This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
Many readers have inquired as to how the illusions from this year’s contest work in the brain. Here is a brief explanation of each.
First Place went to “Motion integration unleashed:” a motion perception illusion by Mathew T. Harrison and Gideon P. Caplovitz from the University of Reno, Nevada, USA.
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Sine-wave grating blobs called “Gabors”—a favorite stimulus that visual scientists use in the lab--can be arranged in various configurations that give rise to dramatic motion effects. Gabors can drift while not actually moving from one position to another. A static square seems to rotate, chopsticks appear to gnash, and waves appear to roll: all without physical displacement. Usually, all the parts of a given moving surface have a single coherent global motion. Our visual system determines this motion by integrating the local motion signals from the points around the surface’s edges. Harrison and Caplovitz’s illusion showcases this principle by employing strong local motion signals (the drift inside each stationary Gabor) that fool the brain into thinking that the whole object is moving. In the real world, barber poles are subject to the same illusory mechanisms: The poles rotate horizontally around their axes, yet the stripes appear—falsely—to move vertically. The barber poles’ apparent vertical motion arises from the same neural wiring that is responsible for Harrison and Caplovitz’s discovery.
See also: Past illusion contest contributions from this and other labs, such as the Infinite Regress Illusion (2005), the Curveball Illusion (2009), and the Steerable Spiral (2010), all of which used drifting gratings to demonstrate the interaction between global and local motion signals.
Second Place went to “Ambiguous Cylinder Illusion:” a perspective illusion by Kokichi Sugihara from Meiji University, Japan.
Sugihara has built outstanding sculptures of impossible objects, that appear as very different shapes depending on their vantage point. At any other angle, the illusion fails. Scientists refer to this as the accidental view, but there is nothing accidental about it. To perceive the illusion, the view must be carefully staged and choreographed, or else the audience will fail to see the “impossible” sculpture.
See also: Ambiguous Cylinders: A New Class of Impossible Objects. K Sugihara. Computer Aided Drafting, Design, and Manufacturing. 3 September 2015 25(3):1.
Third Place went to “Silhouette Zoetrope:” a motion illusion by Christine Veras from Nanyang Technological University, Singapore.
This illusion is an homage to early tropes that have tricked our senses since the Victorian Era. Unlike in traditional zoetropes in which the rotating objects within a cylinder are viewed through moving slits in the cylinder, Veras inverted the structure, so that the slots strobe behind the cutout images, thereby animating them. Interestingly, the animation nevertheless appears to take place inside the empty slotted cylinder, to create the illusion of moving silhouettes. Each cutout image was attached to a stick and placed outside the slotted cylinder. The interior of the slotted cylinder was white. The resulting animated image only exists in our brain. The illusion not only helps us better understand how our brain perceives movement but illustrates that the impression of an image can be created at an arbitrary depth without the use of projection or digital technology. It is a unique combination of what can be characterized as shadow puppets and an inside-out zoetrope, enabling the viewer to see moving silhouettes. The fields of optics, physics, neuroscience, and stereoscopy come together in this invention. Veras says that she has always been fascinated by the zoetrope, and was working to try to design a new way to combine shadow puppets with the zoetrope when she had a dream about a different way to structure the device. The result was exciting and astonishing.
See also: Victorian Theatrics. S Martinez-Conde, S Macknik. Scientific American Mind. 2015.
I have described the remaining Top 10 Finalists below, in no particular order:
Lights and Darks in Vision” is a brightness illusion by Jose-Manuel Alonso from the State University of New York, USA
This is a study in how the visual system differentially processes information about light (against a dark background) versus darkness (against a light background) and uses the two types of information for different perceptual purposes. If we split a picture of a face into just the dark pixels versus just the light pixels, we see that the ‘whites’ of the eyes in the dark-half picture have the same luminance as the ‘dark’ lips in the light-half image, though the lips appear as much darker than the eyes. This strong contrast illusion occurs with any face or visual scene photographed with a standard digital camera and processed in this manner. Notice that the crisp dark image seemingly contains most of the detail of the face, whereas the light image appears to have relatively high contrast: as if the two different polarities of visual contrast have different roles in vision. Alonso came up with this demonstration while writing a grant application for the National Institutes of Health and thinking about ways to illustrate the function of ON and OFF visual pathways (which are specialized to see lights versus darks) in the brain. When he created the two versions of the face, he noticed that the light-half image paradoxically appeared darker than the dark-half image, and vice-versa. Puzzled, he measured the physical luminances of both images to ensure he hadn’t made a mistake. Ultimately, he realized that he had discovered a powerful brightness illusion that helps us now to understand how the visual system processes darks vs lights.
See also:
Principles underlying sensory map topography in primary visual cortex. J Kremkow, J Jin, Y Wang, JM Alonso. Nature 533, 52-57, doi:10.1038/nature17936.
Neuronal nonlinearity explains greater visual spatial resolution for darks than lights. J Kremkow, J Jing, SJ Komban, Y Wang, R Lashgari, X Li, M Jansen, Q Zaidi, JM Alonso. Proceedings of the National Academy of Sciences (USA). 2014.
“A New Illusion At Your Elbow” is a tactile illusion by Peter Brugger and Rebekka Meier from University Hospital Zurich, Switzerland
Move your finger slowly along the inside of a friend’s forearm from the wrist towards the elbow with their eyes closed, and he or she will erroneously report that you have reached the elbow well before you actually do reach the crook. This illusory anticipation may rest on our experience of tactile velocities that are usually much faster, and make us believe that we feel touch at a body location not yet reached. Neural characteristics of skin receptors specialized for slow motion may also contribute to the anticipation error. Like previously described illusions, the elbow crook illusion is larger on the non-dominant arm. Women showed a smaller illusion than men, confirming their reportedly superior cutaneous sensitivity. The illusion may rely on the interplay between two factors, one psychological and influencing tactile perception in a “top-down” way, the other a “bottom-up” neurophysiological mechanism. The psychological factor is based on our everyday experiences of motion on the skin, which is mostly faster than that experienced in the elbow game. This makes us anticipate touch at a body location in the direction of motion. The neurophysiological factor comprises especially long after-discharges of cutaneous mechanoreceptors, which can lead to a subjective enlargement of slow-motion tracks on the skin. The authors learned about this illusion from Brugger’s daughter, who played this game on the playground with other children.
See also: A new illusion at your elbow. P Brugger, R Meier. Perception, 2015, 44:219–221. doi:10.1068/p7910
“The Shrunken Finger Illusion” is a visual-tactile illusion by Vebjørn Ekroll, Bilge Sayim, Ruth Van der Hallen and Johan Wagemans: University of Leuven, Belgium
You can make your finger feel shorter than it is by putting it into a halved ping-pong ball. Look at your finger directly from above, with the fingertip extended upwards towards your eyes. Then, place the halved ping-pong ball on top of your finger, such that your fingertip is hidden inside the half-ball. Now, when you look at the halved ping-pong ball from above, it will look like a complete ball. But what about your finger? It feels shorter as if to make space for the unseen half of the illusory ball! The shrunken finger illusion shows that our experience of the hidden backsides of objects are “real” to us in the sense that they can affect the experience of our own body in a dramatic and almost bizarre way – even against better knowledge. From this, we can conclude that our experience of the hidden backsides of things is not only a product of our conscious thinking but also automatically constructed by perceptual mechanisms that are impervious to conscious knowledge. When you look at the world around you, it rarely appears as an empty façade, though there is no way you could know that it was not, and the brain evidently disambiguates these possibilities by concluding that objects are 3D solids. The Shrunken Finger Illusion shows that this perceptual shortcut extends to sensory modalities other than vision. The creators discovered the illusion during a research study of magicians. They were specifically studying a trick known as “multiplying balls”, in which the magician fools the audience by using a halved ball such as the one used in this illusion.
“Remote Controls” is a perceptual alternation illusion by Arthur G. Shapiro from American University, USA
Two physically identical rectangular bars alternate their lightness and darkness in synchrony but appear as if they wink in alternation in certain conditions. The appearance of winking (alternating) or blinking (bars in sync) can be controlled by rectangles placed in the vicinity of the modulating bars: the bars blink when the rectangles are far away or adjacent to the bars, but wink when there is a gap between the bars and the rectangles. The effect is remarkable because of the sudden change from wink to blink or vice versa, and because the change can occur across large distances. Remote Controls may look like a standard brightness illusion, but it is not! In a standard brightness illusion, two identical test patches appear differently from one another, whereas in Remote Controls, the two identical patches appear in illusory modulation with respect to one another. The key to the Remote Controls illusion is contrast. When the bars are both bright, one bar has a high contrast relative to its background, and the other bar has a low contrast relative to its background. When the bars are dark, the contrast relationships are reversed.
The main point is that the visual system represents the contrast apart from the appearance of color. That is, the difference between the bars and the background is just as salient to our perception as is the color of the bars. Shapiro predicted this illusion was predicted from his theoretical work about brightness perception in the brain.
“The Dalesmen Singers Illusion” is a motion illusion by Mike Pickard and Gurpreet Singh, from Sunderland University, UK
The moving letters in the illusion are actually rock steady on screen! All that changes is the filled middle of each letter. This alternating contrast change at the edges nevertheless creates an impression of movement. As the luminance alternation cycles, it sometimes matches the background and becomes momentarily indistinct, whereas at other points in the cycle the apparent contrast reverses, as if the letters’ shadows were moving. Pickard and Singh’s illusion shows that there are situations in which we cannot be sure of where things are, or if they are moving or not. Both illusion creators are academics whose interest lies is in linking visual science knowledge to design practice, in a process known as Visually Directed Design. They use multimedia software to examine and test illusory phenomena and then use them in creative concepts applied to design practice. The Dalesmen Singers Illusion incorporates multiple visual factors that work together to create a maximum illusory effect.
“Caught Inside a Bubble” is a color adaptation filling-in illusion by Mark Vergeer, Stuart Anstis, and Rob van Lier, from the University of Leuven, Belgium, the Radboud University Nijmegen, The Netherlands, and the University of California, San Diego, USA
In this illusion, you see colors that are not actually present on the screen. An image of colored bullseyes alternates with an image containing different sized greyscale bullseye circles (bubbles). Although the bubbles are colorless, they appear to be colored. These illusory colors are the afterimages of the previous bullseye’s colors. The intriguing thing is that the colors that fill each bubble change, depending on the size of the bubble. One colored image causes multiple, completely different afterimages, and each bubble ‘captures’ the afterimage of the bull’s eye color that matches the bubble’s size. When your brain is confronted with the same color at the same location for a prolonged period of time, it will become less sensitive to this color: this is called color adaptation. As a consequence, if you stare at a colorful image, and then look directly at a white screen (or wall), you will perceive the colors opposite to the colors in the initial (adapted) image: a color afterimage. One property of afterimages is that they become stronger, and more salient when the contours of the adapted color match the contours of the colorless image presented after the colored image. In the illusion created by Vergeer, Anstis and van Lier, you see the afterimage of the bull’s eye color that matches the contour of each bubble, which varies depending on the bubble’s size. Your brain uses contours to reinforce your color experience. The authors have worked together on the visual science underlying the interaction between colors and contours for several years now. “Caught Inside a Bubble” is the product of some of the perceptual principles unveiled by their collaboration.
“Millusion” is an ambiguous perspective illusion by Sylvia Wenmackers, from the University of Leuven, Belgium
Because this illusion involves windmills, the author calls it a Mill-illusion, or “Millusion”. The “Millusion” occurs when we view windmills, under conditions in which we can only see their silhouettes (for instance, due to fog). During the day and without fog, shadows and light help us to determine whether each windmill’s blades are turning in front of the turbine or behind it, so the illusion does not occur. When we see only silhouettes, however, we lack strong depth information and may interpret the mills as pointed in the wrong direction. The author experienced this illusion while driving past a windmill farm in the evening. One turbine seemed to be turning “the wrong way” compared to the others. By the time she got home that night, she had puzzled out what must have happened: her perception was wrong, not the direction of the windmill. She later asked a scientific illustrator, Pieter Torrez (Scigrades), to help her animate the illusion.
See also:
A column about the illusion (with a still of the animation) in the Dutch language, in a popular science magazine (Eos) and in March 2016 on their online portal (link).
A machine-translation is available, which may give you a rough indication of the contents (link).