The Psychology of Visual Illusion

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Physiological illusions, such as the afterimages following bright lights or adapting stimuli of excessively longer alternating patterns contingent perceptual aftereffect, CAE , are the effects on the eyes or brain of excessive stimulation of a specific type - brightness, tilt, color, movement, and so on. Unlike those demonstrating a physical or physiological basis, cognitive illusions are those that occur through misdirecting stored knowledge and assumptions.

The illusion occurs perceptually but is under some degree of conscious control cognitive illusions can be reversed at will. The following list includes many of those visual illusions that have been researched and are well known to the general public. Auditory illusions involve the sense of hearing. Hearing is achieved through sensitivity to the movement of molecules through a medium in the environment outside the organism.

The Psychology of Visual Illusion by J. O. Robinson - Book - Read Online

Individual species of organisms have sensitivities to frequencies that fall within a particular range. For example, humans are generally limited to frequencies between 20Hz to 20 kHz, which are commonly called audio or sonic frequencies. There are various illusions in which a listener may hear sounds that are not present in the stimulus, or "impossible" sounds. In short, auditory illusions highlight areas where the human ear and brain , as organic tools, differ from perfect audio receptors. While not as well known or as well studied as visual illusions, a number of auditory illusions are quite common.

Octave Illusion - Discovered by Diana Deutsch in , the octave illusion is an auditory illusion produced by playing an alternating sequence of two notes that are spaced an octave. The tones are played over headphones, with each ear receiving the tones simultaneously, except that when the right ear receives the high tone the left ear receives the low tone, and vice versa. Many people perceive a single tone that switches in pitch and from ear to ear, hearing, for example, "high tone - silence - high tone - silence" in the right ear while hearing "silence - low tone - silence - low tone" in the left ear.

Surprisingly, right-handed people tend to hear the high tone in the right hear, while left-handers seem to show no tendencies. Glissando Illusion - Produced and demonstrated by Diana Deutsch in , the glissando illusion is produced by an oboe tone played together with a sine wave that glides up and down in pitch.

These two sounds are repeatedly switched between left and right, such that whenever the oboe tone is to the left, a portion of the glissando is to the right, and vice versa. When played over stereo loudspeakers, the oboe tone is heard correctly as jumping back and forth from ear to ear, whereas the segments of the glissando appear joined together. People localize the glissando in a variety of ways. Right-handers most often hear it as traveling from left to right as its pitch glides from low to high, while left-handers tend to obtain different illusions.

Tritone Paradox - First published by Deutsch in , this paradox is constructed by two computer -produced tones that are related by a half- octave a tritone. When the two tones are played consecutively, some listeners hear the tones as ascending, while others hear the same tones as descending. This experience can be particularly astonishing to a group of musicians who are all quite certain of their judgments, and yet disagree completely as to whether such a pair of tones is moving up or down in pitch.

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McGurk Effect - This illusion is a perceptual phenomenon that shows the relation between hearing and seeing in speech perception, suggesting that speech perception relies on more than one modality. It was first described by McGurk and McDonald in The effect is robust, persisting even with knowledge of the illusion, unlike certain optical illusions that can break down once one 'sees through' them. Shepard Tone - Created by psychologist Roger Shepard at Bell Labs, this illusion consists of a set of tones that seem to perpetually rise or fall.

The illusion is created by super-positioning pure tones sine waves an octave apart. The brain's inability to identity the fundamental tone causes it to "slip" periodically, thereby creating the illusion, much like an eye looking at a barber pole pattern. Knowledge of illusions in the physical senses of taste, smell , and touch is limited. There exist very few examples of illusions, perhaps due to the slower temporal resolution compared to that of vision and hearing.

The following are examples of those that have been studied. Phantom Limb - This tactile illusion is the sensation that an amputated body part, most commonly a limb, is still attached to the body. Most sensations are that of pain, but may include itching, warmth, cold, squeezing, and burning, although the limb may also feel as if it is shorter or in a distorted and painful position.

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Initially reasoned to be the product of inflamed nerve endings, phantom limb sensations have been found to be due to the reorganization of the somatosensory cortex. Stroking different parts of the face leads to perceptions of being touched on different parts of the missing limb.

Thermal Grill - The thermal grill refers to a tactile illusion that was first demonstrated by T. Thunberg in Physical contact with this mixture of relatively mild temperatures elicits sensations of painful heat. Haptic Illusions - Haptic illusions are created by mixing force cues with geometric cues to make people feel shapes that differ from the actual shape of the object. Art is illusionistic by nature.

How and Why do Optical Illusions Work?

Even from the earliest forms of art, cave paintings that relied on outlines to suggest forms, illusion has been the bedrock of artworks. Modern science has found that such outline drawings can actually be recognized by the brain faster than a photograph of the object. Seemingly obvious principles of art, which are based on actually visual illusions, were not developed until the Italian Gothic time period c.

The Renaissance saw numerous discoveries of artistic principles that artists utilized to suggest reality. A mathematical approach to perspective was an especially crucial one; beforehand, artists inconsistently raised and enlarged objects and figures to suggest depth, which resulted in an unrealistic and flat image. Color and contrast were also used to suggest depth; distant objects were rendered with lower contrast replicate the grayish cast caused by the scattering of light through distance in the atmosphere.

Softer edges to suggest curvature, such as on bodies, were also popularly employed by artists who followed the Venetian school of the Italian Renaissance. This involves extremely realistic paintings which create the illusion that the objects depicted really exist as three-dimensional entities, sometimes within the dimension of the viewers themselves. This technique was frequently used during the Mannerist and Baroque artistic periods, though use of this technique dates back much further.

It was often used to optically open up the domes or ceilings of a basilica to "reveal" the sky, upon which Jesus ', Mary 's, or various saint 's ascensions were painted sotto in su, meaning "seen from below" in Italian. Mediums have expanded to include walls and even furniture, such as a deck of cards painted on a table, for example. While there are countless artists who deliberately employed visual illusions to evoke a sense of reality in their artworks, there are many others who employed illusions for the sake of exhibiting the illusionary nature of art and perception, including Dutch graphic artist M.

Performance magic has also long been an art form, relying on baffling and amazing illusions to entertain audiences by giving the impression that something impossible has been achieved. Though the effect is that the performer seems to have supernatural abilities, the illusion of magic is created entirely by natural means. Illusions and acts may be characterized as production, vanish, transformation, restoration, teleportation, levitation, penetration, or sleight of hand illusions.

Mimes are also known for a repertoire of illusions that are created by physical means. The mime artist creates an illusion of acting upon, or being acted on by, an unseen object. These illusions exploit the audience's assumptions about the physical world. Well known examples include "walls," "climbing stairs," "leaning," "descending ladders," "pulling and pushing," and so forth. New World Encyclopedia writers and editors rewrote and completed the Wikipedia article in accordance with New World Encyclopedia standards. This article abides by terms of the Creative Commons CC-by-sa 3.

Credit is due under the terms of this license that can reference both the New World Encyclopedia contributors and the selfless volunteer contributors of the Wikimedia Foundation. The study of visual illusions dates further back than the recognition of psychology as a separate discipline and a large proportion of all that is in this book was described by Helmholtz in his Handbuch der Physiologischen Optik. Boring and Titchener have both reviewed the early literature. The volume of writing on the subject, particularly the geometrical optical illusions, in the late nineteenth century can give the impression that interest faded after the turn of the century.

8 Mind-Bending Optical Illusions (And What They Reveal About How Our Brains Work)

This impression is expressed by Tolanski , but Zusne did a count of all such publications over the years. This clearly shows that the number of papers published on the geometrical optical illusions has increased steadily, quite closely in step with the total number of papers published in psychology. The steadily increasing number of papers has not, however, found an echo in an increase in the publication of books.

There seems to be a need for a work drawing together the very large literature on illusions and attempting to assess the various theoretical positions which have more recently been described. The information consists of cues about, for example, whether the display contains objects, how many objects, whether they are large or small, near or distant.

The perceptual decision is biased by states of the organism such as hunger, emotion, expectancy and so on. There is a large literature on such bias, but it is beyond the scope of this book. More important for my argument is ambiguity in the visual display. More than one perceptual decision is possible for most displays. Displays can vary from complete ambiguity to partial ambiguity. The latter are ones in which a particular interpretation is by far the most likely one, but other interpretations are probable and still others, although bizarre, are possible.

Paucity of cues, brought about, for example, by the brevity of the period of sampling or by reduced illumination, can increase ambiguity. One potent source of illusions is misinterpretation of cues in a situation in which there is at least a small amount of ambiguity.

But most of the illusions of this sort which one meets in everyday life involve situations which are ambiguous mainly because of brevity of sampling or paucity of cues. In these circumstances the perceptual decision is more difficult and cues arriving after the first perceptual decision often cause a revocation of it.


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Here the first sample of cues was brief. We resolved any ambiguity it contained and made a decision, but subsequent cues showed the decision to be wrong. Cues in the illusion displays described in this book are generally rather sparse. Line drawings have nothing of the richness we are used to in visual scenes, nor have small lights and luminous lines in the dark.

Ambiguity is therefore both more likely to be present and more likely to be resolved in a way which, by other evidence, is erroneous.


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Another way in which illusions are thought to occur is by the inappropriate operation of some mechanism which generally works in favour of veridicality truthfulness. This probably accounts for most of the illusions involving simple line-figures, though the precise nature of the process is unknown.

Gregory, for example, supposes that constancy, a mechanism which generally helps us to judge size and distance, is accidentally set off by the line illusions and leads to distortions see Chapter 6. Other writers maintain that a process which helps us to see edges, lateral inhibition in the visual system, also misleads us when lines run close together in the visual field.

A third way in which illusions occur is by the failure of the visual system to cope with input. The most common everyday example of this is probably ordinary visual blurring of fast-moving objects, but I would hesitate, perhaps unreasonably, to call this an illusion. That psychologists have taken such a steady and enduring interest in visual illusions might be taken as an indication of long-standing intellectual perversity, but there are arguments against such a view.

A very important strategy in finding out how correct perception operates is to observe situations in which misperceptions occur; to test, that is, the limits of the satisfactory function of perception whilst carefully altering the conditions under which it is working. It is fairly easy to formulate a theory which is consistent with the facts of correct perception, but it is a much more demanding task to produce a theory which is capable of predicting the failures as well as the successes of the perceptual system. People have long been aware of this and probably the chief cause of the continued interest in illusions is that they have been used as test instruments for theory, particularly by the Gestalt school.