SoundEagle would like to invite all and sundry to contemplate the following queries:
👁 Is seeing always believing? ❇️
👁🗨 Is viewing indeed perceiving? ✳️
👀 How robust is human vision? ❇️
😵 How tricky is optical illusion? ✳️
A person naturally possesses many different types of receptors called sensory neurons, which are specialized cells capable of detecting light, sound, smell, taste, temperature, pressure, pain or balance by transmitting sensory signals or stimuli to the brain, which interprets or makes sense of the stimulations to produce perception, which “is the organization, identification, and interpretation of sensory information [by the brain] in order to represent and understand the presented information, or the environment.” An illusion is a distortion of the senses resulting from the misidentification or misinterpretation of the sensory information.
Of all the illusions that accompany or plague the human senses, optical illusions, also known as visual illusions, are the most common and best understood, given that any human possessing complete and healthy senses is visually oriented by nature, and that visual information is critical in everyday activities such as posture, movement and communication. Some performance arts exploit and convey visually encoded human experiences to such an effective degree that optical illusions have become the lingua franca of certain artforms. For example, prodigiously proficient in the theatrical technique of suggesting action, character or emotion without words or speech, professional mime artists can seamlessly engage and entertain audiences as they convincingly titillate the human imagination using only gesture, expression and movement, transforming what is ostensibly a featherweight balloon into a cumbersome object with enough inertia to resist strenuous pulling and pushing:
The best mime I've ever seen! https://t.co/8niRo9xuy5—
Optical Illusion (@Way_Things_Work) January 07, 2017
By the same token, illusionists have all along tricked audiences into believing that their performances are the results of magical rendering rather than the adept manipulations of human perceptual habits and cognitive limits via the deployment of adroit staging, clever lighting, sleight of hand, masterful setups, ingenious contraptions and psychological controls. Yet, even in the complete absence of magic tricks, the human eye and perceptual device can colour, distort and misinterpret what we see anytime and anywhere, sometimes in astonishing degrees and unexpected fashions, arousing much curiosity and deserving further investigation. It is no wonder that some undergraduate psychology textbooks published as early as the late 1980s feature optical illusions as the first topic of discussion in the first chapter. Beginning in the early 20th century, optical illusions have contributed to scientists’ understanding of the perceptual system in general and the visual system in particular. Experimental investigations into the psychological phenomena and physiological aspects of optical illusions have revealed that such illusions are not so much the side effects or (by)products of vision itself but how visual signals are processed and interpreted by the brain.
One cannot escape from experiencing optical illusions in everyday life because the perceptual system of the brain is so busily occupied with the real-time processing of incoming signals from the eyes that it constantly relies on shorthand representations and “spends cognitive energy as effectively as possible, using assumptions about visual information, to create a tailored, edited vision of the world”, according to the following educational video explaining “how optical illusions trick your brain”:
Why optical illusions trick your brain is approached from an evolutionary perspective in the following demonstrative video entitled “Why do we see illusions?”, recognizing that the brain has evolved to deal with natural stimuli in motion, which often leave optic flow in the form of trails or blurred lines in the visual field. Under this scenario, optical illusions are the results of the compensatory mechanisms of the visual system for dealing with neural delays averaging 100 milliseconds, between the moment when any visual cue first enters the eyes and when it is processed by the brain, which compensates for the delays by projecting images 0.1 second into the future. According to Mark Changizi, a theoretical cognitive scientist and neurobiologist, this exaggerated extrapolation of certain cues allows the viewer to perceive the future in order to perceive the present, at the expense of causing the viewer to intuit motionlike characteristics to static images. Considering the ubiquity, persistence and permanence of such optical illusions, they are indeed perceptual trade-offs universally favourable to human survival, as much as they can improve the odds of catching or hitting a thrown ball. Such illusions of movement and curvature reveal how the visual system constantly anticipates what will happen and dynamically predicts the future to counteract or offset the slight delay between a visual cue about to be processed by the brain and the ensuing conscious perception of the cue. In other words, since the brain requires one-tenth of a second to translate the visual cue into a visual perception of the world, the visual system has evolved an automated strategy to compensate for neural delays by generating images of what will happen 0.1 second into the future. Such neural foresight allows humans to react to events in the present, especially with respect to fast, action-oriented, reflexive acts requiring speed and precision, such as playing sports and manoeuvring smoothly through a crowd. However, this neural foresight is also the source of many unintended visual side-effects, given that it can readily generate a large variety of optical illusions under certain conditions.
The illusion of depth and motion (such as contraction, expansion, undulation, pulsation and/or rotation) can be best illustrated by optical art (also called op art), a subset of visual art deploying optical illusions in their subject matters or design principles. Explaining “what happens when your eyes and brain don’t agree” in the Smithsonian magazine of the Smithsonian Institution on 22nd August 2013, an optical artist named Gianni Sarcone uses his own exemplarily designed graphic showing a seemingly pulsating rosette to demonstrate the confluence of several illusory effects as follows:
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.…
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.
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.
Two days later, the comment of a reader named David Navarrete connects the artful creations and scientific discussions of optical art to the therapeutic deployments of static virtual reality for increasing or enhancing wellness-inducing properties of the indoor or built environment, such that the sense of motion, depth and realism of certain surfaces or imageries can be imparted by the optimum use of optical illusions and design principles, as informed and united by the biophilia hypothesis, which propounds that humans, being innately affiliated with Nature throughout the evolution of the upright species, have various instinctual proclivities to establish connections with Nature and other lifeforms:
Using similar principles, the art of illusion has also been demonstrated to help patients heal faster in hospital settings and improve productivity in the workplace.
Rather than using patterns to create the illusion of movement, one can use high resolution digital photography and the principles of composition to make unique sky compositions that, when back-lit using the proper color light temperature (6500 Kelvin high CRI), give the illusion of depth. In other words, virtual skylights.
These virtual skylights or biophilic illusions of nature, can reproduce the subtle hues and saturation of real skies. When these virtual skylights are installed in enclosed hospital rooms and treatment suites, conference rooms or any environment with little or no access to the outdoors, they create the illusion of depth.
These illusions of real sky provide quantifiable improvements in cognitive function, mitigating stress and fatigue, and triggering a relaxation response in observers who would otherwise become more lethargic when remaining in artificial and enclosed surroundings for long periods of time (the average work day).
The art of illusion is used extensively in healthcare settings to aid in healing and comfort patients by giving them a view to nature that creates biophilic engagement.
The art and science of illusion go way past the amusing, they are fascinating and very useful in enclosed architectural spaces.
In the two optical arts devised here by SoundEagle, the optical illusion of straight rectangle edges appearing to be curved is due to the visual perception of shadowing. In addition, the two identical faces appear to be different in size relative to the respective rectangles framing the faces.
There are several ways to understand the gist of optical illusion in a mere sentence. According to Wikipedia, it is “an illusion caused by the visual system and characterized by visually perceived images that differ from objective reality.” In other words, an optical illusion arises when there are anormalies, discrepancies or deviations in what is being seen or perceived, in contrast with, or as opposed to, what is actually shown or present(ed) in the objective reality:
The information gathered by the eye is processed in the brain to give a percept that does not tally with a physical measurement of the stimulus source. There are three main types: literal optical illusions that create images that are different from the objects that make them, physiological illusions that are the effects of excessive stimulation of a specific type (brightness, color, size, position, tilt, movement), and cognitive illusions, the result of unconscious inferences. Pathological visual illusions arise from a pathological exaggeration in physiological visual perception mechanisms causing the aforementioned types of illusions.
At Newopticalillusions.com, the resource website created by a group of university students to host the largest collection of optical illusions sorted in various categories, an optical illusion is defined as “any illusion that deceives the human visual system into perceiving something that is not present or incorrectly perceiving what is present.” Another definition of optical illusions is given as “the dissociation between how an object or an event looks physically and how it gets perceived subjectively.” The definition is embedded within a lengthy explanatory essay entitled “What is an Optical Illusion?”, in which the readers are provided detailed summaries of the following areas of discussion:
- The representational gaps between the human perceptual system and the reality.
- The links between optical illusions and cognitive or perceptual illusions, as well as certain mental illnesses and mind-altering substances.
- The explorative works of scientists and the creative endeavours of visual artists based on the concepts of optical illusions as transformative agents, phenomenological tools and experimental media.
The abovementioned essay is quoted here as follows:
Optical illusion is also called visual illusion. In this illusion, a person perceives images in a very different way than in reality. The eyes gather information and send it to the brain. The perception of the object for which the information is sent to the brain does not match with the physical attributes of the same object. Optical illusions can happen naturally or could be demonstrated by using some visual tricks which show some specific assumptions in the human perceptual system. Many of us have experienced or heard about mirage in desert areas which is a very good example of natural illusion and is an optical phenomenon. The size of the Moon varies for example it looks smaller when it is over the head and it looks larger when it is near the horizon. This is also an example of natural illusion but it is not an optical phenomenon but it could be categorized as cognitive or perceptual illusion.
The Herman grid or Necker tube is the example of Developed or discovered illusions. When we learn and understand these types of optical illusions, it becomes easier for us to understand the limitations of the visual system of the human beings. Sometimes people do get afterimages when they are exposed to bright lights then different types of physiological illusions are experienced. Another type of optical illusion which is quite well known and interesting is the Cognitive illusions. In this there is interaction between different levels of perceptual processing and the knowledge which is in-built gets misdirected. It is further classified into different subtypes. When the pictures or objects get switched between the alternative interpretations they show ambiguous illusions. Rubin vase or Necker cube are good examples for this type of illusion. When the objects are represented with distortions of length, position, size and curvature they create Geometrical-optical or distorting illusion. The good examples are Café Wall illusion, Ponzo illusion. Penrose triangle or impossible staircases are the example of Paradox illusions. Sometimes optical illusions are introduced due to schizophrenia or hallucinogenic medicines where a person hallucinates about an object which is not present there.
One of the tools which the neuroscientists use to understand the working of the brain to study how the brain has the sense of reality created is the optical illusion or the visual illusion. Earlier many visual artists and the illusionists used the optical illusions to create deep insight into the internal workings of the visual system of humans. Artists have been successfully used the techniques to fool the brain to make it think a flat canvas to be a three-dimensional one long before scientists finished studying the properties of neurons. Optical illusions are defined as the dissociation between how an object or an event looks physically and how it gets perceived subjectively. WE a person experience visual illusion, the person may see a thing which is not there or the person may not see a thing which is there. The person may see a thing in an entirely different way also. As there is a disconnection between the perception of an object or image and the reality, optical illusions are able to demonstrate the way in which the brain can fail to recreate the physical world. When all these failings are studied thoroughly, we are able to known more about the computational methods used by the brain to build visual experience.
There are many artists who are known to have worked with the concepts of optical illusions such as Bridget Riley, Salvador Dali, M.C.Escher, Marcel Duchamp, Victore Vasarely and Charles Allan Gilbert. Some modern artists who have tried out various experiments with the optical illusions are Ocatavio Ocampo, Shiegeo Fukuda, PAtrik Huges, Dick Termes, Rob Gonsalves, Ben Heine, Akiyoshi Kitaoka. In movies also, the concept of visual illusions have been used. There is a popular hypothesis about the Optical illusion called Cognitive processes hypothesis. It is considered as a framework to understand the optical illusions as the signature of the experimental statistical method of the evolution of vision to solve the inverse problems. Scientists and others have been working and experimenting on visual illusions a lot and all these will definitely help us to understand more about this interesting phenomenon of the human body.
In the other extended essay querying “How do Optical Illusions Work?”, the readers are exposed to the centrality of visual representations and cognitive interpretations arising from the complex mediations between the human perceptual system in the subjective realm and the physical reality in the objective world with respect to the pattern-matching and pattern-seeking mechanisms of the brain as well as the functional features of the eye, the aberrations or abnormalities of which result in optical illusions:
Everything that we see around us is the illusion only. What we see around us is actually not the way they are in reality. For us, the sky appears flat to us, the sun seems to move across the sky to us. All these are nothing but the optical illusions which our eyes have created. Generally Optical illusions can be attributed to the visual perceptions of ours or the intellectual interpretations of ours. According to the study the past experiences of a person, the desires and the influences from different persons or events are also responsible for the creation of the optical illusions.
Due to erroneous vision or mistaken judgments the visual illusions may take place. When there is an error in estimating the distance of an object and the viewer it is an example of judgment. Sometimes, a bright object appears larger than a dark object even if the dimensions are the same. When the objects having the contrasting colors are kept closer, one may mistake one color for the other. It is an error which happens during sensing the object. Our brain has the tendency to group things into four different types which are similarity, continuity, closer and proximity. The similar color objects are grouped together to get a particular shape. Our brain always searches for patterns. The brain tries to find the continuity in whatever it sees. When short lines kept at space are arranged in a circular way the brain perceive it as a circle. The objects kept closely are grouped together. One of the natural phenomena which is commonly talked about or can be seen in the nature is the formation of mirage. In case of mirage the light rays coming from an object bend and the object appears in a place where it is not exactly present. For some there seems to be no difference between the hallucination and optical illusion but there is a very minute different between the two. In case of hallucination, an object appears though it is not there and it is more of an individual’s experience.
According to Wundt, the laws which deal with the formation of retinal image and the eye movement are responsible for the optical illusions. When the light falls on an object ,the light gets reflected by the object and an inverted image of the object gets created on the retina in the eye. The brain then interprets the image and then we are able to perceive the object the way it is really present. When the perception does not match with the actual physical dimensions of the object , it is considered as the optical illusion. When a flat surface is represented by a shade of colors, it seems to be very deep. When an object has some specific geometrical structures these create an illusive effect on the viewer. When the eyes are raised in a horizontal plane, the effort involved is more than that of the effort needed to turn the eyes. This difference of effort makes the eyes to feel that the vertical distances are greater than that of horizontal one of the same magnitudes. The involuntary movements of the eyes cause angular movements.
The chromatic and spherical aberrations are caused by the lenses. Eyes have abnormalities such as eccentricity and astigmatism. All these aberrations or abnormalities lead to optical illusions. The wave lengths of different colors vary so their angle of reflection and refraction also vary. Due to these differences chromatic aberrations happen and as a result optical illusions like “advancing” or “Retiring” colors are caused. When one keeps the eyelids almost closed, because of the movement of the eyelids, objects seem to be moving. When one eye is closed, the distance and direction of an object cannot be judged accurately. The distance and direction of objects seem to vary depending on the concentration of the objects placed in the area. If a line is divided into smaller partitions then the partitions seem to appear shorter than a single line of the similar length as the partition. We can conclude by saying that optical illusion works based on the principles of light and the how the light is interpreted by the eyes.
The ensuing educational videos and exemplary images demonstrate not only the idiosyncrasies of sight but also the wonders of optical illusion, which are inherent aspects of the visual apparatus and cognitive dimension of human beings throughout their lives. These examples convincingly reveal that the very fabric of seeing the world through the perceptions of colour, form (or shape), edge, orientation, constancy, brightness, contrast, depth, perspective and motion can be readily altered, distorted or even compromised by certain cues or contexts, and by certain interactions with, or assumptions about, the world. They uncover numerous innate biases and compensations of the visual system, which are surprisingly robust and culture-invariant, quite independent of intelligence and personality, but predictably dependent on biological sensory structures within the human body, and on external conditions in the physical environment, insofar as individuals are socioculturally defined by, and draw behavioural meanings from, their interactions with(in) the environment, a complex ecology in which the human brain has evolved mechanisms for finding information, patterns and relationships associated with those behavioural meanings.
Whilst the visual faculty has been sufficiently robust and dependable in ensuring the survival of the human species, it does plays tricks with (the more rational parts of) the mind from time to time. Investigations into optical illusions have uncovered the patterns and limitations of the visual system, which has to deliver what we perceive to be seamless vision as efficiently as biologically possible with certain concessions over unavoidable sensory distortions, so that the neural networks and perceptual devices in our brain can process, filter, approximate and even anticipate visual cues to produce the most likely interpretation of what we see with our naked eyes. In that regard, seeing is certainly not always believing.
All in all, illusions are akin to special windows and intriguing corridors through which the intricate workings of the perceptual system can be discovered and gauged. They provide important insights into the nature of the sensory and cognitive processes entailed in normal perceptions, which are usually rather accurate even though they are invariably prone to distortions, which are themselves replete with context-dependent variability. Whilst such distortions in all their diversity can be very fascinating to behold by all and sundry, it is even more valuable to understand that the systematic inquiry into the normal functioning of the perceptual system by studying the conditions under which the system falls short or breaks down is a highly effective model or approach for investigating specific phenomena or pieces of evidence to identify empirical regularities and generalizations in the scientific fields, including behavioural and medical sciences. Such a model or approach generally entails the following steps:
- Observing the initial occurrence of the optical illusion.
- Formulating hypotheses, postulations or predictions about the fundamental aspects or essential elements of the illusion.
- Testing whether these aspects or elements impart illusory effect to laboratory subjects randomly chosen from a sample population.
- Postulating or modelling some mechanism to explain the effect.
- Iterating the whole process to accommodate new data or further evidence.
- Refining the theory or improving the accuracy of the explanation.
Those who are keen to learn more about the different types of optical illusion should peruse the 👁🗨 List of Optical Illusions 👁.
Go on. Stare at the red dot for ten seconds https://t.co/6MskruW2f9—
Optical Illusion (@Way_Things_Work) October 02, 2017
Triangle trippy illusion. https://t.co/aD2GqkjWPl—
Optical Illusion (@Way_Things_Work) September 25, 2017
My Modern Met (@mymodernmet) October 02, 2017
Perspective at its best https://t.co/Mz5YJdKvy3—
Optical Illusion (@Way_Things_Work) September 17, 2017
Cool drawing optical illusion https://t.co/FtLAWYSHKu—
Optical Illusion (@Way_Things_Work) September 16, 2017
Perspective is everything https://t.co/jQA2O3TwR5—
Optical Illusion (@Way_Things_Work) September 09, 2017
Illusion stairs bad day. https://t.co/8mVyszxaDI—
Optical Illusion (@Way_Things_Work) August 31, 2017
Put a fist in the centre of the gif. It speeds it up. https://t.co/OYfR3uv1XB—
Optical Illusion (@Way_Things_Work) August 30, 2017
All dots are running in a straight line. https://t.co/riLK8wtlXR—
Optical Illusion (@Way_Things_Work) August 10, 2017
Rt when you see a word. https://t.co/2Bml5NCJEn—
Optical Illusion (@Way_Things_Work) August 02, 2017
It’s not a parrot! It’s a woman covered in very meticulous body paint by artist Johannes Stoetter https://t.co/Vrc8k9CIGY—
Optical Illusion (@Way_Things_Work) June 14, 2017
Fix your eyes on this black dot and the grey stripe will turn blue. https://t.co/3hCUXiZOI1—
Optical Illusion (@Way_Things_Work) June 03, 2017
We may not be seeing what's actually there, all the time. https://t.co/POdmUfBwm9—
Optical Illusion (@Way_Things_Work) February 12, 2017
These rotating circles. Just don’t focus on one dot! https://t.co/dhVFe5jQn1—
Optical Illusion (@Way_Things_Work) January 27, 2017
Graph: Rabitness vs Duckness. https://t.co/q6Buxp0YH8—
Cliff Pickover (@pickover) September 23, 2017
Cliff Pickover (@pickover) September 22, 2017
|Afterimage illusion||An afterimage or ghost image is an optical illusion that refers to an image continuing to appear in one’s vision after the exposure to the original image has ceased.|
|Afterimage on empty shape (also known as colour dove illusion)||This type of illusions is designed to exploit graphical similarities. The effect is related to a class of effects referred to as contrast effects.|
|Ambiguous image||These are images that can form two separate pictures. For example, the image shown forms a rabbit and a duck. Ambiguous images or reversible figures are optical illusion images which exploit graphical similarities and other properties of visual system interpretation between two or more distinct image forms. These are famous for inducing the phenomenon of multistable perception, the occurrence of an image being able to provide multiple, although stable, perceptions.|
|Ames room illusion||An Ames room is a distorted room used to create an optical illusion, such that a person standing in one corner appears to the observer to be a giant, while a person standing in the other corner appears to be a dwarf. The illusion is so convincing that a person walking back and forth from the left corner to the right corner appears to grow or shrink.|
|Ames trapezoid window illusion||A window is formed in the shape of a trapezium. When observed, it appears to the human eye as a rectangular window, but is in fact a trapezoid. It is often hung and spun around to provide the illusion that the window rotates through less than 180 degrees.|
|Autokinetic effect||The autokinetic effect or autokinesis is a phenomenon of visual perception in which a stationary, small point of light in an otherwise dark or featureless environment appears to move.|
|Autostereogram||An autostereogram is a single-image stereogram (SIS), designed to create the visual illusion of a three-dimensional (3D) scene from a two-dimensional image in the human brain. An ASCII stereogram is an image that is formed using characters on a keyboard. Magic Eye is an autostereogram book series.|
|Barberpole illusion||A visual illusion that reveals biases in the processing of visual motion in the human brain. When a diagonally striped pole is rotated around its vertical axis (horizontally), it appears as though the stripes are moving downwards in the direction of its vertical axis rather than around it.|
|Benham’s top||Named after the English newspaper-man and toymaker Charles Benham, who in 1895 sold a top painted with the pattern shown. When the disk is spun, arcs of pale colour called Fechner colours or pattern-induced flicker colours (PIFCs) are visible at different places on the disk.|
|Beta movement||An optical illusion, first described by Max Wertheimer in 1912, whereby a series of static images on a screen creates the illusion of a smoothly flowing scene. This occurs when the frame rate is greater than 10 to 12 separate images per second. It might be considered similar to the effects of animation.|
|Bezold Effect||When small areas of colour are interspersed, an apparent change of tone of a colour due to the alteration of the colour of the background. A colour may appear different depending on its relation to adjacent colours. In the example, the red seems lighter combined with the white, and darker combined with the black.|
|Blivet||An impossible trident, also known as an impossible fork, a blivet, poiuyt or devil’s tuning fork, is an impossible image because in reality the shape cannot exist. It is a drawing of an impossible object (undecipherable figure), a kind of optical illusion. It appears to have three cylindrical prongs at one end, which then mysteriously transform into two rectangular prongs at the other end.|
|Café wall illusion||This geometrical-optical illusion is a pattern where different coloured squares on a wall appear to form horizontal curved lines. It is named such because this is the type of artwork often seen on café walls.|
|Catoptric cistula||A box with insides made of mirrors so as to distort, magnify or multiply images of objects put into the box. Also called a catoptric theatre or chest.|
|Checker shadow illusion||The checker shadow illusion shows that when a shadow is cast onto a checked board, the colours of squares A and B in the photos appear to be different, when in fact they are the same.|
|Chubb illusion||The Chubb illusion is an optical illusion or error in visual perception in which the apparent contrast of an object varies substantially to most viewers depending on its relative contrast to the field on which it is displayed.|
|Colour constancy||Colour constancy is an example of subjective constancy and a feature of the human colour perception system which ensures that the perceived colour of objects remains relatively constant under varying illumination conditions. A green apple for instance looks green to us at midday, when the main illumination is white sunlight, and also at sunset, when the main illumination is red.|
|Colour phi phenomenon||The colour phi phenomenon is a perceptual illusion in which a disembodied perception of motion is produced by a succession of still images.|
|Contingent aftereffect||An illusory percept that is apparent on a test stimulus after exposure to an induction stimulus for an extended period.|
|Convergence micropsia||A type of micropsia characterized by the reduction in apparent size of objects viewed when the eyes are more converged than they need to be for the distance of the object from the eyes.|
|Cornsweet illusion||An illusion where two colours can obviously be seen to be different when placed directly beside each other; however, when the two colours are separated by a thick black line, they appear to be of the same hue.|
|Delboeuf illusion||An optical illusion of relative size perception. The two black circles are exactly the same size; however, the one on the left seems larger.|
|Disappearing Model||A trompe-l’œil body painting by Joanne Gair to “…make people disappear by painting them right into a background or paint clothing on a human body that is virtually undistinguishable from actual fabric!”.|
|Ebbinghaus illusion||The Ebbinghaus illusion, or Titchener circles, is an optical illusion of relative size perception. The two orange circles are exactly the same size; however, the one on the right appears larger.|
|Ehrenstein illusion||The Ehrenstein illusion is an optical illusion studied by the German psychologist Walter Ehrenstein in which the sides of a square placed inside a pattern of concentric circles take an apparent curved shape.|
|Figure-ground (perception)||The faces–vase drawing that Danish psychologist Edgar Rubin described exemplifies one of the key aspects of figure–ground organization, edge-assignment and its effect on shape perception, which depends critically on the direction in which the border (edge) between the black and white regions is assigned. This is an instance of multistability (or multistable perception), the tendency of ambiguous perceptual experiences to pop back and forth unstably between two or more alternative interpretations.|
|Filling-in||In vision, filling-in phenomena are those responsible for the completion of missing information across the physiological blind spot, and across natural and artificial scotomata. When steadily fixating the central dot for many seconds, the peripheral annulus will fade and will be replaced by the colour or texture of the background.|
|Flash lag illusion||When a visual stimulus moves along a continuous trajectory, it may be seen ahead of its veridical position with respect to an unpredictable event such as a punctuate flash. This illusion tells us something important about the visual system: contrary to classical computers, neural activity travels at a relatively slow speed. It is largely accepted that the resulting delays cause this perceived spatial lag of the flash. Still, after several decades of debates, there is no consensus regarding the underlying mechanisms.|
|Forced perspective||A technique involving optical illusion, used in photography, filmmaking and architecture to make an object appear farther away, closer, larger or smaller than it actually is, by manipulating human visual perception through the use of scaled objects and the correlation between them and the vantage point of the spectator or camera.|
|Fraser spiral illusion||The Fraser spiral illusion, or false spiral, or the twisted cord illusion, was first described by the British psychologist Sir James Fraser in 1908. The overlapping black arc segments appear to form a spiral; however, the arcs are a series of concentric circles.|
|Gravity hill||A place where the layout of the surrounding land produces an optical illusion, making a slight downhill slope appear to be an uphill slope. Thus, a car left out of gear will appear to be rolling uphill against gravity.|
|Grid illusion||Any kind of grid that deceives a person’s vision. The two most common types of grid illusions are the Hermann grid illusion (1870) and the scintillating grid illusion (1994). The first is characterized by “ghostlike” grey blobs perceived at the intersections of a white (or light-colored) grid on a black background. The grey blobs disappear when looking directly at an intersection. The second is constructed by superimposing white discs on the intersections of orthogonal gray bars on a black background. Dark dots seem to appear and disappear rapidly at random intersections, hence the label “scintillating”. When a person keeps his or her eyes directly on a single intersection, the dark dot does not appear. The dark dots disappear if one is too close to or too far from the image.|
|Hering illusion||The Hering illusion (1861): When two straight and parallel lines are presented in front of radial background (like the spokes of a bicycle), the lines appear as if they were bowed outwards. The Wundt illusion produces a similar, but inverted effect.|
|Hollow-Face illusion||The Hollow-Face illusion is an optical illusion in which the perception of a concave mask of a face appears as a normal convex face.|
|Hybrid image||A Hybrid image is an optical illusion developed at MIT in which an image can be interpreted in one of two different ways depending on viewing distance.|
|Illusory contours||Illusory contours or subjective contours are visual illusions that evoke the perception of an edge without a luminance or colour change across that edge. These are instances of reification, the constructive or generative aspect of perception, by which the experienced percept contains more explicit spatial information than the sensory stimulus on which it is based.|
|Illusory motion||An optical illusion in which a static image appears to be moving due to the cognitive effects of interacting color contrasts and shape position. Apparent motion is the most common type of illusory motion, and is perceived when images are displayed in succession at a specific frame rate such as in a movie. Other types include induced movement, motion aftereffect, Stroboscopic effect and optical art.|
|Impossible object||A type of optical illusion consisting of a two-dimensional figure which is instantly and subconsciously interpreted by the visual system as representing a projection of a three-dimensional object. A cube whose edges cross in an inconsistent way is an example of an impossible object, specifically an impossible cube (cf. Penrose triangle).|
|Irradiation illusion||An illusion of visual perception in which a light area of the visual field looks larger than an otherwise identical dark area. It arises partly from scattering of light inside the eye. This has the effect of enlarging the image of a light area on the retina.|
|Isometric illusion||An isometric illusion (also called an ambiguous figure or inside/outside illusion) is a type of optical illusion, specifically one due to multistable perception.|
|Jastrow illusion||An optical illusion discovered by the American psychologist Joseph Jastrow in 1889. It is also called the ring-segment illusion, Wundt area illusion or Wundt-Jastrow illusion. In the illustration shown, the two toy railway tracks are identical, although the lower one appears to be larger.|
|Kanizsa triangle||The Kanizsa triangle is an optical illusion first described by the Italian psychologist Gaetano Kanizsa in 1955. It is a triangle formed of illusory contours (also called subjective contours), which are visual illusions that evoke the perception of an edge without a luminance or colour change across that edge. The spatially separate fragments give the impression of a bright triangle defined by a sharp illusory contour, occluding three black circles and a black-outlined triangle. These are instances of reification, the constructive or generative aspect of perception, by which the experienced percept contains more explicit spatial information than the sensory stimulus on which it is based.|
|Kinetic Depth Effect||The Kinetic depth effect refers to the phenomenon whereby the three-dimensional structural form of a silhouette can be perceived when the object is moving. In the absence of other visual depth cues, this might be the only perception mechanism available to infer the object’s shape. Additionally the direction of motion can reverse due to the existence of multiple 3D visual solutions.|
|Leaning tower illusion||The Leaning tower illusion is an optical illusion that presents two identical images of the Leaning Tower of Pisa side by side.|
|Lilac chaser||Lilac chaser is a visual illusion, also known as the Pac-Man illusion.|
|Liquid crystal shutter glasses||A technique of displaying stereoscopic 3D images by only presenting the image intended for the left eye while blocking the right eye’s view, then presenting the right-eye image while blocking the left eye, and repeating this so rapidly that the interruptions do not interfere with the perceived fusion of the two images into a single 3D image.|
|Lunar terminator illusion||Lunar terminator illusion is an optical illusion where the apparent source of sunlight illuminating the moon does not corresponding with the actual position of the sun.|
|Mach bands||Named after the physicist Ernst Mach, Mach bands is an optical illusion that exaggerates the contrast between edges of the slightly differing shades of gray, as soon as they contact one another, by triggering edge-detection in the human visual system.|
|McCollough effect||The McCollough effect (1965) is a phenomenon of human visual perception in which colourless gratings appear coloured contingent on the orientation of the gratings. It is an aftereffect requiring a period of induction to produce it.|
|Missing square puzzle||The missing square puzzle is an optical illusion used in mathematics classes for helping students to reason about geometrical figures; or rather to teach them not to reason using figures, but to use only textual descriptions and the axioms of geometry. It depicts two arrangements made of similar shapes in slightly different configurations. Mitsunobu Matsuyama’s “Paradox” uses four congruent quadrilaterals and a small square, which form a larger square. The apparent paradox is explained by the fact that the side of the new large square is a little smaller than the original one.|
|Moon illusion||The Moon illusion is an optical illusion in which the Moon appears to be 50% to 75% larger near the horizon than it does while higher up in the sky. The same illusion applies to any sizeable object in the sky, such as the sun or a constellation.|
|Motion aftereffect||A visual illusion experienced after viewing a moving visual stimulus for a time (tens of milliseconds to minutes) with stationary eyes, and then fixating a stationary stimulus. The stationary stimulus appears to move in the opposite direction to the original (physically moving) stimulus.|
|Motion illusion||An optical illusion in which a static image appears to be moving due to the cognitive effects of interacting colour contrasts and shape position. Also known as illusory motion.|
|Müller-Lyer illusion||The Müller-Lyer illusion is an optical illusion consisting of a stylized arrow. When viewers are asked to place a mark on the figure at the midpoint, they invariably place it more towards the “tail” end.|
|Multistability||In a dynamical system, multistability is the property of having multiple stable equilibrium points in the vector space spanned by the states in the system. Multistable perceptual phenomena are a form of perceptual phenomena in which there are unpredictable sequences of spontaneous subjective changes. Perceptual multistability can be evoked by visual patterns that are too ambiguous for the human visual system to recognise with one unique interpretation. Famous examples include the Necker cube, Schroeder staircase, structure from motion, monocular rivalry and binocular rivalry, but many more visually ambiguous images are known. Since most of these images lead to an alternation between two mutually exclusive perceptual states, they are sometimes also referred to as bistable perception.|
|Musion Eyeliner||A proprietary high definition video projection system that allows moving images to appear within a live stage setting.|
|Necker cube||An optical illusion first published in 1832 by Swiss crystallographer Louis Albert Necker, comprising an ambiguous line drawing representing a cube, drawn with its edges as line segments. Since the cube can be interpreted as being in either of two different three-dimensional orientations, this illusion is an instance of multistability (or multistable perception), the tendency of ambiguous perceptual experiences to pop back and forth unstably between two or more alternative interpretations.|
|Numerosity adaptation effect||A perceptual phenomenon in numerical cognition which demonstrates non-symbolic numerical intuition and exemplifies how numerical percepts can impose themselves upon the human brain automatically. This effect was first described in 2008.|
|Orbison illusion||First described by the psychologist William Orbison in 1939, the optical illusion comprises a two dimensional figure such as a circle or square superimposed over a background of radial lines or concentric circles. Both the figure and the rectangle containing it appear to be distorted; in particular, squares appear slightly bulged, circles appear elliptical, and the containing rectangle appears tilted.|
|Pareidolia||Pareidolia is a psychological phenomenon in which the mind responds to a stimulus, usually an image or a sound, by perceiving a familiar pattern where none exists (e.g. in random data). Common examples are perceived images of animals, faces, or objects in cloud formations, the Man in the Moon and the Moon rabbit.|
|Penrose stairs||The Penrose stairs was created by Lionel Penrose and his son Roger Penrose. A variation on the Penrose triangle, it is a two-dimensional depiction of a staircase in which the stairs make four 90-degree turns as they ascend or descend yet form a continuous loop, so that a person could climb them forever and never get any higher.|
|Penrose triangle||The Penrose triangle (also known as the Penrose tribar or impossible tribar) was first created by the Swedish artist Oscar Reutersvärd in 1934. The mathematician Roger Penrose independently devised and popularised it in the 1950s, describing it as “impossibility in its purest form”. A triangle whose edges cross in an inconsistent way is an example of an impossible object (cf. impossible cube).|
|Pepper’s ghost||An illusion technique used in theatre, amusement parks, museums, television, and concerts. An audience views a stage or room with various objects in it. On command, ghostly objects appear to fade in or out of existence in the room, or objects in the room magically transform into different objects.|
|Perceived visual angle||An optical illusion where the physical and subjective angles differ is then called a visual angle illusion or angular size illusion.|
|Peripheral drift illusion||A motion illusion (1979/1999) generated by the presentation of a sawtooth luminance grating in the visual periphery.|
|Phantogram||Phantograms, also known as Phantaglyphs, Op-Ups, free-standing anaglyphs, levitated images, and book anaglyphs, are a form of optical illusion. The illusion of depth and perspective is heightened by stereoscopy techniques.|
|Phi phenomenon||The optical illusion of perceiving a series of still images, when viewed in rapid succession, as continuous motion.|
|Poggendorff illusion||The Poggendorff illusion (1860) is a geometrical-optical illusion that involves the misperception of the position of one segment of a transverse line that has been interrupted by the contour of an intervening structure (here a rectangle).|
|Ponzo illusion||A geometrical-optical illusion (1911): two identical lines across a pair of converging lines, similar to railway tracks, are drawn. The upper line looks longer because we interpret the converging sides according to linear perspective as parallel lines receding into the distance. In this context, we interpret the upper line as though it were farther away, so we see it as longer – a farther object would have to be longer than a nearer one for both to produce retinal images of the same size.|
|Rubin vase||A famous set of ambiguous or bi-stable (i.e., reversing) two-dimensional forms developed around 1915. This is an instance of multistability (or multistable perception), the tendency of ambiguous perceptual experiences to pop back and forth unstably between two or more alternative interpretations.|
|Sander illusion||In Sander’s parallelogram (1926) the diagonal line bisecting the larger, left-hand parallelogram appears to be considerably longer than the diagonal line bisecting the smaller, right-hand parallelogram, but is in fact the same length.|
|Silencing||Silencing is an illusion in which a set of objects that change in luminance, hue, size, or shape appears to stop changing when it moves.|
|Size–weight illusion||The size–weight illusion, also known as the Charpentier illusion or Charpentier–Koseleff illusion, occurs when a person underestimates the weight of a larger object (e.g. a box) when compared to a smaller object of the same mass.|
|Stroboscopic effect||A visual phenomenon caused by aliasing that occurs when continuous motion is represented by a series of short or instantaneous samples.|
|Swept-plane display||A structure from motion technique with which one can create the optical illusion of a volume of light, due to the persistence of vision property of human visual perception.|
|Ternus illusion||An illusion related to human visual perception involving apparent motion. Element motion is characterized as the outer disc in the Ternus display being seen as “jumping over” the other two discs in the display. Group motion gives the perceiver the illusion that all of the discs within the display are moving simultaneously to the right and then back again.|
|Thaumatrope||An optical toy popular in the 19th century. A disk with a picture on each side is attached to two pieces of string. When the strings are twirled quickly between the fingers the two pictures appear to blend into one due to the persistence of vision.|
|Trompe-l’œil||An art technique that uses realistic imagery to create the optical illusion that the depicted objects exist in three dimensions.|
|Troxler’s fading||Troxler’s fading: When one fixates on a particular point for even a short period of time, an unchanging stimulus away from the fixation point will fade away and disappear.|
|Vertical–horizontal illusion||The Vertical-horizontal illusion is the tendency for observers to overestimate the length of a vertical line relative to a horizontal line of the same length. This even happens when people are aware that the lines are of the same length.|
|Visual tilt effects||The phenomenon that the perceived orientation of a test line or grating is altered by the presence of surrounding lines or grating with a different orientation.|
|Wagon-wheel effect||An optical illusion in which a spoked wheel appears to rotate differently from its true rotation.|
|White’s illusion||A brightness illusion: Rectangles A on the left look much darker than the rectangles B on the right, even though they are printed from the same ink and reflect the same amount of light.|
|Wundt illusion||The two red vertical lines are both straight, but they may look as if they are bowed inwards to some observers. The distortion is induced by the crooked lines on the background. The Hering illusion produces a similar, but inverted effect.|
|Zoetrope||One of several pre-film animation devices that produce the illusion of motion by displaying a sequence of drawings or photographs showing progressive phases of that motion.|
|Zöllner illusion||The Zöllner illusion is a classic optical illusion named after its discoverer, German astrophysicist Johann Karl Friedrich Zöllner. It creates the illusion that the series of parallel, black diagonal lines are not parallel. The shorter lines are on an angle to the longer lines, and this angle creates the impression that one end of the longer lines is nearer to the viewer than the other end.|