Bridget Riley.

"Rhythm and repetition are at the root of movement. They create a situation within which the most simple basic forms start to become visually active. By massing them and repeating them they become more fully present. Repetition acts as a sort of amplifier for visual events which seen singly would hardly be visible. But to make these basic forms release the full visual energy within them, they have to breathe, as it were, to open and close or to tighten up and then relax. A rhythm that's alive, has to do with changing pace and feeling how the visual speed can expand and contract, sometimes go slower and sometimes go faster. The whole thing must live." —Bridget Riley