Psychophysicsquantitatively investigates the relationship between physicalstimuliand thesensationsandperceptionsthey affect. Psychophysics has been described as "the scientific study of the relation between stimulus and sensation"[1]or, more completely, as "the analysis of perceptual processes by studying the effect on a subject's experience or behaviour of systematically varying the properties of a stimulus along one or more physical dimensions".[2]Psychophysicsalso refers to a general class of methods that can be applied to study aperceptual system. Modern applications rely heavily on threshold measurement,[3]ideal observer analysis, andsignal detection theory.

Anoptical illusion(also called avisual illusion) is characterized byvisually perceivedimages that differ from objective reality. The information gathered by the eye is processed in the brain to give aperceptionthat does not tally with a physical measurement of the stimulus source. There are three main types: literal optical illusions that create images that are different from the objects that make them, physiological illusions that are the effects of excessive stimulation of a specific type (brightness, colour, size, position, tilt, movement), andcognitiveillusions, the result ofunconscious inferences.Optical illusions are often classified into categories including the physical and the cognitive or perceptual,[1]and contrasted withoptical hallucinations.

Impossible objectFrom Wikipedia, the free encyclopedia(Redirected fromImpossible objects)For John Frankenheimer's film, seeImpossible Object. For the independent rock band, seeThe Impossible Shapes.Animpossible object(also known as animpossible figureor anundecidable figure) is a type ofoptical illusion. It consists of a two-dimensionalfigure which is instantly andsubconsciouslyinterpreted by thevisual systemas representing aprojectionof a three-dimensional object.In most cases the impossibility becomes apparent after viewing the figure for a few seconds. However, the initial impression of a 3D object remains even after it has been contradicted.Examples of impossible objects

Oscar Reutersvrds optical illusion (1934)

Animpossible cube

Penrose stairs

Penrose triangle

Blivet

Moon illusionFrom Wikipedia, the free encyclopediaThis articleneeds additional citations forverification.Please helpimprove this articlebyadding citations to reliable sources. Unsourced material may be challenged and removed.(May 2008)

Aharvest moonTheMoon illusionis anoptical illusionwhich causes theMoonto appear larger near thehorizonthan it does higher up in thesky. This optical illusion also occurs with theSunandstar constellations. It has been known since ancient times and recorded by various cultures.[1]The explanation of this illusion is still debated.[2][3]Relative size hypothesis[edit]

TheEbbinghaus illusion. The two central circles are the same size.Historically, the best-known alternative to the "apparent distance" theory has been a "relative size" theory. This states that the perceived size of an object depends not only on its retinal size, but also on the size of objects in its immediate visual environment. In the case of the Moon illusion, objects in the vicinity of the horizon moon (that is, objects on or near the horizon) exhibit a fine detail that makes the Moon appear larger, while the zenith moon is surrounded by large expanses of empty sky that make it appear smaller.[14]The effect is illustrated by the classicEbbinghaus illusionshown at the right. The lower central circle surrounded by small circles might represent the horizon moon accompanied by objects of smaller visual extent, while the upper central circle represents the zenith moon surrounded by expanses of sky of larger visual extent. Although both central circles are actually the same size, the lower one looks larger to many people.

Ponzo illusionFrom Wikipedia, the free encyclopedia

An example of the Ponzo illusion. Both of the horizontal yellow lines are the same length.ThePonzo illusionis ageometrical-optical illusionthat was first demonstrated by theItalianpsychologistMario Ponzo(18821960) in 1911.[1]He suggested that thehuman mindjudges an object's size based on its background. He showed this by drawing two identical lines across a pair of converging lines, similar torailwaytracks. The upper line looks longer because we interpret the converging sides according tolinear perspectiveas parallel lines receding into the distance. In this context, we interpret the upper line as though it were farther away, so we see it as longer a farther object would have to be longer than a nearer one for both to produce retinal images of the same size.

Caf wall illusionFrom Wikipedia, the free encyclopedia

The caf wall illusion: the horizontal lines are parallel, despite appearing to be at different angles to each otherThecaf wall illusionis ageometrical-optical illusionin which theparallelstraight dividing lines between staggered rows with alternating black and white "bricks" appear to be sloped.It was first described under the nameKindergarten illusion[1]in 1898, and re-discovered byRichard GregoryExplanationDiagonal lines are perceived because of the wayneuronsin thebraininteract. Different types of neurons react to the perception of dark and light colors, and because of the placement of the dark and light tiles, different parts of the grout lines are dimmed or brightened in the retina. Where there is a brightness contrast across the grout line, a small scale asymmetry occurs whereby half the dark and light tiles move toward each other forming small wedges. These little wedges are then integrated into long wedges with the brain interpreting the grout line as a sloping line.

Fraser spiral illusionFrom Wikipedia, the free encyclopedia

Fraser spiral illusionTheFraser spiral illusionis anoptical illusionthat was first described by the British psychologistSir James Fraser(1863 1936) in 1908.The illusion is also known as thefalse spiral, or by its original name,the twisted cord illusion. The overlapping black arc segments appear to form a spiral; however, the arcs are a series of concentric circles.

Mach bandsFrom Wikipedia, the free encyclopediaFor the shock-wave pattern, seeMach diamond.

Exaggerated contrast between edges of the slightly differing shades of gray, appears as soon as they touch

The Mach bands illusion makesa)the darker areas falsely appear even darker in the illusionary "bands" stretching along boundaries with the slightly lighter areas, andb)the lighter areas falsely appear even lighter along the same boundaries.

The illusion is independent of orientation.Mach bandsis anoptical illusionnamed after the physicistErnst Mach. It exaggerates thecontrastbetween edges of the slightly differing shades of gray, as soon as they contact one another, by triggering edge-detection in the human visual system.

Delboeuf illusionFrom Wikipedia, the free encyclopedia

The two black circles are exactly the same size; however, the one on the leftseemslarger.TheDelboeuf illusionis anoptical illusionof relative size perception. In the best-known version of the illusion, two circles of identical size have been placed near to each other and one is surrounded by anannulus; the surrounded circle then appears larger than the non-surrounded circle if the annulus is close, while appearing smaller than the non-surrounded circle if the annulus is distant. A 2005 study suggests it is caused by the same visual processes that cause theEbbinghaus illusion.[1]

ExplanationThe difference in sizeperceptionis due to the surrounding visual cues (larger or smaller surrounding circles), and the way thebrainprocesses these visual cues.The Ebbinghaus illusion has played a crucial role in the debate over the existence of separate pathways in the brain for perception and action. Experiments have shown that, while adult subjects perceive the center circles as differing in size, they reach out to grasp the circle accurately. In theory, this is due to the process of perception using a different visual pathway than the process of action. While adults rarely misjudge the size of the center disk while reaching for it, experiments have found that young children do, in fact, misjudge size both perceptually and through action. Researchers have proposed that this is because young children rely on both pathways to process tasks, instead of the separate pathways that adults use.

Explanation of cognitive illusions[edit]Perceptual organization[edit]

Reversible figures and vase

Rabbitduck illusionTo make sense of the world it is necessary to organize incoming sensations into information which is meaningful.Gestalt psychologistsbelieve one way this is done is by perceiving individual sensory stimuli as a meaningful whole.[7]Gestalt organization can be used to explain many illusions including therabbitduck illusionwhere the image as a whole switches back and forth from being a duck then being a rabbit and why in thefiguregroundillusion the figure and ground are reversible.

Kanizsa triangleIn addition, Gestalt theory can be used to explain theillusory contoursin the Kanizsa Triangle. A floating white triangle, which does not exist, is seen. The brain has a need to see familiar simple objects and has a tendency to create a "whole" image from individual elements.[7]Gestalt means "form" or "shape" in German. However, another explanation of the Kanizsa Triangle is based inevolutionary psychologyand the fact that in order to survive it was important to see form and edges. The use of perceptual organization to create meaning out of stimuli is the principle behind other well-known illusions includingimpossible objects. Our brain makes sense of shapes and symbols putting them together like a jigsaw puzzle, formulating that which isn't there to that which is believable.

Depth and motion perception[edit]

Ponzo illusionIllusions can be based on an individual's ability to see in three dimensions even though the image hitting the retina is only two dimensional. ThePonzo illusionis an example of an illusion which uses monocular cues of depth perception to fool the eye.In the Ponzo illusion the convergingparallel linestell the brain that the image higher in thevisual fieldis farther away therefore the brain perceives the image to be larger, although the two images hitting the retina are the same size. The optical illusion seen in adiorama/false perspectivealso exploits assumptions based on monocular cues ofdepth perception. TheM.C. EscherpaintingWaterfallexploits rules of depth and proximity and our understanding of the physical world to create an illusion. Likedepth perception,motion perceptionis responsible for a number of sensory illusions. Filmanimationis based on the illusion that the brain perceives a series of slightly varied images produced in rapid succession as a moving picture. Likewise, when we are moving, as we would be while riding in a vehicle, stable surrounding objects may appear to move. We may also perceive a large object, like an airplane, to move more slowly than smaller objects, like a car, although the larger object is actually moving faster. Thephi phenomenonis yet another example of how the brain perceives motion, which is most often created by blinking lights in close succession.The ambiguity of direction of motion due to lack of visual references for depth is shown in the spinning dancer illusion. The spinning dancer appears to be moving clockwise or counterclockwise depending on spontaneous activity in the brain where perception is subjective. Recent studies show on the fMRI that there are spontaneous fluctuations in cortical activity while watching this illusion, particularly the parietal lobe, because it is involved in perceiving movement.[11]Color and brightness constancies[edit]

Simultaneous Contrast Illusion. The background is acolor gradientand progresses from dark grey to light grey. The horizontal bar appears to progress from light grey to dark grey, but is in fact just one colour.

In this illusion, the coloured regions appear rather different, roughly orange and brown. In fact they are the same colour, and in identical immediate surrounds, but the brain changes its assumption about color due to the global interpretation of the surrounding image. Also, the white tiles that are shadowed are the same color as the grey tiles outside the shadow.Perceptual constancies are sources of illusions.Color constancyand brightness constancy are responsible for the fact that a familiar object will appear the same color regardless of the amount of light or color of light reflecting from it. An illusion of color or contrast difference can be created when the luminosity or color of the area surrounding an unfamiliar object is changed. The contrast of the object will appear darker against a black field that reflects less light compared to a white field even though the object itself did not change in color. Similarly, the eye will compensate for color contrast depending on the color cast of the surrounding area.In addition to the Gestalt principles of perception, water-color illusions contribute to the formation of optical illusions. Water-color illusions consist of object-hole effects and coloration. Object-hole effects occur when boundaries are prominent where there is a figure and background with a hole that is 3D volumetric in appearance. Coloration consists of an assimilation of color radiating from a thin-colored edge lining a darker chromatic contour. The water-color illusion describes how the human mind perceives the wholeness of an object such as top-down processing. Thus, contextual factors play into perceiving the brightness of an object.[12]

contrast effectis the enhancement or diminishment, relative to normal, ofperception,cognitionor related performance as a result of successive (immediately previous) or simultaneous exposure to astimulusof lesser or greater value in the same dimension. (Here, normal perception, cognition or performance is that which would be obtained in the absence of the comparison stimulusi.e., one based on all previous experience.)Chubb illusionFrom Wikipedia, the free encyclopedia

Figure 1: An example of the Chubb illusion. The center areas of two rectangular fields are identical, but appear different because the background fields are different.TheChubb illusionis anoptical illusionor error invisual perceptionin which the apparentcontrastof an object varies substantially to most viewers depending on its relative contrast to the field on which it is displayed.[1]These visual illusions are of particular interest to researchers because they may provide valuable insights in regards to the workings of humanvisual systems.

Simultaneous contrastChecker shadow illusionFrom Wikipedia, the free encyclopediaThechecker shadow illusionis anoptical illusionpublished byEdward H. Adelson, Professor ofVision ScienceatMITin 1995.[1]The image depicts a checkerboard with light and dark squares. The optical illusion is that the area of the image labeled A appears to be a darkercolorthan the area of the image labeled B on the 2D plane of the rendered 3D projection. However, they are actually exactly the same color on the 2D plane of the image file (but not necessarily in the 3D projection) which becomes especially obvious if the projected 3D scenery is rendered partially or entirely defective.That the two squares are of the same color on the 2D plane can be proven using the following methods: Opening the illusion in an image editing program and using the eyedropper tool to verify that the colors are the same. Cut out a cardboard mask. By viewing patches of the squares without the surrounding context, you can remove the effect of the illusion. A piece of cardboard with two circles removed will work as a mask for a computer screen or for a printed piece of paper. Connecting the squares with a rectangle of the same color, as seen below in the middle figure. Using aphotometer. Print the image and cut out the squares. Cut out each square along the edges. Remove them. Hold them side by side.[2] Isolating the squares. Without the surrounding context, the effect of the illusion is dispelled. This can be done by using the eyedropper tool inimage editing programs, such asGimpto sample the values of A & B, and to color in the newly adjacent rectangles using the paint bucket tool.Areas of the image A and B are the same colorThe original image plus two vertical stripes of the same shade of gray to make the colour equivalence more clearA rectangle of the same color has been drawn connecting the two areas of the image

Explanation[edit]As an explanation as to why the illusion works, Adelson writes:The visual system needs to determine the color of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray "paint" that belongs to the surface.The first trick is based on local contrast. In shadow or not, a check that is lighter than its neighboring checks is probably lighter than average, and vice versa. In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is light when compared to its neighbors. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look dark by comparison.A second trick is based on the fact that shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges. The visual system tends to ignore gradual changes in light level, so that it can determine the color of the surfaces without being misled by shadows. In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible.The "paintness" of the checks is aided by the form of the "X-junctions" formed by 4 abutting checks. This type of junction is usually a signal that all the edges should be interpreted as changes in surface color rather than in terms of shadows or lighting.As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view.[3]