Visual Analytics - An Interaction of Sight and Thought - JMP

Visual perception works according to
its own rules.
The effect has now been reversed: we see the objects on the left as concave and those
on the right as convex. All I did, however, was turn each sets of objects upside down—I
didn’t switch them. The reason that we now see those on the left as concave is
because, through eons of evolution, visual perception learned to assume that light was
shining from above, which causes us to see the objects on the left as concave, because
the shadows are on the top, and those on the right as convex, because the shadows
are on the bottom.