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This is another image I found on Google+
All lines are absolutely straight, parallel and perpendicular but why does it appear to have a curvature?
Related: How does this illusion work?
Like these questions :) Many of these illusions come from Prof. Akiyoshi Kitaoka, a japanese Psychologist and expert for Gestalt Psychology. On his website you'll find some more fascinating illusions and questions to ask here ;)
The illusion above is named Cafe Wall illusion and the newest model to explain those illusions is the contrast-polarity model. Short explanation from his webpage:
The paper explained it better to me:
Kitaoka, Pinna, and Brelstaff (2004) proposed a phenomenal model to explain the Café Wall illusion, which stressed the importance of contrast polarities of a solid square and its adjacent line segment. When a dark/light square is accompanied by a dark/light line segment, the apparent tilt is the direction of contraction of the square angle (Figure 18a,b). In contrast, when a dark/ light square is accompanied by a light /dark line segment, the apparent tilt is the direction of expansion of the square angle (Figure 18c,d).
This explains why you perceive a tilt. If you position the smaller squares now in distinct edges of the big squares, you can achieve 2- and 3-dimenional illusions. Here you see a increasing of the tilt due to more smaller squares:
Here you can see that the positioning of the smaller squares is critical to achieve the 3D effect of the orignial bulge effect in your question:
Notice that Gestalt Psychology is a non-reductionistic theory approach and investigates mainly the phenomenology and underlying Gestalt Laws of visual perception. How these Gestalt Laws developed on a deeper level is a question of neurobiological evolution similar to, "why have some species of apes color-vision and some not". The ellipses in the explaining picture above show you, that our cognitive visual machine somehow tries to group divided objects (square and line of same contrast/brightness) in one line and we see a tilt. I'm guessing here, but this is probably due to cognitive brain algorithm that saves things and objects we see and perceive mainly by countor and shapes, rather than pixel by pixel like a computer and digital camera do it, which of course don't perceive any tilt or 3D illusion in any of those trick images :)
Read the papers for more explanations and examples, not behind a paywall:
Apparent contraction of edge angles, A Kitaoks, Perception, 1998
Tilt illusions after Oyama (1960): A review. Kitaoka, A, Japanese Psychological Research, 49, 7-19.
This optical illusion tricks you into seeing different colors. How does it work?
These levitating spheres may appear red, purple or green at first glance, but in actuality, all 12 orbs are the same bland shade of beige.
Shrinking the image exaggerates this illusion, while zooming in minimizes the effect, according to David Novick, the creator of the image and a professor of engineering education and leadership at the University of Texas at El Paso. But why do we perceive the spheres as anything but their true color, beige?
This skewed perception stems from a phenomenon known as the Munker-White illusion, Novick told Live Science.
In essence, the illusion works because "our acuity for shape is better than our acuity for color, which means that we perceive the shapes with more detail and the colors with less detail," Novick said.
So, while the outlines of the spheres all appear identical, as they are, "the color sort of bleeds over, or assimilates, to adjacent spaces," Novick said. Specifically, the color of the spheres gets "pulled" closer to the color of the stripes crossing over them, in the foreground. In this particular image, called "Confetti Spheres 5," an array of green, red and blue stripes cut across the spheres and warp our perception of their actual hue.
The illusion relies on the hue of the foreground stripes, not the colors in the background behind the spheres. So, if you remove the crisscrossing stripes, the illusion disappears, leaving only identical beige balls.
Very nice! This neatly matches the original with the foreground stripes removed. pic.twitter.com/ZcwflVhj6OJune 15, 2019
The illusion works very similarly when you convert all of the colors to grayscale. In fact, "White's illusion," in isolation, refers to the perceived changes in the lightness of a shape &mdash ranging from white to black &mdash caused by the lightness of shapes overlapping it, according to a 2010 report published in the journal Colour: Design & Creativity. For instance, when you run white stripes over a gray rectangle, the gray appears lighter, or closer to white but when you run black stripes over the same rectangle, it appears darker, or closer to black.
Psychologist Michael White became known for describing this effect in the 1960s. And in 1970, Hans Munker, another psychologist, demonstrated a similar effect with colored shapes and stripes, in which the foreground stripes warp the perceived hue of the background shape, according to the 2010 report.
Scientists have competing theories about what's happening in the brain to cause this shift in perception. Some think the illusion sets in early during visual processing, when light first hits the retina, while others think the effect takes hold later as the brain processes the data. It could be a combination of both, the 2010 report notes.
Whatever the exact cause of the illusion, it's fun to play with, Novick said. By tampering with both the lightness and the color of the foreground stripes, you can "amplify" the apparent color change, causing the background shapes to look wildly different from their true colors.
That said, "it's much easier to get differences in apparent colors for some hues than others," Novick noted. For instance, if the background shape and foreground stripes are complementary colors (opposites on the color wheel), like red and green, their colors will actually cancel out, so the spheres will end up looking white or grey instead, he said.
Novick is currently studying which color combinations generate the largest apparent change in the color of the background shape to maximize the effects of the illusion. In addition, he and his collaborator Akiyoshi Kitaoka, a psychology professor at Ritsumeikan University in Kyoto, Japan, are comparing how the Munker-White illusion affects 3D shapes, as in "Confetti Spheres 5," as compared to flat, 2D shapes.
"The illusion seems more perceptible, or vivid, with the spheres rather than the flat discs. And we don't know why," Novick said. "At this point, I don't think anybody does."
Novick was originally inspired to craft illusions after seeing and reading about Kitaoka's work. Kitaoka's website dances with vivid patterns that appear to swirl and undulate as you stare at them text along the top of the page warns, "Should you feel dizzy, you had better leave this page immediately."
Upon studying these baffling illusions, Novick became interested in folding them into his own research program on human-computer interaction. In summer 2017, he began crafting new versions of the illusions on his own and, for fun, posted some to his Twitter account. One of these posts &mdash a 2D version of the confetti illusion &mdash went "unexpectedly viral" on July 18, 2018.
"I first learned about this when my wife came in and said, 'You're in the newspaper in England,'" Novick said, noting his surprise. To this day, Novick tweets about two new illusions each week, and his older posts periodically get rediscovered, go viral and sometimes make the news, he said.
How does the rubber pencil illusion work?
If you want to see a rigid pencil turn to rubber, just ask an elementary-school student. In a favorite playground trick, an amateur magician picks up a pencil near the tip and lightly jiggles the whole thing up and down. When the illusion is performed correctly, the straight line turns into a wiggling wave.
So, how does the rubber pencil illusion work?
Let's start with the simple explanation: Your eyes and brain just can't keep up. When light enters your eyes, receptors called rods and cones pass a signal along nerves to your brain, which processes it. Think of each of those signals as a photograph. Your brain ties those images together so that they appear to move smoothly, just as they do in a flip-book.
"The eyes tend to sum up light over time," said Jim Pomerantz, a cognitive psychologist who studies visual perception at Rice University in Texas.
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But humans have remarkably slow visual systems, Pomerantz said. Humans can process 50 to 100 individual frames &mdash pages in that flip-book &mdash per second, depending on the size of what we see, according to a 2016 study published in the journal PLOS One. For context, some bird species can process 145 frames every second. There is some evidence to suggest that houseflies can process upward of 270 frames per second, and the fastest flies can process 400 frames a second.
When tracking a fast-moving object, your visual system actually doesn't sense the object moving in real time. Instead, each frame of motion leaves roughly a milliseconds-long impression on your retina, the part of the eye that senses light. That's why, if you wave your hand quickly in front of your face, you'll see a blur, and why fluorescent bulbs appear to cast a steady light. "What people don't realize is that those fluorescent tubes are flickering," Pomerantz said. If you were, say, a pigeon, you'd see a strobe light.
So, when your friend jiggles a pencil up and down, your visual system isn't actually capturing that motion in detail it's giving you a summary, Pomerantz said. This is where things get a bit more complex. When Pomerantz published the first study on the rubber pencil illusion in 1983, he used a computer to graph out each frame of a pencil's movement in detail.
His results, published in the journal Perception and Psychophysics, found that in the simulation, if a pencil is held near the tip and jiggled just so, graphs of each individual frame joined together to form a smooth curve. That's what your visual system picks up. If you were a bird or an insect, you'd see a straight line moving up and down, because those creatures can process more frames per second, Pomerantz said.
But there's more to the trick. More recent research has found that Pomerantz's theory is an important part of the story but doesn't completely answer the question of why the pencil appears to turn to rubber. Working together, teams of scientists in Germany and Ohio had participants move their eyes in specific ways while paying attention to computer simulations of jiggling lines. The idea was that the eye movement would change the "snapshots" these people captured on their retinas. If Pomerantz was completely right, it should be possible to partially "cancel" the pencil's motion, making it look more straight, by tracking it with your eyes, said Lore Thaler, a psychologist at Durham University in England.
The 2007 study, published in the Journal of Vision, found that eye movement did make the line more rigid but not as much as it should have based on Pomerantz&rsquos theory alone. Another experiment further supported the researchers&rsquo suspicion that there was more to the story. A box, drawn around the outside of the line and being waved up and down in tandem also changed the perceived rubberiness of the line. The box provided context, helping the brain discern the motion of the pencil. In effect, when the box and the pencil were waved together, participants saw a straight line moving up and down.
Together, Pomerantz's theory and these results suggest that it's not just about the "snapshots" our eyes capture it also has to do with their context and the way our brains process the snapshots.
It's unclear exactly why our brains are unable to process a straight line moving up and down, Thaler told Live Science. But scientists do know this: The human brain "just does the best it can," she said.
These Patterns Move, But It’s All an Illusion
Focus on the ball at the center of the image above. The scene appears to vibrate. If you move your head slightly forward and backward, the color fields of the rosette appear to pulsate.
Scientists have several theories about how our eyes and brain collaborate to create the illusion of movement—although the precise neural mechanics remain unknown. Still, what we do know makes it possible for artists such as myself to design visual pranks.
This vibrating rosette combines several illusory effects. To begin with, when we fixate on a pattern, it momentarily remains on our retinas as an after-image. One theory is that small, involuntary eye movements cause this ghost image to overlap with the image on the page. The result is what’s called a moiré effect: similar, repetitive patterns merged together at slightly different angles, creating a rippling effect. I enhanced this effect by adding two high-contrast colors, blue and yellow.
This pattern, with a skull in the center, appears to pulsate. By Gianni Sarcone.
Also, when we approach an object, our brain normally makes adjustments so that the object’s size and brightness appear to remain constant. But when you move your head back and forth, the alternating dark and light patterns in my rosette seem to change in both size and brightness. One possible explanation is that our visual system cannot bring the blurred boundaries within the image into focus, and our brain cannot adjust.
Seeing is believing—except when the mind can be tricked into believing what it sees.
In this illusion, the yellow lines seem to waver. By Gianni Sarcone.
As your eyes pass over this design, the pink hearts appear to shift diagonally in opposite directions. The large blue heart pulsates. By Gianni Sarcone.
5 Color Illusions and Why They Work
Think colors are objective facts? Think again! Color is more subjective than you might expect—it’s really all in your head. These illusions show you how.
1. Checkerboard Illusion
In this illusion, both block A and B are the same color.
Don’t believe it? Check this out.
It’s all because of color constancy, which helps the brain recognize objects regardless of the amount of light being reflected. Cone cells in our eyes help us see color. As these cones register different wavelengths of light, special neurons in the visual cortex try to make sense of the cone activity. Seeing that Square B is under a shadow, your brain assumes that the square must be even lighter than it really is.
2. Chubb Illusion
Another example of color constancy: the left inner box appears darker than the box on the right—although they’re the same color. Both squares reflect the same amount of light into your eyes, but they still appear different because of the context.
3. Scintillating Grid Illusion
This one is pretty trippy. Dark dots rapidly appear and disappear at the intersections. However, if you stare at one intersection, the crossroad remains white.
Scientists are still trying to put a finger on this one. One theory, called lateral inhibition theory, suggests that several photoreceptors in the eye send information to a retinal ganglion cell in the brain. As your brain interprets these signals, the most active brain cells inhibit and reduce the activity in neighboring cells, making them less excited. This creates an unequal black-white contrast.
4. The Cornsweet illusion
Lateral inhibition strikes again! Both panels are the same color. Just cover the fold with your finger to see it.
5. The Bezold Effect
Wilhelm von Bezold discovered that a color may appear darker depending on its context. In this picture, there’s only one shade of red, although the right side appears darker.
Scientists are still puzzled by this one. Some think lateral inhibition is to blame, although many disagree.
Optical Illusions and Your Brain
Evanston resident and Northwestern University assistant professor of cognitive psychology Steve Franconeri knows all the best optical illusions. His research focuses on visual cognition, or how the way we see affects the way we learn. He is leading a Science Cafe just for teens on Saturday, October 24, from 11:00 am—12:30 pm at Boocoo Cafe in Evanston. Science in Society asked him for a preview, and why he likes to participate in these kinds of events.
Steve Franconeri Why do optical illusions occur?
Your eyes generate a two-dimensional photo of the world, but your brain needs a three-dimensional understanding in order for you to act in the world. Your visual system is very good at figuring out the 3D world, but it’s also doing a lot of hard work that it hides from you. Computer vision researchers have been trying to replicate this process for a long time, and they’ve learned that it’s surprisingly tough.
So do we see the world as it really exists?
You don’t see the image of the world that hits your eyes. Your brain only lets you see its interpretation of that world. This interpretation is based on your past experience.
For example, you and I are sitting across from each other. You see an image of a man who is about six feet tall sitting several feet away from you. But a man who might be 12 feet tall sitting twice as far away would generate the same image on your eye. Your brain decides that it’s a six foot tall man sitting closer to you because you probably haven’t met any 12 foot tall humans.
Illusions trick you by using your own experience against you. Take the checkerboard example. (At this point, Professor Franconeri pulled up the image below on his computer.)
Squares A and B above are actually the same color. Don't believe it? Check out the proof on the right. (Both images courtesy of Edward H. Adelson.)
Look at the squares marked A and B. Do they seem the same color? Of course not - B is clearly brighter. Your brain knows that the cylinder is casting a shadow over square B, and so it believes that B must be brighter. But the two squares actually are the same color. In fact, if you cover up all the other parts of the picture, and look at only the squares marked A and B, you’ll see what I mean. If you count the photons, or the basic units of light, that both squares emit you would see they are exactly the same.
Why does the brain do this with the checkerboard?
Probably because color is more important to people than how many photons something emits. Real color, and not the number of photons, tells us more about, say, if a piece of fruit is ripe or rotten.
You’ve volunteered to conduct several Science Cafes, including one on optical illusions for adults last spring. What are they like?
It’s fun. The audience really engages with this topic. We go through lots of examples of optical illusions and the attendees offer hypotheses about them. Many people have an intuitive understanding of how the visual system works, so lots of people chime in with their theories. Intuition is right about half the time, and regardless, we get to the how and why in a fun manner.
Why do you volunteer to conduct Science Cafes?
One of the things I like about Science Cafes is that I get to talk to people outside my field about my work. It challenges me to think about my work differently and express myself more clearly. It’s very healthy and helpful.
And this upcoming Science Cafe is for young people, which I’m looking forward to. I always liked science as a kid, but I didn’t know what I wanted to do when I grew up. And I had very diverse interests -- I liked science, but I also liked movies and sports and art and lots of other things too. As a young person I was able to meet scientists and it helped me think about my options differently.
The simple explanation for why optical illusions leave us scratching our heads
The dress, the sneaker, and now the retro caravan — they are the modern-day optical illusions confounding millions around the world.
It's been more than six years since a photo of 'that dress' shocked the internet .
It was posted to Tumblr on 26 February 2015 by Caitlin McNeill, a member of a Scottish folk group, Canach, who was performing at a friend's wedding.
The dress belonged to the bride's mother and is blue with black stripes. Yet millions of people saw something different in McNeill's photo.
The dress in two different types of light (left) and the original photo (right) which caused so much debate. (Reddit/Barbarellaf)
In the first week, the dress was talked about in more than 10 million tweets on Twitter, with wide disagreement about its colour.
The hashtags #whiteandgold and #blackandblue quickly trended.
In 2019, Hollywood star Will Smith posted a photo of a sneaker to his Instagram.
"What do you see?" he asked his then-51.8 million followers.
Like the dress, all was not what it seemed. For some people, the sneaker appeared white and pink, for others it was green and grey.
The sneaker debate has been raging since 2017, when celebrity Will Smith reposted it. (@dolansmalik/Twitter)
Last week Melbourne man, Spencer Porter, took to TikTok to show off his retro caravan . Like the sneaker, some people saw the caravan as painted pink and white, others white and blue.
What is an optical illusion?
They happen when the human brain and eyes try to speak to each other in simple language but the interpretation "gets a bit mixed up".
This is the thinking from the Queensland Brain Institute.
"The colour of an orange, the size of a chair, how far away the door is – your brain knows all these things because the eyes told it so in simple language," Queensland Brain Institute's Cedric van den Berg wrote .
Duck or rabbit? Joseph Jastrow discovered this painting in 1900, but while it's not known who drew it, its an early version of a classic illusion. (Supplied)
"But your brain also has to ɿill in the blanks', meaning it has to make some guesses based on the simple clues from the eyes."
Does left brain, right brain dominance play a part?
Well, it seems it's perhaps all a myth.
Research by Harvard found the idea people are "left-brained" or "right-brained" may not be as fixed as first thought. It may just be down to personality and thinking style.
"Those who are right-brained are supposed to be intuitive and creative free thinkers," Harvard Health Publishing's Robert H Shmerling MD wrote .
"Meanwhile, left-brained people tend to be more quantitative and analytical. They pay attention to details and are ruled by logic."
Queensland Brain Institute says no one side of the brain is used more than the other.
A picture of four men sharing a drink while on a bushwalk has left readers scratching their heads. (Supplied) On closer inspection, you can see the fourth man is wearing cammo gear that completely obscures him from view. (Supplied)
So, why do people see different images, colours?
As scientists from Cambridge University found, the brain can be easily fooled.
They found, for instance, if there's a black spot on the left of a screen that fades while a black spot appears on the right, humans will 'see' the spot moving from left to right.
But, if the spot that appears on the right is white on a dark background, humans 'see' the spot moving from right to left.
Others believe it comes down to brain activity.
A study in CORTEX , an international science journal, found "those who perceive The Dress as white/gold have higher activation in response to the image of The Dress in brain regions critically involved in higher cognition (frontal and parietal brain areas.)"
These Optical Illusions Trick Your Brain With Science
To revist this article, visit My Profile, then View saved stories.
To revist this article, visit My Profile, then View saved stories.
Kids love being tricked. I know this because over 1,000 UK schoolchildren just voted a book about optical illusions as the winner of this year's Royal Society Young People's Book Prize for science.
Written by Clive Gifford, Eye Benders: The Science of Seeing and Believing explains how your brain sees colors, sizes, shapes, and even movements that do not actually exist. "You cannot explain how many optical illusions work without giving the reader an idea of the brain’s structure and performance," Gifford wrote in The Guardian. And while we still have a lot to learn about how the brain works, Gifford takes care to point out the theories that best explain how we get fooled.
Some of the oldest illusions use simple colors and shapes to trick our sense of scale and perspective. We see equally-sized circles that seem disproportionate, parallel lines that appear to converge, and staircases that never end. Others, like the elephant with the disjoined legs in the gallery above, likely confuse our proclivity to see patterns and fill visual gaps. Illusions that appear to move are perhaps the most interesting. Our brain is continuously rescanning the things it sees, "Like a twitchy digital camera continually autofocusing and adjusting the eye’s lens," says Gifford. Deployed correctly, color contrasts and sequential shapes could trick the scanning process, causing images that come alive as twisting tie-dyes, tumbling leaves, and spinning pinwheels.
Gifford is a journalist and author who has written a staggering number of books, from choose-your-own-adventures, to computer manuals. The Royal Society's Young People's Book Prize recognizes science writing aimed at kids, and is judged by children under age 14.
The reason why not everyone can see an optical illusion
WE’VE all been there. Staring at the computer screen, struggling to decipher the optical illusion in front of us. Science can explain why it’s so hard.
Spot the panda. Picture: Gergely Dudás Source:Supplied
THEY are the optical illusions and ‘Where’s Wally’ type puzzles that have everyone peering at their screen.
If you’ve spotted the Giant Panda hiding within a sea of zigzags, maybe it’s time to move on to the latest mind game — which is finding the 𠆌’ that’s surrounded by hundreds of circles, or perhaps the cat hiding in the kitchen.
Can you see the ‘C’? Picture: Facebook. Source:Supplied
There’s a bear in there — Can you spot the panda hiding among all these snowmen in this festive ‘Where’s Wally?’ style game? Picture: Gergely Dudás. Source:Supplied
Found it yet? If you’re having trouble, you’re not alone.
Associate Professor Paul E Dux, from the School of Psychology at the University of Queensland, said the difficulty with locating an image like the 𠆌’ among the circles, or the panda hiding among the snowmen is because some people have stronger ‘special attention’ than others.
“Panda among the snowmen is really focusing on visual search, which is a major topic of investigation,” Professor Dux told news.com.au
“This area focuses on how people search through space and a visual environment. Basically, you present stimuli and you’re finding a target in a mass of distracting objects.
“The reason why some people find it and others don’t is because some have better ‘spatial attention’ than others,” he continued.
“The other reason is that people sometimes just get lucky, and the eyes fall on the target immediately. Finding an image like the panda within the snowmen, or the C between the circles shows how we distribute attention across space. When there’s distracters among the target it’s harder to see.”
The panda and snowman image, which was liked by 42,000 people and shared over 100,000 times within days, left many people failing to find the panda at all.
“The panda image here shows how difficult it is to find one slightly different item among other quite similar items,” he said.
“The Panda and Snowman image is similar to if you’re looking for coriander amoongst an aisle of other greens. That’s what we call an inefficient visual search, which is why people struggle to locate the panda.
There’s a reason why some people can you see the panda, and others cannot. Picture: Ilja Klemencov. Source:Supplied
Earlier this week, ‘They can Disappear’ was revealed by Russian-artist Ilja Klemencov, which revealed a giant panda hidden behinds a mass of black and white zigzagged lines.
While the panda can be easily seen by some observers, others struggled to see the black and white bear. For those having trouble seeing the panda, the advice is to take a step back from the screen and squint. Better?
Professor Barton Anderson, from the University of Sydney, said no matter what the excuse, everyone can see the panda behind the zigzagged lines.
𠇊ll people can see these images,” Professor Anderson told news.com.au.
“If they say that they can’t, just tell them to view the image from a distance, or to blur their eyes by squinting, taking off their glasses, or putting on someone else’s glasses to induce blur.
“The eyes are actually made of photoreceptors and neurons, and are best considered as pieces of 𠆎xtruded’ brain. Some of the processing is done in the eyes, and some (a lot more) is done in the brain.”
Just a bunch of bars right? Shake your head from side to side, what do you see? Jesus perhaps? Picture: Supplied. Source:Supplied
Every day sees a new mind puzzle trending on social media sending many of us cross-eyed and even a little crazy if the illusion can’t be seen or solved.
The trick of these visual illusions is the relationship between our eyes and brain.
According to Inside Science, the way some people notice an image in an illusion quicker than others, boils down to picture 𠇏ooling” the brain and taking advantage of “shortcuts.”
“When you look at something, what you’re really seeing is the light that bounced off of it and entered your eye, which converts the light into electrical impulses that your brain can turn into an image you can use,” science filmmaker Kirk Zamieroski wrote.
“This process takes about a tenth of a second, but your eyes receive a constant stream of light, an incredible amount of information, so it’s really difficult for your brain to try to focus on everything at once.
“It would be like trying to take a sip of water from a fire hose. So your brain takes shortcuts, simplifying what you see to help you concentrate on what’s important, which helps compensate for your brain’s tenth-of-a-second processing lag.
“Optical illusions fool our brains by taking advantage of these kinds of shortcuts,” he added.
Classic illusion — can you spot the young woman and the old woman? Picture: Supplied. Source:Supplied
The real boom of optical illusions began in the 19th century, when a school of scientists who studied perception created simple illusions that shed light on to how the brain perceives patterns and shapes. According to Melissa Hogenboom from BBC Future, this study is what “kickstarted the early theories on how our eyes can play tricks on our mind.”
The 20th century saw little in the way of breakthrough in the field of illusions. It was the early 2000s that saw illusions make a real comeback.
“One school of thought suggests that some illusions highlight the way the brain constantly tries to predict what will happen,” Ms Hogenboom wrote.
“The theory goes that many illusions show that we try to predict the future to compensate for the slight delay between an event and our conscious perception of it.
“The light from these words you are reading has to reach your eye, before a signal travels to the brain to be processed — this takes time, which means the world you perceive is slightly in the past.
Not all optical illusions trick our brain into seeing motion. Some can also trick our brains into perceiving colours or shades that aren’t visibly present.
A study that looked at the Necker Cube revealed that the brain can actually flip between two different views. Take the Young woman/Old woman illusion below, for example.
The Hermann grid, which is over 100 years old, is another illusion that sees little grey dots appear between black squares, even though they aren’t actually there. Focus on one of the apparent dots, and it goes away. One explanation as to why this happens is because our “neurons are competing with one another to see the light and dark parts of the image.”
Hope we haven’t sent you cross-eyed!
The Herman Grid leaves people seeing grey dots at each intersection. Picture: Supplied. Source:Supplied
Making Sense of the Hermann Grid Illusion
When viewing the Hermann Grid, you will probably notice the faint dark spots that appear at the intersections of the white lines. But why do they appear? And why do they disappear as soon as you look directly at them? Both answers lie in how the retina converts visual stimuli into electrical impulses.
Posterior neurons convert light stimuli into electrochemical messages that are sent to anterior neurons. The most anterior cells are called ganglion cells. Each ganglion cell receives information from many cells and must decipher what is important and how it will transmit that information. This communication results in unique organization of the ganglion cell known as Center/Surround.
The center/surround organization of ganglion cells explain why the iridescent dark spots appear.
The receptive field of the ganglion cell is depicted here. In this figure, the pink represents inputs that are stimulated by light, while blue represents inputs that are not stimulated The cell is excited by light that falls in the center and inhibited by light falling in the surround. Each plus and minus represents an input from a posterior neuron.
Ganglion 1 has ten of 16 inputs exposed to light. Eight inputs of those stimulated are excitatory, & two of those stimulated are inhibitory. Two excitatory inputs are ‘canceled out’ by two inhibitory inputs, resulting in a net of six stimulated excitatory inputs. The lines at non-intersection points appear bright because there is relatively high excitatory stimulation.
Ganglion 2 is not exposed to light at all. The center is not excited and the surround is not suppressed. Because no input is stimulated, the black background appears dark. Ganglion 3 has twelve of sixteen inputs exposed to light. Eight of those stimulated inputs are excitatory however, four are inhibitory. There is a net of four excitatory inputs stimulated. The intersections appear darker than the lines because there is less net excitatory stimulation.
But why do the dark spots disappear as soon as you look directly at them?
Our central vision is sharp and clear, allowing us to resolve details with great accuracy. Ganglion cells close to the fovea have a very small receptive field, with fewer inhibitory inputs. Therefore, at the fovea, there is less inhibition of the center by the surround, and the dark spots disappear.