Gaze for several seconds at the cross between the above image pair. While fixating, compare with your “inner eye”: the left image is blurred, and the right has high contrast. Ok so far.
State 1/2 or press the space bar, while still fixating the cross and compare the images with your inner eye.
After the change, on the left side you will (initially) perceive super high contrast. This persists only for a few seconds, then it becomes apparent that the two halves are, as indeed the case, identical. The longer you had fixated the initial image pair, the longer the “super high” contrast on the left will persist (within limits, longer than ≈10 s saturates).
There are 2 states for blur and contrast of both images. You can adjust blur (in pixels, higher values more) and contrast in %. For me there is a very wide range of parameters where the effect seems about equally strong.
Contrast adaptation (slow, seconds) and contrast gain control (fast, ≈100 ms) are at work all over the place in our visual system, 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 inspired by Webster et al. 2002); it is also, for instance, specific for the depth plane (Aslin et al 2004).
Incidentally, this effects explains why patients are satisfied with multifocal or “extended focus” intraocular lenses. There, retinal contrast never exceeds 50%, yet patients do not find the images flat. Reason: because of contrast adaptation, as demonstrated here, the grey-grey becomes the “new” black-white, so all seems well. Absolute contrast threshold does not improve, however, posing a serious problem for night driving (at mesopic luminance levels, our contrast threshold is 30× worse, see Hertenstein et al. 2016)
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. Source of image: Wikipedia
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
Hertenstein H, Bach M, Gross NJ, Beisse F (2016) Marked dissociation of photopic and mesopic contrast sensitivity even in normal observers. Graefes Arch Clin Exp Ophthalmol 254:373–384 PDF