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The eyes, and the brain, can fall prey to a range of optical illusions, for a variety of reasons.
One such illusion is known as the Thatcher Effect. Named for former British Prime Minister Margaret Thatcher, since it was a photo of her that was famously used to illustrate the illusion, shows that the brain cannot properly process a photo of a face which is upside-down.
It demonstrates this by modifying upside down faces, flipping the mouth and eyes right side up. Because of this, and how our minds have learned to recognize faces, the brain believes nothing is wrong...until the image is turned right side up. Only at that point does the brain finally realize that yes, something was indeed very wrong with the image.
The Thatcher Effect demonstrates the importance of the brain’s processing of visual information, and the fact that recognizing faces is something we learn to do as our vision develops.
The bar in the center of the image above appears gradated, changing from light to dark gray in the opposite direction as the background gradient.
Have you guessed the illusion yet? Perhaps you have. This is a case of your brain fooling itself. If you cover everything in the image apart from the bar, you’ll confirm that it is actually monochrome.
This illusion is caused by the brain interpreting the ends of the bar as being in different lighting, and from that determines what it thinks the bar would look like if evenly lit. Through this process, it decides that the left end of the bar is a light gray object in dim lighting, while the right is a darker gray that is well lit.
(Image credit: Fibonacci | Creative Commons)
This mind-bending illusion, first discovered in 1861 by German physiologist Ewald Hering, makes it look like the two straight red lines in fact bow outward. According to Hering, the reason for this illusion is that when we see the red lines crossing over the blue ones that radiate outward, our brains overestimate the angle made at the points where the red lines intersect the radiating ones.
Current research hypothesizes that the reason for this miscalculation is that since there is a delay between the time light hits the retina to when the brain perceives it, we evolved to compensate by generating images of what we think will occur one-tenth of a second in the future.
So in this case, that predicting leads to the overestimation of the angles, which makes the red lines look bent when they are truly straight. (And if you don’t believe it, cover up the blue lines and take another look at either of the red ones!)
On this checkerboard, the squares labeled A & B appear to be very different shades of gray, right? However, when we look at the board again, this time with solid gray lines running along both squares, we can see that they were the same shade all along!
This illusion, published by Edward H. Adelson, Professor of Vision Science at MIT in 1995, demonstrates how our visual system deals with shadows. When we try to determine the precise color of something, our brain knows that shadows can be misleading, in that they can make a surface look darker than it truly is. So the brain compensates by interpreting the shadowed surfaces as being lighter, even though it’s the exact same color as an unshadowed object which appears, at first glance, to be much darker.
(Image credit: Akiyoshi Kitaoka)
As hard as it might be to believe, nothing in this image is moving.
Unfortunately, there isn’t too much more we can tell you about this mind-boggling illusion.
To this day, there is no solid explanation for illusory motion. Some experts think it is linked to something called fixation jitter, involuntary eye movements which present the illusion that objects close to something you’re fixated on are moving.
Others believe that as you glance around the image, the motion detectors in your visual cortex get “confused” by dynamical changes in neurons, and thus believe that you are, in fact, seeing movement.
Illusions like this one serve to illustrate to us that while we know much, much more about vision than we did in the past, there is always more to learn!
These are just a few of the many fascinating optical illusions out there; demonstrations of just how fascinating our visual systems truly are. 
Chances are you’ve seen this one somewhere online. In this illusion, you see the silhouette of a dancer, spinning in place. The illusion involves the direction in which you see it spins. Sometimes it appears to rotate in a clockwise direction, and at other times, in a counterclockwise direction. The director in which the dancer spins can potentially be changed at will by the viewer, or it may appear to change direction on its own.
The spinning dancer illusion, created by Nobuyuki Kayahara in 2003, has since become one of the more well known optical illusions. The reason for the illusion is that the lack of visual depth in the animation, and ambiguity regarding the dancer’s anatomy are too ambiguous for our visual systems to process properly, so we can perceive the spinning dancer in differing, even conflicting states.
Another optical illusion you are likely to have run into before, the Rubin’s vase illusion, created by Edgar Rubin in 1915, is among the most famous optical illusions in the world. In the image, we see what can alternatively be a vase or two faces in profile, facing each other.
This illusion is explained through the concept of figure-ground organization. This lets us perceive objects both as figures and backgrounds. In this illusion, however, what we see alters with a change of perspective. If the black area is seen as the background, the vase becomes the figure. Conversely, when we see the white area as the background, the faces become the figure.
Yet another illusion you’re likely to have run into before, this was created by Franz Carl Müller in 1889, and has become extremely recognizable.
In this illusion, you see three horizontal lines, each with differently configured arrowheads on the ends of the line. While the lines may look to be all of different lengths, they are, in fact, all equally long.
Interestingly, some studies show that Western individuals are more susceptible to this illusion, since they are more used to “carpentered” surroundings, meaning that they live and work in areas where straight lines and right angles are commonplace. One explanation for how this illusion works is that the differently aligned arrows make the lines look to be of different lengths. Inward pointing arrows make an object appear closer, while outward facing arrows appear further away. Since the lines are presented side by side, we perceive the “further away” line as longer.
Another illusion which plays with our understanding of perspective, the Ebbinghaus illusion, also known as the Titchener Circles, was discovered by Hermann Ebbinhaus in the 19th century. This illusion challenges your perception of size. In the more common version of the illusion, created by Edward B. Titchener, we see two equally sized circles, one of which is surrounded by larger circles, and another surrounded by a ring of smaller circles.
Even though the circles are the same size, the one surrounded by bigger circles appears smaller than the one surrounded by smaller circles. The reason for this is believed to be related to how we perceive size, specifically, on the context involved. With the changed context in which we see the circles, our perception of their size also changes.
This optical illusion is a famous example of the concept of illusory contours. That refers to our perception of an edge or an outline where there isn’t one. This perception is created by different shapes and edges being presented together and arranged in a way that implies the presence of defined contours or edges.
In Kanizsa’s Triangle itself, the three incomplete black circles and open angles generate the illusion of a white triangle. As is usually the case with this type of illusion, the illusory shape appears both closer to the viewer, and brighter.
It works because the incomplete circles trigger our depth perception, causing our visual system to believe the dark shapes are further away and darker than the apparent triangle.
This illusion was first made in 1892, and has been fascinating people to this day.
The image we see can be alternatively viewed as a duck facing left, or a rabbit facing right, and the illusion operates on the concepts of how our visual system perceives ambiguous images, and the process of mid-level vision.
Mid-level vision is how our brains organize visual information based on the perceived edges of the image. With ambiguous images, the edges are unclear, and so we can perceive two contradictory versions of the same image depending on how we look at it.
There are many more fascinating optical illusions out there which can help us gain a better understanding of our visual system. And, of course, they’re just plain cool to look at. 
In the above image, the blakc heart looks like it’s constantly expanding! However, it never seems to get bigger, and it’s not a looped gif, so what’s going on?
This illusion plays on the fact that our eyes are constantly moving (if you don’t believe us, ask someone to track your eye movements for a bit.)
If we simply saw everything in real time, since our eyes constantly move, we’d have a hard time seeing anything comprehensible. To compensate for this, the brain “edits” all this compiled input into an image we can properly see, similar to how a good movie editor can seamlessly edit scenes together. Our brains also rely on the context of an object to effectively determine what it sees.
In the illusion here, the lined background is used by the brain as a reference point to orient the heart. Between the lines, and our eyes’ constant movement, the heart thus appears to be expanding.
Another image that provides the illusion of movement, the spirals above appear to be slowly spinning by taking advantage of a phenomena known as apparent motion.
It takes one tenth of a second for signals from the retina to reach the brain, and if there is more contrast in what the brain is seeing, the faster the transmission. (For example, a higher contrast signal arrives one twentieth of a second faster than a low-contrast one.) So in this illusion, the contrast gradients are arranged in a way that tricks the brain into believing there is motion, as the high contrast parts of the image arrive faster than the rest.
There are several different images that illustrate this phenomenon, but they all fall into the same general group of impossible shapes. The image depicts an object which, at first glance, appears realistic, but upon closer inspection, its mind twisting nature becomes clear as we realize that this shape could never exist in a real, 3D form.
This illusion is related to the Gestalt laws, which describe how we see and interpret the world around us, with everything being parts of a single image. So we see, in complex scenes, objects against a background, and those objects themselves are made of parts, which in turn are made of smaller parts.
According to one of these Gestalt laws, when we see ambiguous or complex objects, the brain tries to make them look as simple as it can. That is why our brain tries to ignore the impossibility of this shape, enabling us to process the image, and once we force it to acknowledge the impossibility of the shape, it becomes an eye-twisting anomaly.
There is always more to be learned about the visual system, and illusions like these always demonstrate that what we see might not always be what’s actually in front of us.
Cognitive illusions generally show an ambiguous image, and they are caused by a phenomenon known as unconscious inferences, which are made by the brain when looking at ambiguous images. Specifically, with these illusions, the brain infers the presence of an object which doesn’t actually exist.
In one well known example of a cognitive illusion, known as Kanizsa’s Triangle, we perceive a white triangle inside the partial black circles. This is due to the positioning of the pac-man shaped black circles. It looks like a triangle fits in there, and the contrast also implies a bright white triangle, so our brain takes the extra step by actually “seeing” it, inferring its presence despite it not truly being there.
This type of illusion occurs when our eyes take in an excessive amount of visual stimuli for a period of time, and it has an effect on our eyes or brain. Brightness, color, or light flashes can all cause such illusions.
We can illustrate this with the image below. Stare at the image for about 15-20 seconds, then look over at the blank white space while blinking. You’ll see an image there now!
Since we spent a significant amount of time exposing our eyes to the initial images, the photoreceptor cells in the retinas keep sending neural impulses based on that to the brain even once we’ve stopped looking at the image, resulting in what we see in the blank space, known as an afterimage.
You may have experienced something similar if you’ve ever looked at the sun for a moment (try not to do that), or looked at some other very bright light. For a short time afterward, while blinking, you’ll see an object in the shape of that bright light, because your eyes have just been overexposed.
Literal optical illusions are when our eyes perceive an image, and our mind fills in gaps in the image which don’t exist. This leads to the creation of an image different from the objects which make up that image, or it focuses in on certain areas of the image, leading to us perceiving something that isn’t truly there.
In the image here, it looks like the elephant has way too many legs. This is because our eyes rely on the edges of objects to distinguish it, and in the example, the shading and especially the lines that make up the elephant confuse our eyes.
This phenomenon is also what leads us to “see” faces in everyday objects, since we’ve also learned to recognize faces, so our brains tend to find them even when they might not actually be present.
These, and other illusions, demonstrate that our brain plays a major role in vision, as it interprets the input that comes from the eyes based on how it’s learned to understand and organize the world around us. And since this is almost entirely done unconsciously, it is susceptible to being fooled or confused when our conscious examination of something conflicts with the brain’s initial assessment.
Unlike some of our senses, vision is very much learned, and studying and experiencing these illusions helps us better understand that process, along with our visual system as a whole. 
Optical illusions are one of the best reminders that vision is a system made up of more than just the eyes. It’s the brain that organizes the input it receives and determines what we see. Optical illusions take advantage of how our visual system works to create the mind-bending visuals we’ve all run into at some point.
Here is a look into another couple optical illusions
This illusion was discovered by Charles Chubb and his colleagues Sperling and Solomon in the late 1980s, during experiments with perceived contrast. In the image here, the gray circle on the simple gray background and the gray circle on the black and white, textured background, are identical, despite appearing to us as different shades of gray.
This can be explained through the concept of imperfect transmittance, which is where the brain has to see through ambiguity in order to perceive an object. (More practical examples are when we look at something from a distance or through fog.) When there is minimal light around the object, the brain attempts to determine its color and contrast by making an educated guess based on the background around the object.
As is the case here, that interpretation isn’t always correct.
Take a look at the image here. Does the black line look like it lines up with the blue line? The truth is, the black line lines up with the red one, as the second image reveals.
This illusion is named for Johann Poggendorff, the German physicist who first detailed this illusion in 1860. While it is clearly a demonstration of how our minds perceive depth and geometric shapes, there is no universally accepted explanation for it.
The most widely accepted theory, however, is that in this illusion our brain is trying to interpret the two dimensional image with three dimensional properties, and in the process it’s distorting the depth between the lines.
The Shepard’s Table illusion was first created by American psychologist Robert Shepard in a book he published in 1990, and provides yet more evidence that our visual system is primarily influenced by our lived experiences with the world around us, to the point where it can mess with our view of reality.
In the image here, the two tables look to be of very different size and shape. In actuality, they are exactly the same! This animated illustration can prove it to you.
This is another illusion caused by the fact that our brain tries to interpret 2D images in the same way it interprets 3D ones. It perceives different sizes because of perspective foreshortening, which means that the closer an object is, the larger it appears on the retina.
Optical illusions are endlessly fascinating, both because they’re simply interesting to look at, and because they can teach us a lot about the visual system. Go explore some more optical illusions! 
Behavioral optometry employs an integrated approach to treatment that views the individual as more than a refractive error, a patient, or visual issue. Extending traditional eyecare beyond 20/20 vision. What is Behavioral Optometry? We are more than just our eyes. However, some of us may experience vision based problems. When most people think of eye-related […]
Does this person look normal to you? How about now? The eyes, and the brain, can fall prey to a range of optical illusions, for a variety of reasons. One such illusion is known as the Thatcher Effect. Named for former British Prime Minister Margaret Thatcher, since it was a photo of her that was […]
Optical illusions work by exploiting the disparity between what the eyes see and how the brain perceives that visual input. In doing so, they demonstrate that our visual systems “edit” what we see without us even being aware of it, as it decides what is worth paying attention to. Even before humans understood the visual […]
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