Re:That's not an optical illusion (Score 3, Informative)

It's not about lighting at all. It's about lateral inhibition. The neurons in your brain, when connected in a nural network, send inhibition signals to their neighboring neurons. Areas close on you retinas have neurons on the same part of the brain, so the squares surrounding the center square are going to activate neurons close to the ones you use to see the center square. Now the lighter squares send a stronger signal to the neurons surrounding the center square, thus sending MORE inhibition, thus making the square appear darker. w0rd.