Note 2021-02-23: on Windows (any browser) the demo has an artifact, as if the grey stripe were somewhat transparent, thus it is not really homogeneously grey. On MacOS, any browser, it’s fine. Working on it.
Above you see a so-called luminance grating with 4 dark-bright cycles. In the middle there is a horizontal greyish bar. Is the lightness (grey tone) constant when gazing along it? You may observe that the grey in the stripe is slightly darker and brighter along its length, in counterphase to the grating.
Of the many things you can do, let’s start with this:
I initially thought this to be a classical case of lateral inhibition in the retina. However, there is interocular transfer – it still works if you show the stripe to one eye and the background grating to the other, thus it’s cortically based. Further quantitative analyses (see sources below) aslo suggest that this effect has its mechanisms in the primary visual cortex.
McCourt ME (1982) A spatial frequency dependent grating-induction effect. Vision Res 22:119–134
Foley JM, McCourt ME (1985) Visual grating induction. J Opt Soc Am 2:1220–1230