An optical illusion is an image or object that we see is different from what it really is. In other words optical illusions occur when our eyes send information to the brain and which is ultimately deceived and which does not correspond to reality.
Optical illusions are misperceptions of the shape, color, dimensions or movement of certain objects. They are due to the fact that a lot of information perceived by our eyes is transmitted to our brain which will sort and analyze it. If the information reaching the brain is contradictory, it cannot interpret it. We can distinguish several types of optical illusions, including impossible images and real geometric figures that appear to us distorted. Optical illusions show that our view of numbers is always a matter of personal interpretation.
How do optical illusions work?
Optical illusions are visual constructs that interfere with our visual system, from the eyes to the brain. Indeed, our brains are constantly adjusting light information from a point in relation to what surrounds it. This is called lateral inhibition.
Famous Fraser Spiral
The Fraser spiral is an optical illusion named after the British psychologist James Fraser, who first studied it in 1908. It is also called the “false spiral”, or, as the original name implies, “the twisted cord illusion”.
Even though it is made up of concentric circles, the image gives the impression of an endless spiral. This is because regular patterns (circles) are combined with non-aligned patterns (colored bands). The Zollner illusion and the cafe wall illusion are based on this same principle, like many other illusions, which implies that a series of slanted elements causes the eye to perceive fictitious bends and descents.
How Do Optical Illusions Happen?
Illusions or optical effects occur because our brains cannot correctly interpret the objects seen by the eye due to disturbances in the circumstances around the object. Visual illusions that occur due to the nature of the visual areas of the brain as they receive and process information. In other words, your perception of illusion has more or less to do with how your brain works on the optics of your eye.
These scams can use color, light and patterns to create images that can be deceptive or misleading to our brains. The information collected by the eyes is processed by the brain, which automatically creates the perception that in reality, it does not correspond to the correct image.
Some optical illusions are physiological. This means that they are caused by some kind of physical means in the eye or the brain.
The Mach Band illusion is an example of a physiological illusion. The line in the center of the image is one solid color. However, due to how the retina of the eye filters out different shades on both sides of the line, the right side of the line appears darker, while the left side of the line appears and appears lighter.
Another optical illusion is cognitive. Cognitive illusions, such as ambiguity (confusing or objects can be interpreted differently), distortion (flipping) and paradoxical illusions (fantasy), occur when our brain automatically makes assumptions based on information sent from our eyes. This illusion is sometimes called a “mind game.”
THE EYE, OPTICAL INSTRUMENT
The eye behaves more or less like a camera. It has a diaphragm, the iris, which lets in more or less light depending on whether it opens or partially closes. Behind the iris, a deformable lens, called the crystalline lens, allows images to form exactly on a sensitive membrane: the retina. The latter is made up of a fine tissue of special cells – cones and rods – arranged in a small area, called the macula or yellow spot; cones are involved in the perception of colors in normal light – there are four species of cones, receptive to red, green, blue or white. The rods are distributed over the entire retinal surface; they analyze the different shades of gray, and work especially in peripheral vision, when the light is not very intense.
Cones and sticks contain photosensitive chemicals, which decompose under the action of light, producing small amounts of electrical energy. These nerve impulses are conducted by the optic nerve to the brain, which analyzes all the information collected.
THE “PRIMARY EFFECTS”, 1St OPTICAL ILLUSIONS
Innatism is a philosophical and epistemological doctrine that holds that the mind is born with ideas/knowledge.
The eye, as soon as it opens (after sleep, for example), seeks to recognize the world around it; he engages in an attentive exploration of the perceptual field offered to him. Illusions can arise from this ocular activity; called innate by certain psychologists, the Swiss Jean Piaget prefers to give them the name of “primary effects”. They appear around the age of five or six and regress as the subject ages.
CIRCUMSTANCES OF APPEARANCE OF “PRIMARY EFFECTS”
These “primary effects” appear when the eye registers a cursory glimpse of points picked up at random; it only stops on certain parts: angles, axes of symmetry, line breaks, construction irregularities, etc. These areas, perceived in detail by the yellow spot, subjectively take on more importance than the others: they are overestimated.
SOME EXAMPLES OF “PRIMARY ILLUSIONS”
If we construct two equal squares using segments of parallel lines, horizontal in one case and vertical in the other, the first figure takes on the appearance of a rectangle higher than wide, and the second of a rectangle wider than it is tall. In fact, in its exploration work, the eye is drawn to the non-continuous sides of each square; these are overestimated at the expense of the sides executed in solid line.
Perpendicular illusions are part of a similar process. If, from the middle of a line segment, we raise a segment equal and perpendicular to the first, the two segments thus arranged do not appear equal: the unshared line seems longer than the other. The illusion is particularly clear when the cut segment gets closer to the horizontal, and the other to the vertical.
Delboeuf’s illusion is a little different: the subject is presented with two equal circles, one of which remains free while the other is placed in the center of a larger circle. Subjectively, the inscribed circumference appears to have a larger diameter. The disturbing element here is the great circle; the eye gets attached to it, always comes back to it, and the overestimation occurs. Jean Piaget has demonstrated experimentally that the error reaches its maximum when the radii of the contained and containing circumferences are in a ratio equal to three quarters.
The Müller-Lyer illusion is one of the most frequently mentioned. The primary effect is particularly powerful there. At the ends of two equal straight line segments, a tail is drawn; for one of the segments, the tail is open outwards; for the other, it is folded back: subjectively, the second seems shorter.
The eye, in its work of exploration, hardly stops on the segments themselves; on the other hand, the irregularities of each end are the object of all his attention. He keeps coming back to them, and repeatedly centers them on his macula. Lines are now only judged by their respective tail units, and the brain decides that one is larger than the other, for the sole reason that it is included in a larger figure.
SECONDARY OPTICAL ILLUSIONS
If the primary effects decrease in intensity with age (without ever disappearing completely), the so-called secondary illusions are reinforced as the subject ages. They result on the one hand from perceptual learning and, on the other hand, from the acquisition of a certain number of visual automatisms.
THE PERCEPTIVE FIELD: BACKGROUND AND FIGURE
The first of these automatisms consists, for the eye, in knowing how to structure and immediately order its perceptions. When the lighting conditions are normal, the perceptual field is differentiated into a background and a more or less complex figure. The background does not have a precise internal organization: it is vague and indeterminate, and always seems to extend below the figure. This, on the contrary, is a set of points, lines or colors appearing as a homogeneous whole and not as an association of disparate elements.
In some cases, back and face are reversible. Thus, the circle imagined by the psychologist E. Rubin is divided into eight equal sectors, striated alternately in the direction of the rays and in the transverse direction. The subject who looks at the whole perceives sometimes a cross striped lengthwise on a background striped transversely, and sometimes a cross crossed out on a striped background lengthwise. The same surface plays both roles, depending on whether it is centered on the yellow spot (it is then a figure) or whether it is perceived by the peripheral cells of the retina (it becomes background).
THE “GOOD SHAPES”, EASILY PERCEPTIBLE FIGURES
For a figure to be clearly distinguished from the background, a certain number of conditions must be fulfilled. The whole of the lines must be strongly enough structured to be centered inevitably and without hesitation on the yellow spot. The figure, generally closed or very slightly open, is made up of continuous segments; it remains fairly simple, uniformly colored, and often has a center or an axis of symmetry. The circle, spiral, ellipse, parallel lines, triangle, trapezoid, rectangle, square, rhombus, and all regular polygons are among the most noticeable figures. These are good forms, according to the expression of supporters of Gestaltpsychology (or psychology of form) such as Wolfgang Köhler in Germany, and Paul Guillaume in France.
Good form can be more or less stable; it can resist more or less well the disturbances of the perceptual field. When several elements of the background themselves become good shapes, the figure is modified; sometimes it even disappears completely from the viewer’s eyes. Generally, the conflict between two contradictory good forms is resolved by a compromise. Certain features of one of the two figures are distorted, curved or underestimated, and the optical illusion is thus generated.
FROM THE OPPOSITION BETWEEN GOOD FORMS IS BIRTH THE ILLUSION
The sides of a square crossed by several concentric circles (or by a spiral) seem to curve towards the center; those of a square striated by several hyperbolas round outwards. On the other hand, a large number of small straight segments intersecting concentric circles transform them into an illusory but remarkable spiral. Likewise, two pointed arches correctly drawn, but crossed out by a rectangle, one of the long sides of which passes through the summit, no longer seem to meet normally. And a circle, inscribed in a set of nested equilateral triangles, changes and takes on an ovoid appearance.
A group of divergent lines disrupts most good shapes, because they introduce a powerful element of asymmetry. Thus, when we observe two parallel and equal line segments placed inside an angular sector, the one closest to the vertex seems the greatest. According to the same principle, a square, shared by a bundle of divergent lines, is subjectively transformed into a trapezoid. Two parallel straight lines enclosing a point from which radiating lines start appear to bend away from each other. A group of parallels, each striated by segments of straight lines oriented differently, loses its original structure and seems to become a network of arbitrary straight lines. The examples are innumerable, and it is not difficult to conceive of new optical illusions: it suffices for that to put in conflict two good forms of different composition. The strength of the error on the psychological level will be primarily a function of the degree of incompatibility of the various elements of the figures.
In children, the perception of space is far from being innate. Jean Piaget has shown that it depends as much on the subject’s motor activity as on his ability to see. It is by learning to grasp objects that the child gets an idea of their size and their distance; it is by moving that he learns to recognize them from different angles, and under varying lighting conditions.
If monocular vision is sufficient to clearly distinguish a flat figure, the acquisition of the notions of depth and relief requires the work of both eyes. Each of the retinas perceives the visual field from a different angle; this slight disjunction between the two images (called retinal disappearance) is at the origin of the perception of relative distances. But many other physicochemical mechanisms are involved.
PERCEPTION OF THE RELIEF
The visual impression of the relief depends mainly on binocular vision; each eye collects a different image of the examined object; the superposition of these two images creates the sensation of relief. This property is used in stereoscopic binoculars, which exaggerate the relief by setting aside, using prisms, the directions of sight of the two images. The reproduction of relief using plane images has been the subject of many works: stereoscopes require two juxtaposed photographs, each of which is brought back, by mirrors and lenses, in the line of sight of the eye. corresponding. To perceive the relief on a screen, one can project two polarized images at right angles and examine them using polarizing glasses, allowing each of the eyes to see only one image. This process allows the production of relief and color projections. The techniques of holography also make it possible to create the virtual image of a three-dimensional object.
PERSPECTIVE AND DEPTH IN VISION
When we pass judgment on the remoteness of several objects, we do not rely solely on the clues provided by them; the entire perceptual field serves as a frame of reference. The idea between perspective and depth are born both from the binocular vision and from the general aspect of the landscape offered to the gaze. The further away an object is, the more it is perceived at a reduced angle (the light information it transmits to the eye is increasingly sketchy; the details are less and less distinguished). In the dark, it is practically impossible to estimate how far away a luminous surface is. Perspective only comes into play when a whole series of phenomena modify the vision. Thus, when an element of the field partially masks another, it is perceived as being situated in front of this one; moreover, the arrangement of shadows and lights informs the observer about the relative position of the various objects; finally, the colors fade with the distance, and the blue components dominate in the distance.
PERSPECTIVE AND ILLUSIONS
The different perspectives, real or supposed, are at the origin of a large number of optical illusions. When the necessary references are lacking or are uncertain (shadows, colors or points of comparison), one hesitates to judge the relative depth of the objects and their relief. There is thus a quantity of ambiguous drawings, which the subject perceives sometimes flat, and sometimes in relief. The German H. Kopfermann showed, as early as 1930, that it suffices to suggest the third dimension to slightly modify two lines of a plane figure. Likewise, trompe l’oeil paintings give an impression of depth thanks to a skilful play of light and shadow. The so-called Necker’s cube can be seen in two different ways, depending on whether one of its faces is supposed to be in front of or behind the solid. Two contiguous parallelograms can evoke a half-open book, seen either from the back or from the front. A figure formed of equal diamonds – black, gray or white – takes on the appearance either of a pile of cubes on a floor, or of a hanging molding; a drawing of a few steps can just as easily be seen as a corner of a ceiling adorned with moldings. As soon as we have recognized the two possibilities, we can switch from one representation to the other as if by clicking; the change in direction of the drawing is due to a modification of the retinal centration: the element seen by the yellow spot is always estimated to be the closest.
The illusions based on a distortion of perspective are no less numerous and no less irritating. If, in a photograph showing two railroad tracks, two equal rectangles are cut out, the rectangle closest to the horizon appears larger. Likewise, if we draw three identical human figures on a sketch suggesting perspective, the figure closest to the vanishing point seems by far the tallest. As for the exasperating Penrose triangle, whose vertices are not in the same plane, it very effectively brings into conflict the idea of perspective and the good triangle shape. Several skilful designers have used this illusion to create paintings of fantastic or surrealist inspiration: the German Escher, in particular, specialized in this field.
ILLUSIONS OF COLORS AND MOVEMENTS
Illusions of color and movement are usually due to the very structure of the retina and the functioning of visual cells (cones and rods). The photosensitive substances contained in the latter take a certain time to decompose under the action of light; this period, between the arrival of the light ray at the back of the eye and the departure of the nerve impulse to the brain, is called the reaction time. When elements of the perceptual field move rapidly, images overlap on the retina, creating a sense of change. Cinema and television reproduce gestures and movements thanks to this permanence of visual images. But various phenomena can give the illusion of movement, even though there is none.
If we look at two identical light sources in the dark, one fixed and the other mobile, they both seem to move symmetrically. When they are fixed but unevenly bright, the more intense appears stable, while the other gives the impression of moving. These errors are attributable to the work of the eye, which has great difficulty in correctly centering its perceptions in the dark. The image of the luminous points is formed successively in different places of the retina, and the illusion is born. The same phenomenon occurs in certain strobe effects: two light sources, which light up alternately, give the impression of a back-and-forth movement.
The irradiation effects, which are particularly strange, are part of a similar process.
Two equal circles, one black and the other white, contained in equal squares respectively white and black, do not appear not the same size. The white circle seems of higher dimension: this is because, in the black square, the eye has difficulty in centering the image of the white circle, and the impression of whiteness persists on the retina, even though the gaze has already crossed the limit of the dark zone. The opposite effect occurs in the illusion of black squares: a figure formed of dark squares, delimiting white alleys, appears to be adorned with gray spots at the crossroads of these alleys. Op’art (optical art), a contemporary artistic movement, makes extensive use of the phenomena of irradiation. The technique consists of establishing periodic structures in two colors, capable of forcing the eye to perpetual motion. Consequently, the permanence of the retinal images causes a vibrating effect: such a drawing of a zebra, for example, thanks to the prolongation of the stripes of the animal, seems to come alive and throb; and such a set of curves, cleverly arranged, remarkably evokes running water.
The illusions of colors and movements are numerous and easy to obtain, the “floating hearts”, red on a blue background, by the German Hermann von Helmholtz, seem to move when one shakes the drawing: it is only time to reaction of receptive cones to two colors are not equal. A gray circle, drawn half on a green background and half on a red background, appears red on the green side and green on the red side: the permanence of visual impressions is the cause. Psychologists have carried out a number of experiments capable of provoking false chromatic and kinetic perceptions. The so-called Fechner’s toton (composed of a black semicircle, and a white semicircle embellished with concentric black streaks) gives rise, when it turns, a curious visual impression: the outer streaks appear red, while the inner streaks appear brown, green and blue. Jean Piaget has shown that the rotary movement of a small white square (presented on a black background) gives rise to variable sensations: at low speeds, the square is perceived as such; then, as the rotation accelerates, the eye sees successively large, rather fuzzy crosses, then a single stationary cross, then a flickering double cross, and finally a stationary double cross, located inside a square whose angles are rounded.
Many visual errors, caused by faulty or too hasty reasoning, are called psychological illusions.
THE EMERGENCE OF SENSE THROUGH THE ORGANIZATION OF THE VACUUM
The brain is responsible for synthesizing all perceptions, so as to give them a meaning that is related to the general activity of the individual; also, habit, memory or emotions can be at the origin of many visual distortions. Man placed in front of a senseless perceptual field tends to organize it according to his memories or his desires; all perception is essentially subjective and personal. The so-called Bühler figure, a sort of curved surface with four angles, is almost always interpreted as a volume (veil, spearhead, etc.). A psychologist tried to project the image of an empty frame onto a very dimly lit screen, asking colleagues what they saw: almost all of them noticed everyday objects where there was absolutely nothing.
THE SENSES ARE SELECTIVE
The brain not only organizes what is meaningless, but also refuses absurdity. Talented designers have designed completely illogical paintings: two-dimensional houses, multiple perspectives, palaces supported by interwoven colonnades, waterfalls feeding themselves, etc. Faced with these representations, intelligence capitulates; the brain “forgets” the illogicalities and retains only the normal parts of the drawing; he divides the sketch into coherent zones, and refuses to consider the image as a whole. Two psychologists showed seventy-four children a photograph of a group of houses, one of which, retouched, is seen in two dimensions: only three young observers noticed and pointed out the absurdity.
The so-called Poggendorff illusion offers a good example of the interpretation of sensations in which the brain engages. If we draw two parallel straight lines, with a secant to the right of the right parallel and a secant to the left of the left parallel, and these two secants are not completely in line with each other, the brain will nevertheless imagine that it is a single line. The French psychologist Éliane Vurpillot has applied this illusion in a remarkable way to an evocative drawing: where one believes to see a pole stretched by a man behind a colonnade, there are in reality only two completely independent straight segments one the other.
WHEN LIVING INFLUENCES PERCEPTION
Memory, habits, hunger, pain, personal interests play a large role in perception; many optical illusions are due to these psychological or bodily manifestations. An American researcher, speaking to a group of young Americans and young Mexicans, presents them with a series of very ambiguous double images (a brunette woman and a blonde woman mixed up, a bullfight and a baseball game intertwined, etc. .); however, he notes that each subject only sees the scene corresponding to his socio-cultural habits. Other experiments prove that hungry people retain, from a very long list of words, only those which concern food; they do not even notice the others, and are unable to mention them after the fact. Likewise, social taboos (in particular sexual and religious prohibitions) cause a systematic forgetting of several words during reading experiences. Finally, we have been able to demonstrate the existence of certain constancies of shapes and colors for all men: whatever the light conditions, a tomato is always seen red, a yellow lemon, a green tree leaf, and a gray donkey. As for the human figure, it is always perceived in an identical way, even observed through a pseudoscope (device inverting the reliefs); in this case, the brain returns to the usual shape of the face, although the eye transmits to it that of a hollow mask.