Scientists did not invent the vast majority of visual illusions. Rather they are the products of artists who 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 said, “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 in op art, the link between art and illusory perception is an artistic style in and of itself.
This illusion, created in 1957 by neuroscientist Donald M. MacKay, then at King's College London, shows that simple patterns of regular or repetitive stimuli, such as radial lines (called MacKay rays) can induce the perception of shimmering or illusory motion at right angles to those of the pattern. To see the illusion, look at the center of the circle and notice the peripheral shimmering.
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, follows in the footsteps of the great op artists of the 20th century. Waterway Spirals is a compelling and powerful version of French op artist Isia Léviant's now classic Enigma. Observe the strong illusory motion along the blue spiraling stripe.
THE ENIGMA ILLUSION
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 of 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.
But does the illusion originate in the mind or in the eye? The evidence was conflicting until we found, in collaboration with our 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 (right), also a tribute to Enigma, reflects the role of eye movements in the perception of the illusion.
Neuroscientist and artist Bevil Conway and his colleagues at Harvard Medical School recently demonstrated that pairs of stimuli of different contrasts are able to generate motion signals in visual cortex neurons, and they have proposed that this neural mechanism may underlie the perception of illusory motion in certain static patterns.
BRIDGET RILEY'S MOTION ILLUSIONS
Eye movements, both large and small, can trigger most of the motion illusions in this article. Blaze, a 1964 screen print by English op artist Bridget Riley (left), gives the impression of fast spiraling motion as observers move their eyes around the image. Fall (right), painted by Riley in 1963, has curved lines that create illusory undulations and volume. Both works are in the Tate gallery in London. The 1965 MOMA exhibition “The Responsive Eye” drew worldwide attention to Riley's op art.
In a work reminiscent of 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.
CIRCLES OF COLOR
British artist Peter Sedgley was Riley's partner for a decade and an important figure in the op art world. His paintings explore the optical interaction of concentric colored circles, which echo the geometry of the human eye and seem to pulsate on the black background. Sedgley airbrushed bands of color to create soft, overlapping rings in this 1968 work, YOU.
THE OUCHI ILLUSION
This illusion is by Japanese op artist Hajime Ouchi. 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 of the University of Freiburg in Germany stumbled on the illusion while browsing Ouchi's book Japanese Optical and Geometrical Art, which was first published in 1973. Spillmann then introduced the Ouchi illusion to the vision sciences community, where it has enjoyed immense popularity.
HOMAGE TO OUCHI
This illusion (right) is a contemporary variation on the Ouchi pattern, drawn by Kitaoka in 2001.
THE ROTATING-TILTED-LINES ILLUSION
An illusion (right) 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 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://illusioncontest.neuralcorrelate.com/2005/rotating-tilted-line-illusion).
Artist Miwa Miwa's variant of the rotating-tilted-lines illusion (above) pays homage to Vertigo, the classic 1958 film by Alfred Hitchcock (left).
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, almost three decades after its creation by Léviant, continues to inspire visual science as well as visual arts.
ART MEETS SCIENCE
This recent work by French artist José Ferreira, Nerve Impulse, not only reprises the Léviant 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 passes the message to its neighbor, like a bucket brigade passing a pail of water.