Balancing the expected and the surprising in geometric patterns

I investigate the trade-off between the expected and the surprising in certain geometric patterns. This work is inspired by Bridget Riley's early Op art pieces, White Discs 2 (1964) and Fragment 6/9 (1965). I analyse these two works, investigate a range of variants, and propose hypotheses about the perceptual effects in patterns like these. The key hypothesis is that there is an æsthetically interesting range where between a quarter and a half of a regular pattern is adjusted in some way. I report on a perceptual experiment that tests and supports this hypothesis, and discuss the implications.