Created on 13 October 1997.
Last modified on 5 August 2000.

Illusions Involving Ambiguity Resolution

The principal function of human perception is to ascertain the nature of the surrounding world. The brain deals with the complexity of that world by selecting from a limited set of possible interpretations the one that best fits the available sensory evidence. It is able to compare the evidence collected by the visual and auditory systems with a number of candidates at a fairly low level, and to automatically select the best fit.

It is interesting to observe how the brain selects between two or more possible interpretations when there is insufficient information available to correctly distinguish between them. In some cases the choice is arbitrary, and in others is determined in a more systematic fashion. The illusions in this tour all share the characteristic of having some element of ambiguity that the brain must resolve in some fashion.

Equiluminance and apparent motion
This illusion examines automatic colour associations. It uses the similarity between two pairs of colours to create an illusion of motion. The interactive demonstration allows you to adjust the colours in such a way that the pair-wise colour association can be reversed. In that case, the motion appears to reverse as well. When the colours are adjusted such that the it is unclear which colour pairs with which, the illusion of motion breaks down.
Necker Cube and depth cues
This illusion uses an orthographic projection to display a wire-frame cube in such a way that there is no way to determine which corner of the cube is closer. As you focus on different corners, the orientation of the cube appears to change as your brain jumps from one conclusion to another. The cube can be animated, spinning around a vertical axis, which tends to strengthen the illusion considerably.
Shepard's Tones
This is a very striking auditory illusion in which a series of tones appears to continually ascend in pitch. After a while, however, it becomes apparent that the tones have remained within the same fairly small range of pitches. The tones are carefully crafted in a way that it is difficult to distinguish the relative pitch of any pair. The illusion of ascending tones comes about because the tones are played in an order that permits two interpretations: a rise in pitch of one semitone, or a fall in pitch of eleven semitones. The auditory system naturally picks the shorter distance.
Tritone Paradox
This illusion is a variation of Shepard's Tones in which two tones, evenly spaced within an octave, are played in alternation. Whereas the choice in the Shepard's Tones illusion is between an increase of one semitone or a decrease of eleven, the choice here is between an increase or decrease of six semitones. This presents an ambiguity that cannot be resolved correctly based on the auditory evidence alone (except by those with very sensitive hearing). It is interesting to see how different individuals resolve the ambiguity in a somewhat systematic way, in contrast to the relatively arbitrary way in which the ambiguity of orientation in the Necker Cube illusion is resolved.

Scott Flinn (