Illusions {illusion} are perceptions that differ from actual metric measurements. Brain uses rules to interpret sense signals, but rules can have contradictions or ambiguities. Vision sees bent lines, shifted lines, different lengths, or different areas, rather than line or area physical properties. Visual illusions are typically depth-perception errors [Frisby, 1979] [Gregory, 1972] [Heydt et al., 1984] [Kanizsa, 1979] [Peterhans and Heydt, 1991].
perception
Illusion, hallucination, and perception sense qualities do not differ. Mind typically does not notice illusions.
neural channels
Illusory edges and surfaces appear, because neural channels differ for movement and position. See Figure 1 and Figure 2.
contrast illusions
Contrast can cause illusions. Adelson illusion has grid of lighter and darker squares, making same-gray squares look different. Craik-O'Brien-Cornsweet illusion has lighter rectangle beside darker rectangle, making contrast enhancement at boundary. Mach bands have boundaries with enhanced contrast. Simultaneous brightness contrast illusions have same-gray squares in white or black backgrounds, looking like different grays. White illusion has black vertical bars with same-gray rectangle behind bars and adjacently and translucently in front of bars, looking like different grays.
color illusions
Color can cause color-contrast illusions and color and brightness illusions. Assimilation illusions have background effects that group same color points differently. Fading dot illusion has a green disk with blue dot in center, which fades with continued looking. Munker illusion has blue vertical bars with same-color rectangle behind bars or adjacently and translucently in front of bars, looking like different colors. Neon disk has an asterisk with half-white and half-red bars, which spins. Stroop effect has the word green in red, the word red in green.
geometric illusions
Geometry causes Ebbinghaus illusion, Müller-Lyer illusion, Ponzo illusion, and Zöllner illusion. Café-wall illusion has a vertically irregularly spaced black squares and white squares grid, making horizontal lines appear tilted. Distorted squares illusion has squares in concentric circles, making tilted lines. Ehrenstein illusion has radial lines with circle below center and square above center, making circle and square lines change alignment. Frazier spiral has concentric circles that look like a spiral in a spiraling background. Men with sunglasses illusion (Akiyoshi Kitaoka) has alternating color-square grid with two alternating vertical or horizontal dots at corners, making vertical and horizontal lines tilted. Midorigame or green turtle (Akiyoshi Kitaoka) has a grid with slightly tilted squares in one direction and a center grid with squares slightly tilted in other direction, making vertical and horizontal lines tilted. Poggendorf illusion has two vertical lines with diagonal line that goes behind space between lines, and two vertical lines with diagonal line that goes behind space between lines and dotted line on one side, making behind look not aligned.
size and depth illusions
Size and depth illusions are Ames room (Adelbert Ames), corridor illusion, impossible staircase (Maurits C. Escher), impossible triangle (Maurits C. Escher), impossible waterfall (Maurits C. Escher), Necker cube, size distortion illusion, and trapezoidal window (Adelbert Ames).
figure illusions
Imagined lines can cause illusions. Illusory circle has a small space between horizontal and vertical lines that do not meet, making a small circle. Illusory triangle has solid figures with cutouts that make angles in needed directions, which appear as corners of triangles with complete sides. Illusory square has solid figures with cutouts that make angles in needed directions, which appear as corners of squares with complete sides.
ambiguous figures
Ambiguous figures are eskimo-little girl seen from back, father-son, rabbit-duck, skull-two dancers, young woman and hag, and vase-goblet.
unstable figures
Figures can have features that randomly appear and disappear. Hermann's grid has horizontal and vertical lines with gaps at intersections, where dark disks appear and disappear. Rotating spiral snakes (Akiyoshi Kitaoka) have spirals, which make faint opposite spirals appear to rotate. Thatcher illusion has smile and eye corners up or down (Peter Thompson).
alternating illusions
Illusions with two forms show perceptual dominance or are bistable illusions. Vase-and-face illusion switches between alternatives.
Hering illusion
Radial rays, with two horizontal lines, make illusions. See Figure 4.
music
Music can cause illusions.
Necker cube
Wire cube at angle makes illusions. See Figure 3.
Ponzo illusion
If railroad tracks and ties lead into distance, and two horizontal bars, even with different colors, are at different distances, farther bar appears longer (Mario Ponzo) [1913]. See Figure 7. See Figure 8 for modified Ponzo illusions. See Figure 9 for split Ponzo illusions. Perhaps, line tilt, rather than depth perception, causes Ponzo illusion.
Rubin vase
Central vase has profiles that are symmetrical faces (Edgar Rubin). See Figure 5.
Zollner illusion
Vertical lines have equally spaced parallel line segments at 45-degree angles. See Figure 6.
After concentrating on object and then looking at another object, sense qualities opposite to, or shifted away from, original appear {aftereffect}| (CAE) [Blake, 1998] [Blake and Fox, 1974] [Dragoi et al., 2000] [He et al., 1996] [He et al., 1998] [He and MacLeod, 2001] [Koch and Tootell, 1996] [Montaser-Kouhsari et al., 2004].
afterimage
After observing bright light or image with steady gaze, image can persist {afterimage} [Hofstötter et al., 2003]. For one second, afterimage is the same as positive image. Then afterimage has opposite color or brightness {negative afterimage}. Against white ceiling, afterimage appears black. Colored images have complementary-color afterimages. Intensity is the same as image {positive afterimage} if eyes close or if gaze shifts to black background. Afterimage size, shape, brightness, and location can change {figural aftereffect}.
brain
Perhaps, CAEs reflect brain self-calibration. Orientation-specific adaptation is in area V1 or V2.
curves
Aftereffects also appear after prolonged stimulation by curved lines. Distortions associated with converging lines do not change with different brightness or line thickness.
gratings
Horizontal and vertical gratings cause opposite aftereffect {orientation-dependent aftereffect}, even if not perceived.
movement
Background can seem to move after observer stops moving {motion aftereffect, vision}.
stripes
Alternating patterns and prolonged sense stimulation can cause distortions that depend on adapting-field and test-field stripe orientations {contingent perceptual aftereffect}.
theory
Aftereffects appear because sense channels for processing color and orientation overlap {built-in theory} or because separate mechanisms for processing color and orientation overlap during adaptation period {built-up theory}.
tilt
After observing a pattern at an orientation, mind sees vertical lines tilt in opposite direction {tilt aftereffect}.
time
CAEs do not necessarily decay during sleep and can last for days.
Illusions can have two forms. Illusions {bistable illusion} like Necker cube have two forms almost equal in perceptual dominance.
Size, length, and curvature line or edge distortions can make illusions {cafe wall illusion}.
Illusions {Pepper's ghost} {stage ghost} {camera lucida} can depend on brightness differences. Part-reflecting mirrors can superimpose images on objects that people see through glass. Brightening one image while dimming the other makes one appear as the other disappears. If equally illuminated, both images superimpose and are transparent.
Gray patches surrounded by blue are slightly yellow {color contrast effect}. Black is not as black near blue or violet.
Mind can perceive transparency when observing different-color split surfaces {color scission}.
Blue and green appear closer {color stereo effect}. Red appears farther away.
When two objects have interchangeable features, and time or attention is short, mind can switch features to wrong object {conjunction error}.
Experimenter taps sharp pencil five times on wrist, three times on elbow, and two times on upper arm, while subject is not looking {cutaneous rabbit}. It feels like equal steps up arm [Geldard and Sherrick, 1972].
Minds perceive darker objects as heavier than lighter ones {empty suitcase effect}.
Light and dark checkerboards can have light-color dots at central dark-square corners, making curved square sides and curved lines along square edges {flying squirrel illusion}, though lines are really straight (Kitaoka).
Illusory people perceptions {ghost} can be partially transparent and speak.
Radial rays with two horizontal lines can make illusions {Hering illusion}.
Black squares in an array with rows and columns of spaces {Hermann grid} can appear to have gray circles in white spaces where four corners meet.
Lighter areas have apparently greater size than same-size darker areas {irradiation, perception}.
Orientation-specific color aftereffects can appear without perception {McCullough effect}. McCullough effect does not transfer from one eye to the other.
Moon or Sun apparent size varies directly with nearness to horizon {Moon illusion}, until sufficiently above horizon. On horizon, Moon is redder, hazier, lower contrast, and fuzzier edged and has different texture. All these factors affect perceived distance.
elevation
Horizon Moon dominates and elevates scene, but scene seems lower when Moon is higher in sky.
distance
Horizon Moon, blue or black sky, and horizon are apparently at same place. Risen Moon appears in front of black night sky or blue day sky, because it covers blue or black and there is no apparent horizon.
topographic map
Moon illusion and other perspective illusions cause visual-brain topographic image to enlarge or shrink, whereas retinal image is the same.
Illusions can have two forms, and people see mostly one {perceptual dominance}, then other.
In dark, blues seem brighter than reds {Purkinje shift}. In day, reds seem brighter than blues.
Line segments radiating from central imaginary circle {radial lines illusion} make center circle appear brighter. If center circle is black, it looks like background. If center circle has color, it appears brighter and raised {anomalous brightness}. If center circle is gray disk, it appears gray but shimmers {scintillating luster}. If center circle has color and background is black, center circle appears blacker {anomalous darkness}. If center circle has color and gray disk, center circle shimmers gray with complementary color {flashing anomalous color contrast}.
A vertical line segment in a tilted square frame appears to tilt oppositely {rod and frame illusion}, a late-visual-processing pictorial illusion.
If a rectangle is left of midline, with one edge at midline, rectangle appears horizontally shorter, and midline line segment appears to be right of midline {Roelof's effect} {Roelof effect}. If a rectangle is left of midline, with edge nearer midline left of midline, rectangle appears horizontally shorter, and rectangle appears closer to midline.
Central circle with vertical stripes surrounded by annulus with stripes angled to left appears to have stripes tilted to right {simultaneous tilt illusion}, an early visual processing illusion.
If small and large object both have same weight, small object feels heavier in hand than large object {size-weight illusion}. People feel surprise, because larger weight is lighter than expected.
Lighter color contours inside darker color contours spread through interiors {watercolor effect}.
In zero-gravity environments, because eyes shift upward, objects appear to be lower than they actually are {zero-gravity illusion}.
Figures {ambiguous figure}| can have two ways that non-vertical and non-horizontal lines can orient or have two ways to choose background and foreground regions. In constant light, observed ambiguous-figure surface-brightness changes as perception oscillates between figures [Gregory, 1966] [Gregory, 1986] [Gregory, 1987] [Gregory, 1990] [Gregory, 1997] [Seckel, 2000] [Seckel, 2002].
Figures (Jastrow) with duck beaks and rabbit ears make illusions {duck-rabbit illusion}.
Vases with profiles of symmetrical faces (Edgar Rubin) can make illusions {vase and two faces illusion} {Rubin vase}.
Old crone with black hair facing young girl can make illusions {Salem witch and girl illusion}.
Illusions can depend on brightness differences, sound-intensity differences, or line-length and line-spacing differences {Craik-Cornsweet illusion}. Finding differences explains Weber's law and why just noticeable difference increases directly with stimulus magnitude.
People can see logically paradoxical objects {impossible triangle} {impossible staircase}. People can experience paradox perceptually while knowing its solution conceptually. Pictures are essentially paradoxical.
Impossible triangles can make illusions {Kanizsa illusion} {Kanizsa triangle}.
Wire cubes at angles can make illusions {Necker cube}.
Impossible stairs can make illusions {Schroder stairs}.
Minds can combine two features, for example, color and shape, and report perceiving objects that are not in scenes {illusory conjunction} {conjunction, illusory}.
Mind can extend contours to places with no reflectance difference {illusory contour} {contour, illusory}.
Medium-size circle surrounded by smaller circles appears larger than same-size circle surrounded by larger circles {Ebbinghaus illusion} {Titchener circles illusion}, a late-visual-processing pictorial illusion.
Lines with inward-pointing arrowheads and adjacent lines with outward-pointing arrowheads appear to have different lengths {Müller-Lyer illusion}.
If railroad tracks and ties lead into distance, and two horizontal bars, even with different colors, are at different distances, farther bar appears longer (Mario Ponzo) [1913] {Ponzo illusion}. Perhaps, line tilt, rather than depth perception, causes Ponzo illusion.
Vertical lines with equally spaced parallel line segments at 45-degree angles can make illusions {Zollner illusion}.
In homogeneous backgrounds, a single object appears to move around {autokinetic effect} {keyhole illusion} [Zeki et al., 1993].
If line or spot is moving, and another line or spot flashes at same place, the other seems behind first {flash-lag effect} [Eagleman and Sejnowski, 2000] [Krekelberg and Lappe, 2001] [Nijhawan, 1994] [Nijhawan, 1997] [Schlag and Schlag-Rey, 2002] [Sheth et al., 2000]. Flashed object seems slower than moving object.
Rotating two-dimensional objects makes them appear three-dimensional {kinetic depth effect} [Zeki et al., 1993].
Alternating visual-stimulus pairs show apparent movement at special times and separations {Korte's law} [Zeki et al., 1993].
After continuously observing moving objects, when movement stops, stationary objects appear to move {motion aftereffect, illusion}.
If screen has stationary color spots and has randomly moving complementary-color spots behind them, mind sees stationary spots first, then does not see them, then sees them again, and so on {motion-induced blindness} [Bonneh et al., 2001].
Spokes in turning wheels seem to turn in direction opposite from real motion {wagon-wheel illusion} [Gho and Varela, 1988] [Wertheimer, 1912] [Zeki et al., 1993].
If people view scenes with flows, when they look at stationary scenes, they see flow {waterfall illusion}. Waterfall illusion can be a series of still pictures [Cornsweet, 1970].
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Date Modified: 2022.0225