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

List of Illusions by Category

Contrast Illusions

Among the simplest examples of the effect of the surrounding context on the perception of an item are the contrast illusions in which the appearance of a figure can be altered by the background against which it is viewed.

Center-surround: the influence of background
Attempt to match the colours of two squares displayed against backgrounds of differing intensity. You may already know that the same colour will look different when viewed against different backgrounds; with this demonstration you can investigate how easy or difficult it is to create the illusion convincingly.
Yellow-gray X's: the interaction of colour
The center-surround demonstration above relies solely on variation in the intensity or luminance of the background. By introducing variations in the hue of the background, this illusion demonstrates how apparent hue can be affected by context.

Other Colour Effects

There does not need to be a clear distinction between foreground and background in order for colours to interact. The effects listed here (presently a list of one) illustrate how the visual system associates meaning with colours or attempts to integrate colours into a coherent interpretation of a scene.

Equiluminance and apparent motion
This demonstration explores the natural tendency of the visual system to make associations between colours on the basis of their intensity. In one configuration, the most natural association results in an illusion of motion. As the intensities are varied, the direction and speed of the apparent motion are affected.

Geometric Phenomena

There are a wide variety of well known figures in which specific arrangements of certain elements lead to distorted judgements of distance and orientation. A related category contains figures that can be interpreted in a number of different ways, but generally only one at a time. Many of the simple arrangements in this representative sample of these categories produce very striking effects.

Border locking and the Café Wall illusion
In this illusion, inspired by a real café in Bristol, England, modest changes in the intensity of a small number of pixels have profound effects on the apparent geometry of the scene.
Depth cues and the Necker Cube
Although our visual system can easily distinguish the three dimensions of our world, our eyes can only deliver two dimensional information to the system's higher levels (namely, the two dimensional projection of a scene onto the retina). Reconstruction of a three dimensional world from two dimensional information is a remarkable strength of our visual system. A large class of unusual figures can be constructed by carefully eliminating the depth cues on which our visual system relies, or by bending the rules of perspective projection. The Necker Cube is perhaps the simplest figure in this category. In spite of its simplicity, or perhaps because of it, it creates an especially striking illusion.

Auditory Illusions

The human auditory system is highly developed and, in many ways, is as sophisticated as our visual perception. As raw auditory sensation proceeds from our ears through the various stages of the system, certain transformations and interpretations are automatically applied in a way that normally gives us considerable ability to accurately analyze the current "auditory scene". It is therefore possible to construct sounds that do not follow the usual assumptions precisely, causing us to make inaccurate or inappropriate judgements about our perceptions. The illusions included here begin to explore some of the ways in which this can be done.

Shepard's Tones
If you have never heard Shepard's Tones, their effect can be rather startling. It is a very robust effect that convincingly demonstrates how judgements of relative pitch (i.e., is one tone higher in pitch than another) can go completely astray.
The Tritone Paradox
The Tritone Paradox explores a special case of Shepard's Tones involving two tones from the usual set of twelve. It provides a nice completion for one issue that the Shepard's Tones demonstration leaves outstanding.

Scott Flinn (