on which theories of the functioning of organic visual systems are based

What are visual illusions? In a sense everything we perceive is an illusion since we can never exactly recreate objective reality through our senses

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. Generally the causative factor is some circumstance associated with the scene or the context in which it is viewed

We are not surprised when our muscles fail to do exactly what we want and we know that we can misinterpret the direction of a sound, but we almost never question our visual decisions in fact we routinely trust our lives to them. How is it possible then, that we are so readily misled by visual illusions, even when we know what the true situation is?

It is very unlikely that a single mechanism underlies all illusions, in particular the visual system is able to compensate for the various distortions introduced by the limitations of the imaging system of the eye

Under normal circumstances, while any compensating function might fail, there appears to be enough redundancy to detect and correct the error so that the final perception is faithful to the physical situation

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.

In the Necker cube, two configurations given face in front or given face in back are reasonable interpretations of the imaged data and there are no cues to cause one interpretation to dominate. The perceptual system appears to formulate and offer us in turn these alternative hypotheses. Figure Perceptual Ambiguity: multistable perception (from Fischler and Firschein, 1987)


In the "Impossible" triangle we have two separated phenomena:

  1. when certain visual cues are present, we assume we are viewing a coherent object in 3D space;
  2. once we have made this assumption, our visual system assumes that the object we are viewing is in the general position, i.e., a slight change in our viewing position should leave our basic perception of the object unchanged

This second assumption seems to be hardwired into our processing, even when we know it is invalid we cannot avoid invoking it

Figure The "Impossible" Triangle (from Fischler and Firschein, 1987)

In the case of the subjective contour we use circumstantial evidence (e.g. gaps in the black circles) to deduce the presence of an occluding object; once this decision has been made, the interpretation of intensities, distances, etc., are changed to make the complete interpretation a consistent one. Figure Subjective Contours: the white square that appears to be occluding the black circles is an illusion it is actually the same intensity as the background (from Fischler and Firschein, 1987)

All these examples show the existence of a system capable of deductive reasoning and consistency maintenance operating below the level of our conscious awareness.


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Last modified on April 05, 1999, G. Scott Owen,