You undoubtedly saw a square figure which had a small rather light square area in the center and increasingly darker perimetric strips extending to the edge. You probably also saw bright arms radiating diagonally out from the center. This figure was adapted from a chromatic version designed by V. Vasarely (Arcturus (1970) as reported in Hurvich, 1981.
These brighter diagonal areas are physically not in the figure. That is to say, if you were to use a light measuring instrument (a photometer) and measure the amount of light coming from any of the concentric perimetric strips you would find that the same amount of light is reflected from all points along any one strip. Yes, that includes that part of the strip along the diagonal where it appears brighter. Consequently, that must mean that this brightness illusion is generated in the visual system. Just incase I have confused you, perhaps looking at a photometric plot of the illusion will help.
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