Scientists did not invent the vast majority of visual illusions. Rather artists have used their insights into the workings of the human eyes and brain to create illusions in their artwork. Long before visual science existed as a formal discipline, artists had devised techniques to “trick” the brain into thinking that a flat canvas was three-dimensional or that a series of brushstrokes in a still life was in fact a bowl of luscious fruit. Thus, the visual arts have sometimes preceded the visual sciences in the discovery of fundamental vision principles through the application of methodical—though perhaps more intuitive—research techniques. In this sense, art, illusions and visual science have always been implicitly linked.
It was only with the birth of the op art (for “optical art”) movement that visual illusions became a recognized art form. The movement arose simultaneously in Europe and the U.S. in the 1960s, and in 1964 Time magazine coined the term “op art.” Op art works are abstract, and many consist only of black-and-white lines and patterns. Others use the interaction of contrasting colors to create a sense of depth or movement.
This style became hugely popular after the Museum of Modern Art in New York City held an exhibition in 1965 called “The Responsive Eye.” In it, op artists explored many aspects of visual perception, such as the relations among geometric shapes, variations on “impossible” figures that could not occur in reality, and illusions involving brightness, color and shape perception. But “kinetic,” or motion, illusions drew particular interest. In these eye tricks, stationary patterns give rise to the powerful but subjective perception of (illusory) motion.
This article includes several works of art in which objects that are perfectly still appear to move. Moreover, they demonstrate that research in the visual arts can result in important findings about the visual system. Victor Vasarely, the Hungarian-French founder of the op art movement, once remarked, “In basic research, intellectual rigor and sentimental freedom necessarily alternate.”
Op artists have created some of the illusions featured here; vision scientists honoring the op art tradition have created others. But all of them make it obvious that the link between aesthetics and illusory perception is an artistic style in and of itself.
In a work reminiscent of British op artist Bridget Riley's, vision scientist Nick Wade of the University of Dundee in Scotland created an example that features both streaming and shimmering motion. An eye is clearly visible in the center of the design, and a face becomes visible if you view the illusion from across the room or shake your head. The hidden face is a portrait of Wade's wife, Christine, and the title Chrystine is a reference to the chrysanthemum shape.
This illusion began with a chance observation. MacKay first saw it on the wallboard of a BBC studio: the broadcasting staff had been annoyed by illusory shadows running up and down blank strips between columns of parallel lines.
OP ART IS ALIVE AND WELL
Akiyoshi Kitaoka, a professor of psychology at Ritsumeikan University in Japan, created Waterway Spirals, a compelling and powerful version of French op artist Isia Léviant's now classic Enigma. In Enigma, black lines converge on a central yellow dot, all beneath concentric purple and pink circles, which seem to spin.
Look at the center of the above image and notice how the concentric green rings appear to fill with rapid illusory motion, as if millions of tiny and barely visible cars were driving hell-bent for leather around a track. Neuroscientist and engineer Jorge Otero-Millan, when at the Barrow Neurological Institute in Phoenix, created this image as a reinterpretation of Enigma by Léviant, who unknowingly combined the MacKay rays and the BBC wallboard illusions.
But does the Enigma illusion originate in the mind or in the eye? The evidence was conflicting until we found, in collaboration with our then Barrow colleagues Xoana G. Troncoso and Otero-Millan, that the illusory motion is driven by microsaccades: small, involuntary eye movements that occur during visual fixation. The precise brain mechanisms leading to the perception of the illusion are still unknown, however. One possibility is that microsaccades produce small shifts in the geometric position of the peripheral areas of the image. These shifts produce repeated contrast reversals that could create the illusion of motion. Otero-Millan's Enigmatic Eye (above right), also a tribute to Enigma, reflects the role of eye movements in the perception of the illusion.
Neuroscientist and artist Bevil Conway, with colleagues then at Harvard Medical School, demonstrated that pairs of stimuli of different contrasts are able to generate motion signals in visual cortex neurons and proposed that this neural mechanism may underlie the perception of illusory motion in certain static patterns.
CIRCLES OF COLOR
British artist Peter Sedgley is an important figure in the op art world. His paintings explore the visual interaction of concentric colored circles, which echo the geometry of the human eye and seem to pulsate on the black background. In his work Video Disque YOB, 1968, Sedgley has organized fluorescent color to create a stroboscopic countermovement on a rotating disk.
Move your head back and forth as you let your eyes wander around the image and see how the circle and its background appear to shift independently of each other. Vision scientist Lothar Spillmann, then at the University of Freiburg in Germany, stumbled on the illusion while browsing Japanese op artist Hajime Ouchi's book Japanese Optical and Geometrical Art, which was first published in 1973. Spillmann introduced the illusion to the vision sciences community, where it has enjoyed immense popularity.
HOMAGE TO OUCHI
This illusion is a contemporary variation on the Ouchi pattern, drawn by Kitaoka in 2001.
An illusion developed by vision scientists Simone Gori and Kai Hamburger, then at the University of Freiburg in Germany, is a novel variation of both the Enigma effect and Riley's Blaze. To best observe the illusion, move your head closer and then farther away from the page. As you approach the image, notice that the radial lines tilted to the right appear to rotate counterclockwise. As you move away from the image, they appear to rotate clockwise. This illusion was featured in the first edition of the Best Illusion of the Year Contest, held in 2005 in Spain (see http://illusionoftheyear.com/2005/rotating-tilted-line-illusion).
Artist Miwa Miwa's variant of the rotating-tilted-lines illusion (top) pays homage to Vertigo, the classic 1958 film by Alfred Hitchcock (bottom).
CHRISTMAS LIGHTS ILLUSION
The Christmas lights illusion, by Italian artist and author Gianni A. Sarcone, is also based on Léviant's Enigma. Notice the appearance of a flowing motion along the green-yellow stripes.
TWO IN ONE
Gori and Hamburger's combination of the rotating-tilted-lines illusion and the Enigma illusion is both visually arresting and a powerful demonstration of illusory motion from a static pattern. The Enigma illusion, some three decades after its creation by Léviant, continues to inspire visual science as well as visual arts.
ART MEETS SCIENCE
This work by French artist José Ferreira W., Nerve Impulse, not only reprises the Enigma effect but also illustrates how nerve cells relay information from the eye to the brain: triggered by a flood of chemicals called neurotransmitters, nerve cells (at top) send electrical signals racing down slender structures called axons. At the axon's knoblike terminals, each nerve cell releases its own neurotransmitters, which diffuse across a narrow synapse gap and bind with receptors on the branchlike dendrites of the next nerve cell to trigger a new electrical signal. Each successive neuron conveys the message to its neighbor, like a bucket brigade passing a pail of water.