Gaze for a few seconds at the fixation cross in the centre of the neighbouring image pair. The left image is blurred, and the right has high contrast, ok.
Now move the mouse pointer over the image* and judge the contrast of the two new halves.
After the change, on the left part you will (initially) perceive high contrast, on the right markedly less contrast. This persists only for a few seconds, then it becomes apparent that the 2 halves are, as indeed the case, identical.
Contrast adaptation (slow, seconds) and contrast gain control (fast, ≈100 ms) are at work all over the place, and be it ‘just’ to make up for the inhomogeneity of retinal sampling. The basic idea is to have the contrast transfer characteristic adapt optimally to the mean contrast, and blur reduces the contrast specifically for high spatial frequencies. Contrast gain control is specific for retinal location & spatial frequency as shown here (the idea was taken from Webster et al. 2002); it is also, for instance, specific for the depth plane (Aslin et al 2004).
Katsushika Hokusai “The breaking wave off Kanagawa” (also called “The great wave”). Woodblock print (≈1831–1833) from Hokusai’s series of 36 views of Mount Fuji. The original is at the Hakone Museum in Japan.
Webster MA, Georgeson MA, Webster SM (2002) Neural adjustments to image blur. Nature Neuroscience 5:839–840
Aslin RN, Battaglia PW, Jacobs RA (2004) Depth-dependent contrast gain-control. Vision Research 44:685–693
Heinrich TS, Bach M (2001) Contrast adaptation in human retina and cortex. Invest Ophthalmol Vis Sci 42:2721–2727
*On a touch device, tap the image. To “untap”, tap on the icon top right.