Let us consider an example where our minds are torn between two possibilities by the problem of perspective. It was discovered by a Swiss crystallographer, Louis Alban Necker in 1832. If we stare at the cubes then the sense of perspective that we rely upon to create a good three-dimensional interpretation of the purely two-dimensional image cast upon the back of the retina is confused: there is no unique three-dimensional image that produces this two-dimensional projection. The brain has constructed two models of a solid cube, each with a different orientation in space, and it flits between the two, showing us both possible perspectives. It is as if there is an advantage in occasional to another view of things, just in case the one already chosen is mistaken. Whole artistic movements have grown up exploiting this image-processing ambiguity. Victor Vasarely, and others in the op art movement, have created intricate images that exploit uncertainties in the brain's identification of neighbouring points and the lines it infers between them, so that there is constantly changing perspective. The image never appears static, An example of this dynamic art-form is shown in an Op Art work like Bridget Riley's Fall, 1963. Tate Gallery, London.

Text from ‘Necker Cubes and Op Art’ by John Barrow