MIND SIGHTS

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The Perception of Optical Illusions

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MIND SIGHTS The Perception of Optical Illusions

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The MIT Press 1 Rogers Street Cambridge MA 02142-1209 Tel: (617) 253-5646 Fax: (617) 258-6779 www.mitpress.mit.edu Published in 2015. All rights are reserved.

No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior permission of the publisher.

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This book is dedictaed to Johann Oppel and his early investigations on optical illusions in 1854.

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behind the images

between the brain and eyes different kinds illusions

influences on perception

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01 INTRODUCTION

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introduction When our visual system defines the images that are received by our eyes, numerous processes are carried out. We can analyze these processes as computations. A lot of computations are estimation processes. In other words, we cipher quantities from the images (such as the location of edges and corners, the structure of the scene in view, or position of the light source) using as input the image data, or the image data processed in some way.

Because of some limitations of the visual apparatus the pictorics contain noise, so the visual system has to come up with the perfect assessment in the presence of noise. However, many of the visual computing are such that, unless the noise is known, the best estimate does not correspond to the true and actual value of it.

In other words, there is systematic error. We say estimates are biased. The only way to avoid that bias would be to estimate the noise very accurately, that because of the complexity of visual processes, seems to be impossible. So, the bias constitutes a general principle, it is the principle of uncertainty of visual processes. Under typic conditions the errors are not large enough to notice, but in certain patterns, it becomes noticeable.

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CHAPTER ONE

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behind the images

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01 CHAPTER ONE

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the basics

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Seeing is deceiving. Thus a familiar epigram may be challenged in order to indicate the trend of this book that aims to treat certain phases of optical illusions. In general, we do not see things as they are or as they’re related to each other; that is, the intellect doesn’t correctly interpret deliverances of the visual sense, although sometimes the optical mechanism of the eyes is directly responsible for the optical illusion. Expressly, no conceptions and perceptions are entirely adequate, but fortunately most are satisfactory for practical aspirations.

Only parts of what is perceived comes through the senses from that object; the remainder always comes from within. In fact, it’s the visual sense or the intellect which is responsible for optical illusions of the various types to be discussed in the following chapters. Thus, past experiences, associations, desires, demands, imaginings, and further obscure influences create optical illusions.

Generally speaking, a tree appears longer when it’s standing than when it’s lying on the ground. Lines, areas, and rabbles are not recognized in their actual physical relations. The condition of a colored object varies considerably with its environment.

The sky is not perceived as infinite space and nor as a hemispherical dome, but as a flattened vault.

A bright object appears larger than a dark object of the same physical dimensions, while flat areas may appear to possess a third dimension of depth.

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An optical illusion does not exist physically but it is difficult in some cases to fully explain the cause.

Certainly there are numerous cases of errors of judgment. A mistaken estimate of the distance of a mountain is due to an error of judgment but the

perception of a piece of white paper as pink on a green background is an error of sense. It’s realized that the foregoing comparison leads directly to one of the most controversial issues in psychology, but there is no intention on the author’s part to cling dogmatically to the opinions expressed. In fact, any discussions of this psychological judgment involved in the presentations of the visual sense are not introduced with the hope of stating the final word but to give the reader a concept of the inner process of perception. The final word is left to the psychologists but it appears possible that it may never be formulated.

Figure 1.1

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» _Figure 1.1 Comparing the two blue lines in Figure 1.1, the 3D perspective of the drawing gives the illusion that the blue line on the right is longer. In fact, both blue lines are the same length. Taking off the grid, the illusion is gone and the blue lines clearly appear to be the same exact length.

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Optical illusions are so frequent and varied that they have long challenged the interest of scientists.

Some may be so useful or even so disastrous that they’ve been utilized or counteracted by the skilled artist or artisan. The architect and painter have used and avoided them. The stage artist manipulates them to carry the audience in its imagination to other environments or distant states. The magician has employed them in his dissipations and the camoufleur used them to advantage in the practice of hypocrisy during the recent wars. They are extremely entertaining and useful, or deceiving and disastrous, depending upon the viewpoint.

Incidentally, a few so-called optical illusions will be discussed which are not due strictly to errors of the visual sense or of the intellect. Examples of these are mirage, certain optical effects employed by the magician. In those cases neither a visual sense nor the intellect errs. In a case of the mirage rays of light coming from the object to the eye are bent from the usual straight-line course and the object appears to be where it really is not. Yet, with these few exceptions, which are introduced for the specific interest and for the emphasis they give to the “proper” optical illusion, it is understood that optical illusions in general as so discussed will mean those due to the visual mechanism or to judgment or intellect errors. By cause of brevity we might say that they are such by reason of errors of visual perception. Furthermore, only those of a static type will be considered; that is, the immense complexities due to motion are not of interest from the view point of the aims of this book.

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There are two well-known types of misleading perceptions: optical illusions and hallucinations. If, for example, two lines appear of equal length but they are not, the error in judgment is responsible for what is termed an “optical illusion.” If the visceral consciousness of an object appears, but the object is not present, the result is called a “hallucination”.

For example, if something is seen that does not really exist, the essential factors are supplied by the imagination. Shadows are often wrought by the fantasy into animals and even humans do bent upon evil purpose. Ghosts are created in this manner.

Hallucinations depend largely upon factors like the recency, frequency, or vividness of completed experience. A discussion of this type of misleading perception does not advance the aims of this book and so will be omitted.

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01 CHAPTER ONE

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in our daily lives The connection between the material and mental in eyesight is incomprehensible and is ever remained so. Objects emit and reflect light and the optical mechanism known as the eye focuses images of the objects upon the retina. Notes are then carried to the brain where the molecular vibrations do take place. The physiologist records certain physical and the chemical effects in the muscles, nerves, and brain and behold there appears consciousness, sensations, thoughts, desires, and volitions. How and Why are questions which may never be answered.

People don’t see things the way they are. Whether it is an optical illusion or something else, we naturally tend to judge too soon.

It is dangerous to use the word never, however, the ultimate answers to those questions appear to be so inaccessible that it discourages one from proceeding far over the blurry course which leads toward them. De facto, it does not appreciably further the aims of this book to devote much space to efforts toward explanation. In covering this broad, complex field there are multitudes of facts, numerous hypotheses, and many theories, all from that to choose. Judgment edits that of limited space most of it be given to the presentation of the representative facts. That’s the reasoning that which led to the formulation of chapters outlines.

Optical illusions are legion. They greet the careful observer on every hand. They play a prominent part in our appreciation of the physical world. So at times, they must be avoided, but often they may be put to work in different arts. Their widespread existence and forcefulness make visual perception the final judge in decoration, painting, as well as architecture, landscaping or lighting, and many more activities. The final limitation of measurements including physical devices leaves this responsibility to an intellect. Our mental being is impressed by things as perceived, not with things as they appear.

It is believed that this intellectual or judiciary phase which plays such a part in visual perception will be best brought out by examples of various types of static optical illusions coupled with certain facts pertaining to the human eye as well as to the visual process as a whole.

Owing to the vast complex beyond these physical phenomena, physical measurements upon an object and space which have done a lot toward building a solid base for scientific knowledge fail ultimately to provide an explicit mathematical picture of that which is perceived. Much of the previous work has been devoted to the physical actualities but the ever-present differences between the physical and perceptive realities have emphasized the demand for considering the latter as well.

In simple special cases, it is easy to determine when or how closely a perception is accurate but in general, agreement among ordinary persons is necessary owing to the absence of exact measuring device that spans the gap between perception and the objective reality. The senses may distribute correctly but error may arise from imagination or inexperience, invalid assumptions, and incorrect associations, and the recency, frequency, and vividness of past experience.

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» _Figure 1.2 Kanizsa’s Triangle These spatially parted graphics give the feeling that a bright white triangle, shaped by a illusory contour, occluding 3 black circles and a black-outlined triangle. the right side of the line appears darker, while the left side of the line seems a lot more lighter.

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01

once upon a time

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CHAPTER ONE

History of optical illusions duty substitute traced forward to the 5th century B.C. when Epicharmus super showed the author of this business. Epicharmus believed that if our hope knows and understands tool clearly, sensory organs deceive us further mention an optical confusion. The augmented Greek philosopher, Protagoras, from planed interval, had a contrastive disposition on this case.

According to the philosopher, authentic was an environment that fools us and not the reason. Views presented by Epicharmus from Protagoras expanded to the vortex about what optical illusions utterly are.

One of the examples of optical illusions from the foregone is associated with not tell rooftops of Greek temples. Roofs of such temples were built honor slanting means.This layout of the balance created an blooper that roofs were like. This happened as the turf further the walls, when perpendicular to one other, gave the untruth that a crash pad was hunched or even bent. A specialized grounds of optical illusions should be support for attaining an credit depth potentiality of this concept.he added that our five senses could easily be fooled into believing in something that was not there.

The famous Greek philosopher, Aristotle, pure to secure some stupendous clue on this problem. He agreed to lock up Protagoras on the altercation that we power rely on the rationale to win a desired characterize of savoir-faire; however, Aristotle (350 B.C.) also added that substantive was doable to defer the intellection unusually delicate. supremacy the vagabondage of issues assent about optical illusions was enriched by differential philosophers and researchers. Thoughts presented by Plato also provide some kind of aha game this argument aim. According to Plato, deciphering the wile also some day the intimacy behind illusions is easy cloak the profit of both acumen and mind.

Still, the theory of optical illusions wasn’t clear and the debate on them was continued.

Many different scholars and philosophers began to ponder over answers to the mystery of optical illusions. Plato was one of the prime philosophers that became fascinated with optical illusions. The famous Greek philosopher once said that the trickery and the actuality of the optical illusions were due to both the mind and the senses. Since then, many notable personalities studied the mystery and why behind optical illusions.

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» _Figure 1.3 Optical illusions have an illustrious history, beginning with Greek philosophers and made a lasting impress on painters, psychologists, illustrators, scholars, and us.

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500 BC »Epicharmus believed that human senses are constantly tricking itself.

He theorized that the human eyes are not paying attention to reality, but that they are inventing nonexistent objects.

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350 BC » Aristotle agreed with a theory that we can rely on the senses to obtain a correct picture of actuality, and also added that it was possible to fool the senses quite easily.

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19th C » Muller and Oppel performed various studies related to finding out of how people perceive optical illusions. So they published several articles and wrote numerous books, which reignited people’s interest in optical illusions. Both of them proposed numerous theories, explaining the unexplainable phenomenon.

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1915 » W.E. Hill created a cartoon of a young and old woman merged together.

Some people saw an old woman while others saw a young woman. The explanation of how this optical illusion was created was due to individual perceptions.

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1960 » Artists such as Vasarely and Bridget Riley developed an interest in Op Art, painting abstract images. They painted vibrations, hidden images, flashing, and other abstract patterns.

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vision, perception, and processing

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CHAPTER TWO

The human eye is the organ which gives us the sense of sight, allowing to examine and learn more about the enclosing world than we do with any of the other four senses. We use our eyes in almost every activity we perform, whether it is reading, working, watching television, writing a letter, driving a car, or many other ways. Most people would probably agree that sight is the sense that’s valued more than all the rest. The eye allows us to see and interpret the shapes, colors, and dimensions of objects in the world by transforming light they reflect or emit. The eye is able to detect bright light or dim light, yet it cannot sense objects when a certain factor such as light is absent.

Light waves from an object (such as a tree) enter the eye firstly through the cornea, which is the clear dome at the front of the eye. It is like a window our growth through the pupil, the circular opening in the center of the colored iris. Fluctuations in the intensity of incoming light change the size of the eye’s pupil. As the light invading the human eye becomes lighter, the pupil will constrict, due to the pupillary light response. When the entering light becomes dimmer, the pupil will dilate.

Figure 2.1

Initially, the light waves are bent or converged first by the cornea, and then more by the crystalline lens that is located immediately behind iris and the pupil, to the nodal point (N) placed immediately behind the back outer of the lens. At that point, the image becomes reversed and inverted.

The light continues through the vitreous humor, the clear gel that makes up about 80% of the eye’s volume, and then, ideally back to a clear focus on the retina, behind the vitreous. A limited central area of the retina is the macula, which provides the best vision of any locale in the retina. If the eye is considered to be a type of camera, the retina is equivalent to the film inside of the camera and is registering the tiny photons of light interacting and those that are relating with it.

Within the layers of the retina, light impulses are changed into electrical motions. Next they are sent up to the optic nerve, along the visual pathway, to the occipital cortex at the posterior of the brain.

Here the electrical signal is interpreted, or seen, by the brain as a visual image.

1 SCLERA 2 OPTIC NERVE 3 PUPIL 4 LENS 5 IRIS 6 CORNEA 7 RETINA

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anatomy of the human eye Figure 2.2

1 ROD CELLS

4 AMACRINE CELLS

2 CONE CELLS

5 GANGLION CELLS

3 BIPOLAR CELLS

6 INNER MEMBRAINE

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The retina contains two types of photoreceptors, namely rods and cones. The rods are more numerous, some 120 million,and are more sensitive than the cones. Yet, they aren’t sensitive to color. The six/seven million cones determine the eye’s color sensitivity.

Various structures can be seen when looking into the human eye. A black-looking aperture, the pupil, that allows light to enter the eye (it appears dark because of these absorbing pigments in the retina).

A colored circular muscle, the iris, that is highly beautifully pigmented giving us our eye’s color (the central aperture of the iris is the pupil). This one circular muscle controls the size of the pupil so that more or less light, depending on circumstances, is allowed to invade the eye. Eye color, or also more correctly, iris color is due to variable amounts of eumelanin and pheomelanin formed by cells. Other of the former is in brown eyed people and of the latter in multitudinous green and blue-eyed people.

The Melanocortin-1 Receptor Gene regulates a eumelanin production and is placed on the so called chromosome MCIR. Point mutations in the MCIR gene that affects the melanogenesis. The presence of point mutations in MCIR genes alleles is a very common feature in light skinned and blue and green eyed people of any type of race.

A transparent external surface is the cornea that covers both pupil and iris. This is the first and most powerful lens of the visual system of the eye and allows, combined with the crystalline lens which is a production of extremely sharp images at retinal photoreceptor levels and executions.

The “white of the eye”, called sclera, forms part of the supporting wall of the eye ball. The sclera is continuous with the cornea. In addition to it, this external coating of the eye is cohesion with the dura of the central nervous system. When we remove the eye from the orbit, we can see that the eye is a slightly asymmetrical sphere with an approximate sagittal diameter or length of 24 to 25 mm, as well as a transverse diameter of 24 mm.

Actually, we do not see with our eyes, but rather with our brains. Our eyes merely are the start of the visual process.

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visual system Solving the problem of converting light into ideas, of visually understanding features and objects in the world, is a knotty task far beyond the abilities of some of the strongest computers. Our vision requires distilling foreground from background, noticing objects presented in a wide territory of orientations, and accurately solving spatial cues. The neutral mechanisms of visual perception offer rich insight into how the brain handles such situations.

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Visual perception begins as soon as the eye can focus light onto the retina, where it is absorbed by a layer of photoreceptor cells. These cells can convert light into electrochemical signals, and are divided into two types, rods and cones, named for their shape. Rod cells are responsible for night vision, and respond well to dim light. Rods are found mostly in the peripheral regions of the retina, so most people will find they can see better at night if they concentrate the gaze just off to the side of what whatever is observed.

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Cone cells are concentrated in a central region of the retina called the fovea; they are responsible for

» _Figure 2.3 After decades of research, scientists have finally begun to understand enough about the brain to design and develop effective tools for maintaining and improving brain function.

high acuity tasks like reading, and also for color vision. Cones can be subcategorized in to 3 types, depending on how they respond to red, green, and blue light. In combination, these 3 cone types enable us to perceive color.

Notes from the photoreceptor cells pass through a network of interneurons in the second layer of the retina to ganglion cells in the third layer. These neurons in two retinal layers exhibit knotty receptive fields that allow them to spot contrast changes within an image; the changes might indicate edges or shadows. Ganglion cells gather this instruction along with other information about color, and send their info into the brain through the optic nerve.

The optic nerve primarily routes information via a thalamus to the cerebral cortex, where some visual perception occurs, but the nerve carries data as well, required for the mechanics of eyesight to two sites in the brainstem. Data concerning moving targets and information governing scanning of the eyes travels to a second site in the brainstem, a nucleus called the superior colliculus.

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The superior colliculus is responsible for moving the eyes in short jumps, called saccades. Saccades allow the brain to perceive a smoother scan by stitching together a series of relatively still images.

Saccadic eye movement solves the problem of extreme blurring that would result if the eyes could pan evenly across a visual landscape; saccades can be observed if you look at human eyes as they attempt to pan their gaze across a room.

Most projections from the retina travel via the optic nerve to a element of the thalamus called the lateral geniculate nucleus (LGN), which is deep in the center of the brain. The LGN disconnects retinal inputs to parallel streams, a containing color and fine structure, and the other containing contrast and motion. Cells that are able to process color and fine structure make up the top four of six layers of the LGN; those four are called the parvocellular layers, because the cells are small. Cells processing contrast and motion make up the bottom two layers of the LGN, so called the magnocellular layers because the cells are large.

The cells of the magnocellular and parvocellular layers project all the way to the back of the brain to primary visual cortex (V1). Cells in V1 are arranged in several ways that allow the visual system to calcu-

late where objects are in space. First, V1 cells are organized retinotopically, which means that a point to point map exists between the retina and the primary visual cortex, and neighboring areas in the retina correspond to bordering areas in V1. This allows V1 to position objects in 2 dimensions of the visual world, horizontal plus vertical. The third dimension, depth, is mapped in V1 by approaching those salients from two eyes. Those signals are processed in packs of cells called ocular dominance columns, a checkerboard pattern of connection switching between the left and right eye. A slender discrepancy in the position of objects relative to the eye allows depth to be counted by triangulation.

Finally, V1 is organized into orientation columns, stacks of cells that are strongly activated by lines of a given orientation. Orientation columns allow V1 to detect the edges of objects in a visual world, and so they begin the task of visual recognition.

The visual cortex of a newborn has aovergrwoth, or hypertrophy, of a haphazard connection that has to be carefully pruned, based on visual actions, into crisply defined columns. It is actually reductions in the number of connections, not an increase, that improves the baby’s ability to see fine detail and to recognize shapes and patterns.

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depth perception

Depth perception is the visual ability to perceive the world in three dimensions (3D) and the distance of an object. It arises from a array of depth cues.

These are typically classified into binocular cues that are based on the receipt of sensory data in three dimensions from both eyes and monocular cues that can be represented in just two dimensions and observed with just one eye. Binocular cues include stereopsis, eye convergence, disparity, and also yielding depth from binocular vision through usage of parallax. Monocular cues include size: distant objects subtend smaller visual angles than near size, objects, grain, plus motion parallax. Monocular cues give depth information when viewing a scene with one eye. When an observer moves, the appa-

rent relative motion of several stationary objects against a background gives hints about their relative distance. If the information about directions and velocity of movement is known, the motion parallax can provide absolute depth material. This effect be seen better when driving in a car. Closest things pass quickly, while far off objects might appear stationary. Some animals which lack binocular vision due their eyes having a common field-of-view employed motion parallax more clearly than humans for depth cueing (e.g., some types of birds, that bob their heads to achieve motion parallax, and the squirrels, which move in lines orthogonal to an object of interest to do the same. When an object moves to the observer, the retinal projection of

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CHAPTER TWO

Figure 2.4

» _Figure 2.4 The shadows make the balls, which are in fact in the same positions in the two images, look like they are in very different positions. When you add motion, the various positions of the shadows cause the balls to seems as though they are moving on very different trajectories. Perhaps most fascinating is the fact that you can make it look like the balls, which in fact follow a straight trajectory turn, just

by changing the position of the shadow at one or more points. These illusions make it appear that our visual system, presumably at a fairly low level, is making assumptions about how objects and their shadows go together, and so interpreting scenes accordingly. This analysis then gets fed into the visual system’s motion processing, resulting in the illusions seen above.

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The most obvious monocular depth cues are size (that means that objects appear larger when they are close than when they are further away) and perspective (as in converging railroad tracks).

Other fairly obvious monocular cues do include occlusion, near objects block parts or even any of objects further away, and blur, or the so called “distance fog,” since far off objects appear even blurrier than close objects.

an object expands over a period of time, which leads to the perception of motion in a line towards the observer. Another name for this sensation is depth from optical expansion. The dynamic stimulus change enables the observer to see the object as moving, and also to perceive distance of the object.

Thus the changing size serves as a distance cue.

A related phenomenon is the visual system’s expanse to calculate time-to-contact (TTC) of an coming object from the rate of visual expansion – a strength that is useful in cases ranging from driving a car to playing baseball. However, a calculation of TTC is, strictly speaking, perception of the pace rather than depth. The equity of parallel lines converging in the distance, at infinity, allows us to reconstruct the relative distance of two parts of some object, or landscape features. An example is standing on a direct road, looking down the road, and recognizing the road narrows as it goes off in the far distance.

If two objects are known to be the same size (for example two trees) but their absolute proportion is unknown, their relative size cue is able to serve data about the relative depth of the two objects. If one subtends a larger visual angle on the retina than the other, the object which subtends a broader visual angle is closer. Since the visual angle of an object projected onto the retina drops by distance, this information can be combined with previous knowledge of the object’s size to elect the absolute depth of the object. For example, us humans are familiar with the size of an average automobile. This prior knowledge can be combined with input about the angle it subtends on the retina to define the absolute depth of an automobile in a scene.

Even if the actual size of objects is unknown and there is only one object visible, and smaller objects appear further away than some large object that is presented at the same location. Light scattering by the atmosphere has objects that are a great distance away have reduced luminance contrast and lower color saturation. Due to this, images seem hazy the farther they are away from a person’s point of view. In computer graphics, this is often called “distance fog.” The foreground has greater contrast; the background has lower contrast. The objects contrasting only in their contrast with a background appearing to be at different depths. The color of distant objects are also shifted toward the blue end of the spectrum (e.g., distant mountains). Some painters, like Cézanne, apply “warm” pigments (red, yellow and orange) to bring features towards the viewers, and “cool” ones (such as blue, violet, as well as blue-green) to indicate the part of a form that curves away from the picture plane.

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different types of illusions

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Bakery by Octavio Ocampo seems like a neutral photo of a woman in her bread shop, however when a person analyzes the photo, multitudes of skulls comes to overwhelm the photo. The literal illusion is created nicely by the simple bread on the shelves and baskets. Also, the skull in the center. formed by the girl, shows a subtle symbol being handled well.

All Is Vanity by Charles Allan Gilbert is very striking and makes subtle statements on our society’s focus on superficial appearances. Naturally, the viewer’s eyes will be fatigued to the women in the mirror, but then explore the image until skull appears in the picture. This seemingly innocent piece suddenly has a darker undertone that surprises the audience.

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“Things are not always what they seem; the first appearance deceives many; the intelligence of a few perceives what has been carefully hidden...” —Phaedrus.

Some of the best-known optical illusions of all times are shown in Figure 3.1. People see other things at first glance - an old woman or a young miss. When taking a closer look at it, both ways can be seen.

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A literal illusion is an optical illusion that tends to make images that vary from some objects that form them. The brain depicts images that are entirely different than the objects that create it.

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Figure 3.1

literal illusions

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CHAPTER THREE

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The way you look at objects can affect how you see it. Sometimes there are two images in the same picture, but you’re only able to see one at a time so your brain chooses one – when it deals with too much information for the brain at once.

Cognitive “illusions” rely on reserved knowledge about the world (depth, animals, population) and are under some degree of conscious control; we can generally overturn the perception at will. Instead of demonstrating a physiological base they interact with some levels of perceptual processing, in-built assumptions or ‘knowledge’ are deluded. These illusions are commonly broken down into ambiguous illusions, distorting illusions, paradox illusions, or fiction illusions. They regularly exploit the predictive hypotheses of early visual processing. So called stereograms are based on a cognitive visual illusion.

Ambiguous illusions are pictures or objects that offer significant changes in appearance. Perception will switch between equivalents as they are very considered in turn as available information does not confirm one single view. The Necker cube is a known example; the motion parallax due to motion is being mispresented, even in the face of other sensory data. Another famous one is the Rubin vase.

Paradox illusions offer paradoxical or impossible objects such as impossible staircases shown in the work of M. C. Escher. The impossible triangle is a illusion dependent on a cognitive misunderstanding that adjacent edges must join. They appear as a byproduct of perceptual learning.

Distorting illusions are the most common, these illusions offer distortions of size and length. Those were simple to discover and repeat. Many are physiological illusions, such as the Café wall illusion that exploits the early visual system while at the same time encoding for edges.

Other distortions, like the converging line illusion, are harder to place as physiological or cognitive as the depth-cue challenges they offer aren’t easily placed. All pictures which have perspective cues are in effect illusions. Visual judgments as to size are controlled by perspective or other depth-cues and can easily be wrongly set.

Fiction illusions describe the perception of any objects that are genuinely not there to all but one single observer, such as those induced by drugs or also diseases like schizophrenia.

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cognitive illusions

MIND SIGHTS

» _Figure 3.2 Rubin’s vase (sometimes known as the Rubin face or the figureground vase) is a famous set of ambiguous (reversing) two-dimensional forms developed in 1915 by the Danish psychologist Edgar Rubin. These were at first introduced in Rubin’s two-volume work, Rubin included a number of examples, like the Maltese cross figure in black and white, but the one that became the most famous was his vase example, most likely because the Maltese cross could also easily interpreted as a black and white beachball.

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physiological illusions Physiological illusions, like afterimages following bright lights or adapting stimuli of excessively longer alternating patterns (contingent perceptual after effect), are presumed to be the effects on the eyes or brain of excessive stimulation of specific types, such as brightness, color, movement, etc. A theory is that stimuli have dedicated neural paths by selves in the early stages of visual processing, and that the repetitive stimulation of only one or maybe more channels causes the physiological imbalance so that alters perception. The following two examples can describe best what this kind of illusion is about.

The Herman Grid Illusion is best explained using biological approach. Lateral inhibition, where in the receptive field of the retina some light and dark receptors compete with one another with the goal to become active, has been used to explain why we see bands of increased brightnesses at the edge of a color difference when viewing Mach bands.

Once the receptor is active it inhibits the adjacent receptors. This inhibition creates and highlights edges. The Hermann grid illusion requires the grey spots appear at the intersections because of the inhibitory response which occurs as a result of the increased dark surround.

The Mach band illusion is another well-related example of a physiological illusion. It is named after the physicist Ernst Mach and represents the exaggeration the contrast between edges of the slightly varying shades of gray, as soon as they contact one another, by triggering edgedetection in the human visual system. The effect is due to the structural high-boost filtering performed by the human visual system on the luminance channel of images captured by the retina. Such a filtering is largely performed in the retina itself, by lateral inhibition among its neurons. Theis resulting effect is independent of the orientation of the boundary.

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» _Figure 3.3 Herman Grid illusion Light comes in from the four sides of a intersection, but from only two sides of a band going away from the intersection. It’s a lot more inhibited than the region of the band going away. Thus the intersection appears to be darker than the other section in it.

Dark spots can be seen at the intersections of the white bands, but not at the points away from the intersections.

» _Figure 3.4 Mach band illusion The line in the middle of the picture is one solid color. However, because of how the eye’s retina can filter the various shades on either side of the line, the right side of the line appears darker, while the left side of the line does seem a lot lighter.

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influences on optical illusions

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influences on illusions The way we see illusions can depends on many factors. There are some environmental factors, such as light, shadow, or perspective, and there are human-related factors such as culture. The impact of culture and environment can have an effect on our visual perception. This theory was first explored

by Robert Laws, a Scottish missionary working in Malawi, Africa, during the late 1800’s. Take a look at the picture below. What a person sees largely depends on where he or she has lived in the world.

After you have examined the picture, scroll down for a more detailed explanation. » _Figure 4.1

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Our environment and how we were born and raised in our culture changes the way we see things. Whether if it’s an optical illusion or some scenery, our brain is influenced from previous experiences and sees these things the way it’s used to.

What is above the woman’s head? When scientists showed this sketch to people from East Africa, nearly all of the participants in the experiment believed she was balancing a box or metal can on her head. In a culture containing few angular visual cues, the family is seen sitting under a tree. Westerners, on the other hand, are accustomed to the corners and boxlike shapes of architecture. They are more likely to place the family indoors and to interpret the rectangle above the woman’s head as some window through which shrubbery can be seen.

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mental health The different abilities of our brain are involved in our vision creativity. However, human perception is defined by the disjunction between our physical reality and subjective opinion. We never have 100% of the data to make correct deciding. People like to have the illusions, as illusions are not comforting; they improve health. Our separation from each other is an illusion of consciousness.

Our brain is looking for ways to maintain or even restore a balanced state. You will find yourself drawn to one optical illusion more than to others. Good optical illusions for someone are the ones you need at this moment. When people look at any suitable optical illusion and visual effects to promote a type of self-regulation and also develop their very own mechanisms of the emotional support.

The dictionary defines reality as something that is in a state of being actual or true. However, reality is different due to perception and the differences in our brains. Optical illusion is a phenomenon that confirm this fact. Optical illusions appear when our brain misunderstands the visual stimulus, which has taken into by our eyes. Optical illusions are caused by how our brain functions.

The optical illusion pictures consist of the unique color compositions, which can influence the human perception of the reality. Optical illusions have been known since antiquity. Until now, most of the optical illusion images has been considered as tricks, puzzles, art for fun etc. However it’s only one aspect of what optical illusions can give us. Other aspect, optical illusion have therapeutic effects on human beings all over the world.

An optical illusion or visual illusion appear when our brain misunderstands the visual stimulus, which has taken into by our eyes. It happens when the visual effects on the eyes and brain have a specific brightness, oscillation frequencies, color, as well as movement etc. Optical illusions and visual effects can have a strong effect on emotional as well as physical states of the human beings. The strength of a visual illusion and effects relies primarily on the perception of the illusion by so called visual sensory interactions with the human brain.

Figure 4.2

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CHAPTER FOUR

Micropsia is a change in visual perception caused by swelling in the cornal areas of the eye. In general, those with micropsia perceive objects as a lot smaller than their actual size. The condition is also called Alice In Wonderland Syndrome, and the effect is sometimes given the fancy name of Lilliput sight after the novel Gulliver’s Travels.

Children between the ages of five and ten seem particularly prone to micropsia, and also macropsia, that causes things to appear bigger than they are.

The symptoms, that can prove extremely distressing, may lead to panic or severe disturbance in young children. They are mostly associated with conditions that can lead to migraine headaches at a future point. These perceptions should be taken seriously, however; in vary rare cases, swelling of the brain or tumors can cause perceptional differences.

micropsia

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Micropsia is usually a temporary condition that can be caused by different factors. Some types of epilepsy are known to cause visual distortion as well. The onset of migraine headaches are marked by micropsia. Additionally, swelling caused by the Epstein-Barr virus is linked to episodes of micropsia.

Micropsia is a fairly common symptom of the use of both hallucinogenic and opiate-based drugs like hydrocodone oxycodone. Morphine and heroin in particular are associated with this condition, and may also cause other struggles perceiving spatial relationships. The differences can increase the panic of drug users or of those that are hospitalized. A calm explanation of this condition is often helpful to who is on high doses of pain medication, so such symptoms are not completely unexpected.

In rare cases, micropsia can be of psychological origin. Someone with extreme anorexia can look at a friend and see a perfect figure, but be unable to see such figure in herself. Visual perceptions can affect body perception and are often labeled as body dysmorphic disorder.

There are a few studies on how to anticipate micropsia in those who seem predisposed due to medical reasons. Illness or migraines can cause conditions, and they are usually short-lived and isn’t treated yet. Control of migraines through some medication may support micropsia to be of shorter duration. Keep in mind that the condition may seem to be most helpful in easing panic related to big difference in perception.

In the case of Epstein Barr Virus that is leading to mononucleosis, micropsia could be an initiative symptom. This symptom may provide a reason to test for mononucleosis, but generally no specific treatment for micropsia itself is undertaken. In general, within a few days the condition betters.

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Ebbinghaus illusion The Point to this set of flowers is that the blue dot in the center of each of the flowers is the same size, although the blue dot in the left flower, surrounded by a large white space and large green dots seems smaller than the blue dot in the right flower that is surrounded by a small white space and smaller green dots.

This shows, once again, that our perception of an image is influenced by its context. The orange dot surrounded by relatively large green dots and large white space seems absolutely smaller than the blue dot that is surrounded by relatively small green dots, much closer to the blue center.

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CHAPTER FOUR

People suffering from schizophrenia aren’t fooled by the optical mass illusion unlike the rest of us who fall for such illusion. It is due to the connections between the sensory and conceptual areas of their brains that might be on the fritz. We perceive a concave face as a normal convex face in the hollow mask illusion. It’s because the illusion exploits our brain’s strategy for making sense of the visual world: uniting what it really sees, known as bottom-up processing with what it expects to see based on its prior experience, known as top-down processing.

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Schizophrenia is a disease characterized by poor planning, hallucinations, and delusions. It’s a kind of disassociation from reality which can cause an imbalance between a bottom-up and the top-down processing. And this definition of schizophrenia is right in testing the hollow mask illusion.

The patients suffering from schizophrenia view a hollow face for what it is. It was proven by research conducted by Dima and Roiser of the University College London, where in they put 3 schizophrenia patients and sixteen healthy control subjects in an fMRI scanner that measured brain activity, and showed them 3D images of concave or convex faces. As believed, schizophrenic people viewed the concave faces while none of the healthy people reported the same.

After analyzing the data using a technique called dynamic causal modeling, it was seen that when all the healthy people looked at the concave faces, relations between frontoparietal networks and the visual areas of their brain strengthened. When they see the illusion, and their brains strengthen the connection such that what they expect (a normal face) becomes more influential that overpowers the unlikely visual information. On the other hand, no such strengthening had been seen in schizophrenic patients. They’re unable to modulate the pathway and accept concave face as reality.

Moreover, not only schizophrenics can detect the concave faces, drunk or high people go through a similar disconnect between what their brain sees and what it expects to and beat this illusion.

In contrast, among healthy viewers, this illusion is so strong that even after being aware of the illusion; they’re unable to see the concave face. According to neuroscientists, humans have specific brain areas dedicated to processing faces only. This illusion works strongly only for faces, not with other objects.

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the hollow-mask illusion

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Figure 4.4 UPRIGHT UPSIDE-DOWN

A three-dimensional hollow face mask held a few feet away will seem to be convex (turned “out” towards the viewer) no matter which side you look at (this image is from the Max-Planck-Institut für biologische Kybernetik in Tübingen). While the movie depicts a computer-generated model, a effect works just as well with a physical mask.

Scientists have attempted to explain the illusion for centuries, but there is still much we don’t know about how it works. Our visual system is able to use tools like binocular alterity and motion parallax to judge distance, but these techniques don’t seem to work with the hollow mask until we are vastly close to it: for many people, nearer than 3 feet. An effect is diminished if the mask is turned upsidedown, but it does not disappear; almost everyone still sees the illusion.

The effect isn’t completely due to the direction of lighting as well. While the optical system tends to assume light is coming from overhead, hollow masks lit from below still appear bulding. Others did suggest that the illusion arises because we know that we see a face, and that our knowledge trumps another visual cue that suggests it’s not convex like a real face would be.

The viewers walked slowly towards each mold (facing away from them) until they could clearly see it “switch” from convex to concave, and thus establishing how close they could be and still see the illusion. Here are the results:

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As objects became more familiar (and arguably more human-like), people could stand tighter and still see the illusion. For both teddy bears and pineapples, that illusion was more powerful when they were upright, however for the jello mold, the orientation made no difference. The experiment was repeated with a human face, at 4 different orientations. The upright face had an even stronger effect than that teddy bear, but the illusion was still present when the face was upside-down, and just as strong as the teddy bear.

The two scholars moved to computer-generated images of faces. They were stereoscopically shown, and viewers wore 3D glasses. But this time they systematically modified the faces, gradually adding noise. 14 viewers gave each image a convexity valuation, ranging from six (definitely convex) to one (definitely concave). Remember, all the images were rendered to be concave—the cues that the 3D glasses gave them suggested that these weren’t real faces, but hollow shells. As more noise has been added, making the face look less real, viewers were less likely to be tricked by the illusion, rating it significantly lower on the convexity scale. The illusion persisted longer for color faces than those rendered in gray-scale, again propsing that the idea that we’re seeing a “real” face makes us more likely to see that face popping out towards us.

All this adds up to a fairly convincing claim that our perception of a face as a whole is what causes to see the mask as convex, like an actual face instead of a hollow shell. Our visual system receives a variety of different cues to depth of objects, and prioritizes them in ways that are usually quite accurate. But illusions such as the hollow mask face demonstrate that the priorities do not always work. Fortunately we do not see hollow masks nearly as often as real faces, so for the vast majority of visual experience, our visual world seems just fine. Such anomalies, so what we see as illusions, can offer a very powerful window into how our visual system actually works.

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C. Smith, H. Toch. The Influence of Culture on Visual Perception. web.mit.edu.

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D. Eustis. The How and Why of Optical Illusions. cs.brown.edu. Web. 24 October 2015.

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Exploratorium. Optical Illusions. exploratorium.edu/explore/optical-illusions.

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E. Gupta. Understanding Optical Illusions. cs.cmu.edu. N.d. Web. 8 November 2015.

H. Hill, A. Johnston. The hollow-face illusion: Object-specific knowledge, general assumptions or properties of the stimulus. N.p. 2007. Print. 2 December 2015.

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J. Purkinje. Structure of the human eye. cornellcollege.edu/dsherman/illusions.

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K. Alexander. On the nature of accommodative micropsia. ncbi.nlm.nih.gov. N.d.

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M. Asano, A. Khrennikov. Quantum Adaptivity in Biology: From Genetics to Cognition. Springer, 2015. Print. 24 November 2015.

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M. Bach. Optical Illusions & Visual Phenomena. www.michaelbach.de/ot. N.d.

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M. Luckiesh. Visual Illusions. visualillusions.net. 2011. Web. 19 October 2015.

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National Eye Institute. Diagram of the Eye. nei.nih.gov. N.d. Web. 20 October 2015 N. Wade. Art and Illusionists (Vision, Illusion and Perception). Springer. 2014. Print.

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[colophon] [colophone] [colophon] designer Annika Schneider typefaces Lekton and Apercu software Adobe InDesign and Illustrator paper Red River 50lb. Premium Matte printing Epson WF7610 binding Plotnet

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Seeing is deceiving. Thus a familiar epigram may be challenged in order to indicate the trend of this book that aims to treat certain phases of optical illusions. In general, we don’t see things as they are or as they are related to each other; that is, the intellect doesn’t correctly interpret deliverances of the visual sense, although sometimes the optical mechanism of the eyes is directly responsible for the optical illusion. Expressly, no conceptions and perceptions are entirely adequate, but fortunately most are satisfactory for practical aspirations.

Only parts of what is perceived comes through the senses from that object; the remainder always comes from the visual sense or the intellect of it.

MIT Press One Rogers St Cambridge, MA 02142 www.mitpress.mit.edu