Major examples of Riley's 1960s monochrome works include: Fission (1963, Museum of Modern Art, MOMA, New York), as well as Movement in Squares (1961), Crest (1964), Cataract 3 (1967), all British Council, London. In Fission, a square covered in black dots is distorted by progressive warping of the circles and compression of their spacing. The effect on the brain of the viewer is such that it cannot establish a fixed image of the picture which appears to hover upsettingly between the painting and the viewer. In this context, it should be noted that, while carefully programmed, Riley's graphic patterns are not strictly derived from scientific calculation, and she refuses to differentiate between the physiological and psychological responses of the eye.