Created on 13 October 1997.
Last modified on 5 August 2000.
List of Illusions by Category
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.
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.
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
- Border locking and the Café Wall
- 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.
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
- 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.