Craik-O'Brien-Cornsweet illusion

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 1960s.

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 band, 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 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.

The images above show the usual presentation of the effect. A far more convincing and dramatic version of the effect can be seen in Purves, Lotto, and Nundy (2002) where it is presented within a quasi-realistic image of solid, illuminated objects. These writers give a (teleological) explanation of this and other illusions, in which the visual system and brain are posited to perform a kind of Bayesian analysis of the likelihood of various real scenes producing the observed brightness distributions. In their words, "[perception] accords not with the features of the retinal stimulus or the properties of the underlying objects, but with what the same or similar stimuli have typically signified" in the past.

According to this wholly empirical way of understanding the relationship between luminance and brightness, whenever a given stimulus is consistent with the experience of equiluminant targets signifying differently reflective objects, the brightness of the returns should appear different. If this idea is correct, then the same perceptual effect elicited by the stimulus profile in DEMONSTRATIONS #02-05 should be generated by any stimulus in which regions with the same luminance would have typically would have turned out to be differently reflective objects in different amounts of light. We therefore sought to test this prediction by examining other sorts of brightness 'illusions'.

In SEE FOR YOURSELF DEMONSTRATION #07, the two equiluminant territories adjoining an opposing pair of luminance gradients that meet along a linear boundary appear differently bright. This stimulus, called the Cornsweet edge, belongs to a larger class referred to as Craik-O'Brien-Cornsweet edges, after the several psychologists who devised such stimuli(Figure 1). Although this illusion has also been rationalized in terms of lateral interactions among neurons in the input stages of the visual system, the region that appears brighter in DEMONSTRATION #07 actually borders the gradient of higher average luminance, whereas the region that appears darker is next to the gradient of lower average luminance, a profile that is in this respect opposite the arrangement of luminances in standard simultaneous brightness contrast stimuli (see DEMONSTRATIONS #02-05).

Despite these apparent differences, the effects elicited by both types of stimuli can be understood in the same statistical terms (Purves, et al., 1999). The common denominator of the Cornsweet stimulus in DEMONSTRATION #7 and the simultaneous brightness contrast stimulus in DEMONSTRATIONS #2-5  is simply what the equiluminant 'targets' in the two luminance profiles have typically turned out to be. The empirical reason for the perception elicited the Cornsweet stimulus in DEMONSTRATION #7 is that the surfaces bordering the gradients will sometimes have been generated by similarly reflective surfaces under the same illuminant; however, the same stimulus will often have been generated by differently reflective surfaces under different intensities of illumination (Figure 2).

These possible sources influence the reflex response to the stimulus according to their relative frequencies of occurrence, with the result that the two equiluminant regions adjoining the Cornsweet edge look different, the territory adjoining the light gradient appearing brighter than the territory adjoining the dark gradient.

Consistent with this explanation, the effect can be increased-or decreased-by the relative probabilities of the possible sources of the stimulus (Figure 3 and Figure 4), as in the simultaneous brightness illustrated in DEMONSTRATIONS #02-05. In both cases, the target that appears brighter is the one more consistent with the experience of a more reflective surface in relatively less light, whereas the target that appears less bright is more consistent with a less reflective surface in relatively more light. In DEMONSTRATION #07 the variety of information in the stimulus is all consistent with the adjoining territories being differently reflective surfaces in different illuminants, making the Cornsweet effect especially powerful.