A Figment of Your Imagination

Who says science isn’t fun? Visual illusions make great eye candy. But they also serve a serious purpose for researchers. How? Illusions push the mysterious and wondrous brain into revealing its secrets.

Illusory Motion Akiyoshi Kitaoka - Rotating Snakes
Illusory Motion

Some stationary patterns generate the illusory perception of motion. This unsettling effect is usually stronger if you move your eyes around the figure. For instance, in this illusion created by Akiyoshi Kitaoka, a professor of psychology at Ritsumeikan University in Japan, the “snakes” appear to rotate (it works better with the large image). But nothing is really moving other than your eyes!

If you hold your gaze steady on one of the black dots in the centre of each “snake”, the motion will slow down or even stop. Because holding the eyes still stops the false sense of motion, eye movements must be required to see it. Vision scientists have shown that illusory motion activates brain areas that are similar to those activated by real motion.

It is a fact of neuroscience that everything we experience is actually a figment of our imagination. Although our sensations feel accurate and truthful, they do not necessarily reproduce the physical reality of the outside world. Of course, many experiences in daily life reflect the physical stimuli that send signals to the brain. But the same neural machinery that interprets inputs from our eyes, ears and other sensory organs is also responsible for our dreams, delusions and failings of memory. In other words, the real and imagined share a physical source in the brain. So take a lesson from Socrates: “All I know is that I know nothing”.

One of the most important tools used by neuroscientists to understand how the brain creates its sense of reality is the visual illusion. Historically, artists as well as illusionists have used illusions to gain insights into the workings of the visual system. Long before scientists were studying the properties of neurons, artists had devised a series of techniques to deceive the brain into thinking that a flat canvas was three-dimensional or that a series of brushstrokes was indeed a still life.

Visual illusions are defined by the dissociation between the physical reality and the subjective perception of an object or event. When we experience a visual illusion, we may see something that is not there or fail to see something that is there. Because of this disconnect between perception and reality, visual illusions demonstrate the ways in which the brain can fail to re-create the physical world. By studying these failings, we can learn about the computational methods used by the brain to construct visual experience. Brightness, colour, shading, eye movement and other factors can have powerful effects on what we “see”.

Brightness Illusion
Brightness Illusion

In this illusion, created by vision scientist Edward H. Adelson of the Massachusetts Institute of Technology, squares A and B are the same shade of grey. (If you don’t believe it, print the photo, cut out the two squares and place them side by side.) This trick of the eye occurs because our brain does not directly perceive the true colours and brightness of objects in the world, but instead compares the colour and brightness of a given item with others in the vicinity. For instance, the same grey square will look lighter when surrounded by black than when surrounded by white.

Another example: when you read printed text on a page under indoor lighting, the amount of light reflected by the white space on the page is lower than the amount of light that would be reflected by the black letters in direct sunlight. Your brain doesn’t really care about actual light levels, and instead interprets the letters as black because they remain darker than the rest of the page, no matter the lighting conditions. In other words, every newspaper is also a visual illusion! Not to mention, it could be other forms of illusion as well.

Cupola of St Ignatius of Loyola church in Rome
Cupola of St Ignatius of Loyola church in Rome

The cupola of St. Ignatius of Loyola church in Rome is a great example of Baroque illusionism. The architect of the church, Orazio Grassi, had originally planned to build a cupola but died before finishing the church, and the money was used for something else. Thirty years later, in 1685, Jesuit artist Andrea Pozzo, was asked to paint a fake dome on the ceiling above the altar. Although Pozzo was already considered a master in the art of perspective, the results he accomplished could hardly be believed. Even today, many visitors to the church are amazed to find out that the spectacular cupola is not real but an illusion.

Architects soon realised that they could manipulate reality by warping perspective and depth cues to create illusory structures that defied perception. Need a big room in a small space? No problem. Francesco Borromini accomplished just that at the Palazzo Spada in Rome. Borromini created this spectacular trompe l’oeil illusion of a 37m long courtyard gallery into a 9m long space. There is even a life-size sculpture at the end of the archway! Not really. The sculpture looks life-size but is actually less than a meter tall.

Palazzo Spada Courtyard Gallery
Palazzo Spada Courtyard Gallery

How could we have missed it? Hundreds, perhaps thousands, of visual scientists, psychologists, neuroscientists, architects, engineers and biologists all missed it – until three years ago. The “it” in question is the leaning tower illusion, discovered by Frederick Kingdom, Ali Yoonessi and Elena Gheorghiu of McGill University. In this illusion, two side by side images of the same tilted and receding object appear to be leaning at two different angles. This incredible effect was first noticed in images of the famed Leaning Tower of Pisa, but it also works with paired images of other receding objects.

Leaning Tower Illusion
Leaning Tower Illusion

The leaning tower illusion is one of the simplest visual tricks one can produce, but it is also one of the most profound in relation to our understanding of depth perception. The tower on the right appears to be leaning more than the tower on the left. Yet these two photographs of the Leaning Tower are duplicates.

The illusion reveals the way in which the human visual system uses perspective to help construct our perception of 3-D objects. We say “construct” because the visual system has no direct access to 3-D information about the world. Our perception of depth results from neural calculations based on a set of rules.

These rules include the following: perspective (parallel lines appear to converge in the distance); stereopsis (our left and right eyes receive horizontally displaced images of the same object, resulting in the perception of depth); occlusion (objects near us occlude objects further away); chiaroscuro (the contrast of an object as a function of the position of the light source); and sfumato (the feeling of depth that one gets from the interplay of in- and out-of-focus elements in an image, as well as from the level of transparency of the atmosphere itself). Because the towers pictured in these paired images do not converge as they recede, the brain mistakenly perceives them as nonparallel and diverging.

Stare long enough at the skull in the ad and it will be “burned” into your vision even after you look away.

Yorick's Skull
Yorick’s Skull

To experience this antique illusion, stare at the X in Yorick’s left eye socket for about 30 seconds. Then look away at a flat surface such as a piece of paper, wall, ceiling or sky, and you will see Yorick’s afterimage as a ghostly apparition.

Vision scientists believe that the adaptation effect producing poor Yorick’s ghost largely takes place in the neurons of the retina. How can we know? Close your right eye and stare at the X again. Then look at the wall again to see the afterimage, but this time switch back and forth between closing one eye and the other. Only the left eye – which was open during the adaptation period – will reveal Yorick’s ghost. This means that the adaptation must have taken place only in neurons responding to stimulation from the left eye. If the adaptation had occurred in the binocular neurons of the brain (in the primary visual cortex and higher visual areas), you would see Yorick’s ghost with either eye.

Adaptation, in this case, is the process by which neurons habituate to, and eventually cease responding to, an unchanging stimulus. Once neurons have adapted, it takes a while for them to reset to their previous, responsive state: it is during this period that we see illusory afterimages. We see such images every day: after briefly looking at the sun or at a bright lightbulb or after being momentarily blinded by a camera flash, we perceive a temporary dark spot in our field of vision.

Science fiction author H.P. Lovecraft considered The Colour Out of Space his best story. In this 1927 classic tale of cosmic horror, a small Massachusetts farming community faces unspeakable evil from the outer reaches of the universe. The extraterrestrial villain is not a face-hugging or chest-bursting alien but something far more terrifying: a weird colour.

Slowly but surely the otherworldly colour mutates and destroys crops, insects, wild animals and livestock. It impregnates the land and the water. The unfortunate farmers who encounter the bizarre hue fall prey to insanity and untimely death.

Yellow Moon Blue Moon
Yellow Moon Blue Moon

Here we have two moons out of space. One yellow and one blue. Or are they? Actually both moons are exactly the same colour in this illusion by psychologist Akiyoshi Kitaoka of Ritsumeikan University in Japan; only the surrounding colours are different. If you don’t believe it, print the photo and cut out the two moons – you’ll find them to be identical. The appearance of colours is all about their context.

Neon Colour Spreading
Neon Colour Spreading

The colours from the small crosses appear to spread onto the white expanse surrounding each intersection. The effect resembles the glare from a neon light. This illusion was reported in 1971 by Dario Varin at the University of Milan, Italy, and a few years later by Harrie van Tuijl of the University of Nijmegen in the Netherlands. Its neural causes are currently unknown.

Our brains are exquisitely tuned to perceive, recognise and remember faces. We can easily find a friend’s face among dozens or hundreds of unfamiliar faces in a busy street. We look at each other’s facial expressions for signs of appreciation and disapproval, love and contempt. And even after we have corresponded or spoken on the phone with somebody for a long time, we are often relieved when we meet him or her in person and are able to put “face to the name”.

The neurons responsible for our refined “face sense” lie in a brain region called the fusiform gyrus. Trauma or lesions to this brain area result in a rare neurological condition called prosopagnosia, or face blindness. Prosopagnostics fail to identify celebrities, close relatives and even themselves in the mirror. Oliver Sacks is a well-known sufferer of prosopagnosia – the celebrated neurologist and author wrote about his own experiences in his book “The Mind’s Eye”.

But even those of us with normal face recognition skills are subject to many illusions and biases in face perception.

Illusion of sex
Illusion of sex

The side by side faces are perceived as female (left) and male (right). Yet both are versions of the same androgynous face. The two images are identical except that the contrast between the eyes and the mouth and rest of the face is higher for the face on the left than for the face on the right.

This illusion shows that contrast is an important cue for determining the sex of a face, with low-contrast faces appearing male and high-contrast faces spearing female. It may also explain why females in many cultures darken their eyes and mouths with cosmetics: a made-up face looks more feminine than a fresh face.

The eyes are the windows to the soul. That’s why we ask people to look us in the eye and tell us the truth. Or why we get worried when someone gives us the evil eye, or has a wandering eye. The English language is full of expressions that refer to where people are looking – particularly if they happen to be looking in our direction.

Here's looking at you kid
Here’s looking at you, kid

Vision researcher Pawan Sinha of the Massachusetts Institute of Technology shows us with this illusion that our brains have specialised mechanisms for determining gaze direction. In the normal photograph of Humphrey Bogart (left), the actor appears to be looking to his left,but in the photo negative (right) he appears to be looking in the opposite direction. Yet Bogart’s face does not look backward; only they eyes are reversed. Why? The answer is that we have specialised modules in our brain that determine gaze direction by comparing the dark parts of the eyes (the irises and pupils) with the whites. When the face is negative, the whites and irises appear to swap position. Our knowledge that irises are light rather than dark in a negative does not change our perception of this illusion.

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.

Enigma Illusion
Enigma Illusion

Look at the centre 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. 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 this illusion are still unknown.


Created in 1981 by artist Isia Léviant, the painting titled Engima had long stumped scientists. Nobody knew why the lines appeared to jitter, how the concentric circles could move, or what exactly it was that gave us this two-dimensional illusion its appearance of depth. Even weirder, most people see an alteration in the color of the circle after a few moments of intense staring. Why did we feel so sucked in to the painting? Then in November 2008, neuroscientists at Barrow Neurological Institute in Phoenix, Arizona, discovered most of the blame goes to the microsaccades, the tiny involuntary movements that occur naturally in the eyes at various times.

Nerve Impulse
Nerve Impulse

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.

The Gardner
The Gardner

Are you impressed with meals that look like one food but are actually made of something else? Tofu burgers and artificial crabmeat, for example, are not what they appear to be.

It’s actually an old trick. In medieval times fish was cooked to imitate venison during Lent, and celebratory banquets included extravagant (and sometimes disturbing) delicacies such as meatballs made to resemble oranges, trout prepared to look like peas and shellfish made into mock viscera. Recipe books from the Middle Ages and the Renaissance also describe roasted chickens that appeared to sing, peacocks redressed in their own feathers and made to breathe fire, and a dish aptly named Trojan hog, in which a whole roasted pig was stuffed with an assortment of smaller creatures such as birds and shellfish, to the amusement and delight of cherished dinner guests.

Unwelcome visitors were also treated to illusory food, but not for their own amusement. Instead they were served perfectly good meat that was made to look rotten and writhing with worms. Maybe not good enough to eat, but good enough to send your in-laws packing!

Food illusions are alive and well in the 21st century. Our buffet of contemporary lip-smacking illusions will appeal to both your eyes and your stomach … for the most part. We hope you’ll enjoy the spread. Bon appétit!


Art can be more than just a feast for your eyes. By using solely meats and breads in the image, photographer Carl Warner captures the feel of old sepia postcards from the late 19th century American prairie — complete with a breadstick – rail fence, serrano ham skies and a salami lane. Yum.

Warner’s work is another example of how the brain puts together information from multiple streams. Visual data from every point of the image are converted from light to electrochemical signals in the retina and then transmitted to the brain—where individual features are constructed from the information in the image. These discrete features are broadcast to multiple high-level visual circuits simultaneously: circuits that recognize faces, circuits that detect and characterize motion, circuits that recognize landscapes and places, and circuits that recognize and process food are just a few of the brain paths that receive this basic information.

In Warner’s art, both the landscape and the food-processing circuits are activated (the other circuits receive the information but ignore it as irrelevant because there are no faces, motion or other triggers in the image). And voilà! Our mind recognizes a delicious plate of cold cuts, as well as an overcast sky, in the same visual data.

Foodscape illusion busted :)
Foodscape illusion busted 🙂

Arcimboldo, Summer and Autumn Heads
Arcimboldo, Self-portraits

Arcimboldo’s composite heads demonstrate that, neurologically speaking, the whole can be much more than the sum of its parts. Clever arrangements of individual fruits, flowers, legumes and roots become exquisite portraiture in their entirety, such as in the artist’s self-portraits as Summer and Autumn.

The brain builds representations of objects from individual features, such as line segments and tiny patches of colour. You see a nose in the Summer portrait not because there is retinal cell that perceives noses but because thousands of retinal photoreceptors in your eye react to the various shades of colour and luminance in that area of the painting. High level neuronal circuits then match that information to the brain’s stored template for noses. The output from those same photoreceptors also activates the high-level object-tuned neurons that recognise turnips, figs and pickles, which is what makes images like these so much fun to look at.

Last but not least, Arcimboldo’s masterpieces also bring to mind the old adage that you are what you eat. “Avoid fruits and nuts”, advises Garfield 🙂

But go for broke with jelly beans!

Cherry topped cupcake
Cherry topped cupcake, Jelly Belly Candy Company

Pointillist painters such as Georges Seurat and Paul Signac juxtaposed multiple individual points to create color blends that were very different from the colors in the original dots. But in a very real sense, all art is pointillism. In fact, all visual perception is pointillism. Our retinas are sheets of photoreceptors, each sampling a finite circular area of visual space. Every photoreceptor then connects to downstream neural circuits that build our perception of objects, faces, loved ones and everything else. Thus, vision itself is largely a pointillist illusion, colored by a tremendous amount of “guesstimation” and filling in on the part of our brain. It doesn’t matter whether the painter uses brushstrokes or fields of dots to define surfaces.

The dots that compose the image of a cherry-topped cupcake are made from multicolored jelly beans, a technique that is not only clever but also delicious. Eat your heart out, Seurat.

We might take up jelly bean pointillism… Check this out! If this is not a bear activity, I don’t know what is 🙂

Jelly bean pointillism
Jelly-bean pointillism

If you agree that jelly bean pointillism is a great idea, you’ll also appreciate these replicas of famous masterpieces: Vincent Van Gogh’s Self Portrait in a Grey Felt Hat (left), Edvard Munch’s The Scream (below left) and Rembrandt’s The Anatomy Lesson of Dr. Nicolaes Tulp (below right). Everything in these images is fit for human consumption.

In an impossible figure, seemingly real objects — or parts of objects — form geometric relations that physically cannot happen. Dutch artist M. C. Escher, for instance, depicted reversible staircases and perpetually flowing streams. Mathematical physicist Roger Penrose drew his famously impossible triangle, and visual scientist Dejan Todorović of the University of Belgrade in Serbia created an impossible golden arch. These effects challenge our hard-earned perception that the world around us follows certain, inviolable rules. They also reveal that our brains construct the feeling of a global percept — an overall picture of a particular item — by sewing together multiple local percepts. As long as the local relation between surfaces and objects follows the rules of nature, our brains don’t seem to mind that the global percept is impossible.

Escher, Waterfall
Penrose Triangle
Penrose Triangle
Elusive Arch
Elusive Arch
Escher, Relativity
Escher, Relativity

Lego Homage to Escher
Lego Homage to Escher

Lipson and Shiu worked together on a Lego rendition of Escher’s Relativity. The original version, a popular lithograph first printed by Escher in 1953, depicts a surreal architectural structure in which there seem to be three separate sources of gravity. The stairs are double-sided and each stair is double-treaded. This was their fourth Escher picture rendered in Lego blocks.

Several contemporary sculptors recently have taken up the challenge of creating impossible art. That is, they are interested in shaping real-world 3-D objects that nonetheless appear to be impossible. Unlike classic monuments — such as the Lincoln Memorial in Washington, D.C. — which can be perceived by either sight or touch, impossible sculptures can be interpreted (or misinterpreted, as the case may be) only by the visual mind.

The Impossible Triangle sculpture in Perth forms part of Claisebrook Square and had its beginnings when local artists were invited to submit their ideas for a major public artwork commission as part of the East Perth Redevelopment project. The sculpture was created by local artist Brian McKay and architect Ahmad Abas and is based on the “Penrose Triangle” concept developed by the British Geneticist Lionel Penrose and his son Sir Roger Penrose, a Professor of Mathematics in the 1950’s.

The illusion of a triangle occurs when the sculpture is viewed along the privileged axis at only two locations. Only one location is accessible, as other structures get in the way at the other location.

It’s broken!

A Figment of Your Imagination

It’s a triangle!

A Figment of Your Imagination

It’s broken again!

A Figment of Your Imagination

Story from Scientific American Mind.

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