The Craik-O'Brien-Cornsweet illusion, also known as the Craik-Cornsweet illusion and the Cornsweet illusion, was discovered by Tom Cornsweet in the late sixties.
Adjust your browser so that you are seeing all of the picture below, and describe to yourself what you see.
If the entire region to the right of the "edge" looks slightly lighter than the area to the left of the edge, then you are experiencing the illusion. Of course, as those familiar with the presentation of optical illusions may have guessed, the brightness of the areas is exactly the same, as we can see if we black out the region containing the "edge:"
The following picture shows the actual distribution of luminance in the picture, and the typical perception of luminance:
The illusion is a little subtle. It is similar to the familiar phenomenon of simultaneous contrast and Mach bands, but differs from it in two important respects.
- In most contrast effects, such as Mach bands, the effect is seen only on areas that are close to the the intensity gradient. In Craik-O'Brien-Cornsweet illusion, however, a very small area (the central "edge") affects the perception of entire large areas, portions of which are distant from the "edge."
- In the Craik-O'Brien-Cornsweet illusion, the region adjacent to the light part of the "edge" appears lighter, and the region adjacent to the dark part of the edge appears darker, just the opposite of the usual contrast effects.
External links
- Purves, Dale, R. Beau Lotto, Surajit Nundy, 'Why We See What We Do'", American Scientist,'' 90(3):236-242 (entire article)
- Dramatic image from Purves, Lotto, and Nundy showing a much more convincing presentation of the Cornsweet illusion