What to see
Above you see two checkerboards; the left has a high contrast, the right a low contrast. In the center of each there is a smaller 4×4 checkerboard patch. Compare their contrast, is it equal? Usually, the patch surrounded by low contrast appears to have a higher contrast. Press “cover” to judge the patches alone: they are identical!
What to do
You can use the middle slider to adjust the contrast of the two small checkerboards so they seem similar, and then use the button “cover” to check on the size of the effect. The contrast of the two large checkerboards can also be adjusted.
A case of contrast adaptation across space: Because the left checkerboard has a high contrast, the local contrast gain is reduced to bring the contrast transfer function into optimal range (this is usually a good thing!). On the right side the inverse happens. The shift of the contrast transfer function extends a little over space, here across the center patches. When the identical center patches pass these two different contrast transfer functions, their neural correlate indeed becomes different. Also known as “contrast contrast”.
Chubb C, Sperling G, Solomon JA (1989) Texture interactions determine perceived contrast. Proc Natl Acad Sci USA 86:9631–9635 [PDF]
Solomon JA, Sperling G, Chubb C (1993) The lateral inhibition of perceived contrast is indifferent to on-center/off-center segregation, but specific to orientation. Vision Res 33:2671–2683
Dakin (2005) Current Biol