The Chubb illusion is an optical illusion or error in visual perception in which the apparent contrast of an object varies substantially to most viewers depending on its relative contrast to the field on which it is displayed. These visual illusions are of particular interest to researchers because they may provide valuable insights in regard to the workings of human visual systems.
An object of low-contrast visual texture surrounded by a field of uniform visual texture appears to have higher contrast than when presented on a field of high-contrast texture. This illusion was observed by Charles Chubb and colleagues and published in 1989. An empirical explanation of the Chubb illusion was published by Lotto and Purves in 2001.
Discovery
Chubb and his colleagues researched this illusion by showing various combinations of foreground objects and background fields to human test subjects and asking them to rate the sharpness of the visual contrast in each foreground object. They found that subjects viewing a patch of random visual texture embedded in a surrounding background field were likely to report different perceptions of visual contrast for the central target patch depending on the relative contrast of the background field. Furthermore, the apparent brightness/ dullness of a texture patch varied as the background varied. E.g. bright points of texture patch appeared duller in a high-contrast background, whereas dark points appeared lighter in an even background. This apparent variance in perception is influenced when it is viewed by different observers and when the background and central target patch are placed on "non-overlapping spatial frequency bands", as explained by Chubb et al. This tendency was found statistically significant. Theories point to a perceptual tool to catalyse contrast adaptation (slow, seconds) to contrast gain control (fast,≈100ms) at an early cortical or pre-cortical 'neural locus'.
Chubb contrast effect
The Chubb illusion is similar to another visual illusion, the contrast effect. The contrast effect is an illusion in which the perceived brightness or luminance of an identical central visual target form on a larger uniform background varies to the test subject depending on the ratio of the central form's luminance to that of its background. This illusion, simultaneous contrast, is illustrated in Figure 2. In it, the central target is brighter. I.e., the ratio of the top central rectangle's luminance (A) to its background field's luminance is greater than one. In the bottom rectangle (B), the background field is brighter. That is, the ratio is less than one. Although the two central target patches are equally bright (identical in luminance) the one on a dark background appears lighter and the one on a lighter background appears darker.
Lateral inhibition
Lateral inhibition is one proposal to explain the contrast illusion or simultaneous contrast. Its advocates theorize that neurons strongly stimulated by the background of B suppress the less strongly stimulated neurons of the interior rectangle. In A, they theorize, there is no such inhibition. However, the fact that both A and B appear of "uniform lightness across their expanse" suggests that the process of lateral inhibition is more complex. Chubb et al. assert that the principle of lateral inhibition rests on the assumption that the determining factor of perceived lightness is the ratio, at the rectangle edge, of rectangle luminance to background luminance. However, the lateral inhibition account is not consistent with phenomena such as the Benary cross and White's illusion as well as transparency and assimilation effects. As such, it is an ad hoc explanation of unclear theoretical interest.
The Chubb illusion illustrates an error in contrast rather than luminance. The zero-luminance background of Figure 2 (A) becomes a zero-contrast field in the analogous portion of Figure 1, while the high-luminance field of Figure 2 (B) becomes a high-contrast texture field. Observers empirically perceive the texture disk of the leftmost portion of Figure 1 as having higher contrast than the disk on the right, even though the two are the same. After conducting experiments on contrast and lightness induction, interocular induction and induction between spatial frequency bands, they show that lateral inhibitory effect is monocular and adapted only for spatial frequency. Chubb et al. support "a model in which the output gain of such a band-selective neuron is normalized relative to the average response amplitude of nearby neurons with the same frequency tuning."