As the visual processing takes some time (in the order of 1/10th of a second), the brain will try to extrapolate a given visual stimulus this time into the future. This makes sense, but of course can go wrong sometimes - and in this article this fact is used to explain many of the optical illusions.
Very interesting unifying explanation for many of these phenomena!
See the paper by Mark Changizi Latency correction and a general theory of illusions".