Concepts being illustrated: Visual perception is generated by the brain, is not just a reflection of the elements out in the world. Not every percept has a direct physical correlate. Not every percept accurately reflects what is really going on, because the visual system has to make a number of assumptions, and once in a while, the assumptions fail.
This is a variation of the Hermann
grid illusion, named after L. Hermann. While he was reading a book on sound
by John Tyndall, Hermann (1870) saw spots in the intersections of spaces
among the figures that Tyndall had arranged in a matrix.
Concepts being illustrated: Context dependent visual perception, lateral inhibition.
Which check do you think is brighter,
the check in the shadow or the check outside the shadow? These two
checks are in fact exactly the same gray value.
Concepts being illustrated: Context dependent visual perception.