A nonobjective art movement began in the U.S. after 1960. Taking Pop Art to a further extreme, artists of this style sought to create unconventional, extraordinary images based on the sciences of visual perception. Op Art, as it was thus called, was a style that tried to create an impression of movement on the picture surface by means of optical illusion.
Bridget Riley was among the most prolific of Op Art artists. Her canvases show dizzying images of lines and colors in certain patterns which the human eye perceives to be active. She used gradual changes of color and wavy lines to add a sense of movement in this work, entitled Cataract 3. The effect works best when you enlarge the image (just click on the artwork to view the bigger version). The lines appear to be moving, don't they? I think the trick is to look at the work dead on; your eye naturally glides over the picture, and this, in turn, generates the effect of moving lines.
It is perhaps no coincidence that art of this caliber rose to popularity in the 1960s and '70s, sometimes called "the psychedelic era." While artwork such as this is maybe more communicable to people on drugs, the inherent themes of such a work bring out much of the popular sentiment of that time. Riley herself is known to have taken inspiration from various Modern and Postmodern literary sources and built off of themes of warped reality, unclear morality and purpose in the world, and the perceived ability of science to degenerate humankind as well as to improve it. We gaze into a strange kind of dystopia when we look into these works. By fooling the brain or the eye with deceptive, illusionary images, our perception of the world and reality is brought to the table for questioning; and our personal sense of humanness is challenged as we find that we can no longer even trust our own eyes to accurately see what's painted on a canvas.