Riley began working on a series of black and white paintings in 1961. Although she only intended to make one, she continued to work on them for six years. Paul Moorhouse comments that the dominant theme of these paintings is the, 'concern with expressing emotional states using the progression and modulation of individual, contrasted visual units ... the now rapid devlopment of her work was galvanised further by a growing awareness of the optical energies latent in the units [triangles, lines, circles and curves] she was using. In the interaction of contrasted elements she saw the tendency of the formal structures she was creating to destabilise, dissolving into intense and unsettling perceptual experiences. For example, in those paintings which use periodic structures - the close repetition of similar elements - the eye is overloaded with information, is frustrated in its ability to distinguish between discrete visual sensations and, in effect, is unable to interpret the information with which it is presented. The perceptual crisis thus provoked, in which the eye and mind veer from one contradictory impression to another, results in a range of hallucinatory phenomena - from iridescence to violent movement - which appear to take place in the space between the viewer and the painting' (Exhibition catalogue, Bridget Riley, Tate Britain, London, June - September 2003, p. 15).