  The thin dark bands along the outer edges of the gradients and the thin light bands along the inner edges of the gradients are optical illusions. The cause of this effect is due to contrast perception; we over process at boundaries and under process where there is constancy. In fact sensory response can even stop with unchanging stimulation. Mach is credited for discovering lateral inhibition in our sense organs, the idea that our senses pre-process information before sending it to the brain.