VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL

Visual Illusions

Perception, causes and examples in the field of visual illusions.

Stefan Valentin Apostol

MED 4

Perception Re Examination

February 2015

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VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL

Abstract

This paper deals with the description and exemplification of visual illusions, reasoning on

possible eye factors and external factors that could cause them. Grasping the topic presented

requires understanding of the illusion phenomenon and elements of vision, perception.

Examples of illusions were used to define the concept and explain its appearance.

Introduction

As reported in Goldstein’s (2010) Encyclopedia of perception visual illusions represent the

altered perception of the actual world, where the human eye could be failing at seeing something,

distorting the reality and visualizing something that is not there, or it’s there in a different form.

It is important to notice that visual illusions and optical illusions are closely related, as visual

perception is needed in both cases, but optical illusions do not rely on brain processes , instead

being based on physical proprieties : light, human eye optics . The sections to follow will provide

with examples on several types of illusions.

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VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL

Human vision and perception

Vision is one of humans’ primary senses, linking and coordinating us with the exterior

environment. As Luckiesh (1965, pp.13) notices in his book Visual Illusions, the process of

vision could be divided in several steps:

Defining objects and surrounding based on color, light, shape, position

Forming an image on the retina through the optical mechanism : the eye

Adding defects of the mechanism, sensitiveness to light , color and structure of

the retina variables

Relating the information with events that follow or precede the initial capture

In his early work Luckiesh (1965, pp 16-17) tries to explain some defects of the optical system

that could lead to visual illusions. He mentions spherical and chromatic aberrations as some

concepts that could give an answer to illusions. The first refers to the way that eyes bend the

straight lines at the edge of an image like a wire mash or a check board pattern. Thus this effect

is automatically countered by variable optical density, which lowers the spherical aberration

effect. The chromatic aberration refers to monochromatic light that makes the objects looks

sharper, as the eyes focuses different color wavelengths differently.

Geometrical Illusions

Howe and Purves (2005, pp. 3-5) acknowledge that the

percepts of our vision are dependent on the projection that

light gives to space, objects on the retina. Thinking that the

stimuli and the projections are dependent also on the

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VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL

sources, discrepancies between the images transmitted to the brain and the actual life

measurements. These errors are called geometrical illusions as they refer to a misperception of

space, orientation, size, direction.

A simple example of geometrical illusion belongs to Jospeh Oppel (1895). Called the T illusion

or vertical-horizontal illusion it illustrates two lines of equal lengths. One of the lines is

positioned horizontally, and the other one vertically, starting from the middle of the first one. As

seen in Figure 1 the illusion created is that the structure seems to be taller than wider, even

though the lengths of the lines are equal.

Another important geometrical illusion is the Müller-Lyer Illusion. Created in 1889 by Frantz

Müler-Lyer, the experiment has shown that two identical lines appear having different lengths

when adding arrow tails to one and arrow heads to the other. As seen in Figure 2 the visually

longer line is the one with arrow tails and the ends. Howe and Purves (2005,pp. 71-81) are not

the only ones that tried to find reasoning for this geometrical illusion, but they gave an answer

using statistics of image-source relationship. Firstly some image templates were set to match the

Müller-Lyer patterns and the arrowhead were set

accordingly to pixel values. The heads of the lines

were also changed with squares, dots to see if the

illusion was still maintained, and it was. The study

found out that the illusion sustains the hypothesis of

visual perception as a probabilistic process, evolved

and inherited in retinal stimulus, Howe and Purves.

(2005,pp 83-84)

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Figure 2

VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL

Brightness Illusions

Goldstein (2010) summarizes that brightness illusions come with the apparent change in object

luminosity when the brain compares the main scene with objects that are close to the scene.

This happens as the brain doesn’t directly perceive the wavelength of light from objects. As said,

it compares then with the surrounding objects to define the brightness, Goldstein (2010,pp.

1079 ) . As an example the Hermann grid shows a black background covered by a white grid of

lines. The brain perceives lower intensity spots, grey spots, at the intersection between the grid

and the background. Experiments found out that retinal ganglion cells were firing when seeing

the intersection points. Figure 3 illustrates the Herman grid.

Figure 3

The same principle works with Edward H. Adelson chessboard. One way to explain this is based

on contrast. The light check is surrounded by darker neighbors, even when it’s staying in the

shadow of the cylinder. By this, it looks brighter than the rest, while the top check looks darker

while surrounded by light contrast checks with no shadows. According to MIT educational web

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VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL

portal, a second explanation argues that the shadows created by cylinder provides an important

role, as shadows have soft edges , and we expect sharp edges regarding the check board. Figure

4 presents the illusions and also the way in which the 2 check blocks are indentified as with the

same size and color.

Figure 4

Color Illusions

As the name suggests these illusions handle the different perception of colors compared to the

physical real state .Goldstein (2010, pp. 1027) describes it as a local contrast effect that is based

on a retinal lateral illusions. As a first example, Benham’s Top or Disk represents a black lines

stimulation project on a white background. By rotating the disk with the presented elements,

people could experience light colors, called Fechner Colors (Brown, 1965). Also an important

part of this illusion is the black background used for rotating the white disk. Based on how long

is the exposure between the black background and the black lines of the wheel, or the white

space of the wheel, human can perceive colors like red, green or blue. Brown (1965) argues that

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VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL

all of these are dependent on the length of white illumination. It is important to notice that

changing the direction, speed of the spin result in perception of various colors, of different

intensities. Figure 5 shows a representation of Benham’s top and the rotation given, resulting in

Fechner colors

Figure 5

The second example of color illusion is R. Beau Lotto’s Rubik cube. Here two color blocks of

the Rubik cube are the same color, but the illusion makes them look different. Goldstein (2010)

mentions that the background of the image and the top part of the Rubik cube are lightened by a

bright spectrum, so the orange takes the appearance of brown compared with the surrounding

blocks ( red, yellow, white) . On the other side the front side of the cube, is projected under a

shadow, where the orange cube is symmetrically positioned on the face. Here the perception of

the desired block is orange, as a result of the shadows casted and the neighborhood presented.

Figures 6 and 7 present the basics of this illusion, showing that the luminance playing a main

factor in visual perception.

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VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL

Figure 6 Figure 7

Size illusions

As stated in the Goldstein (2010,pp 1078-1079) size illusions occur when contextual cues of the

objects are changed. One of the most famous illusions in this topic is the Ebbinghaus illusion. As

seen in Figure 8 two identical circles are presented, one surrounded by a background of small

circles and the other one surrounded by a background of large circles. The illusion appears as the

circle surrounded by the large circles looks smaller than the circle surrounded by the small

circles. In their study Aglioty et al. (1995, pp 681-683) present test participants with the illusion

in the form of plastic circles and then asked to chose and grip one if it’s bigger or smaller. The

interest of the study was to measure how much the hand grip is open. The important finding was

whether body stimuli was transmitted wrong by an illusion of size and in conclusion , the brain

commanding to the hand how much to open in order to grab each circle.

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VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL

Figure 8

Another important size illusion is represented by Shepard’s Tables . Here 2 identical table

counters are presented, one that stays horizontally showing the legs, and the other one flipped by

90 degrees and positioned vertically with the legs

facing down ways. The vertically positioned table

looks longer and thinner even if the size is constant

regarding the horizontal one. The effect is explained

in Shepard’s (1990,pp 48,128) known as size-

constancy expansion, an illusory expansion of space

coming with distance change.

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Figure 9

VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL

Shape and Orientation Illusions

Speaking of orientation change or shape change we are also speaking about a distortion effect

where the brain is induced to believe that shapes and orientation of those are different than the

actual state of them, as Goldstein (2010,pp 1079-1080) acknowledges.

One of the first to be discussed here is the Café Wall Illusion, discovered by Richard L Gregory

and Priscilla Heard in St Michael’s Hill, Bristol, England. Here a coffee place had the outside

walls paved with white and black bricks that gave a distortion effect. Gregory & Heard (1979,pp

365-380) see the illusion as a check board figure. It succeeds in deceiving the viewer within the

field of visual perception. Horizontal lines made of white and black bricks, alternate to give the

perception of wedge distortion. In their work Gregory & Heard (1979) describe the illusion to be

a sum of different rules regarding wedge distortion, luminance and the space between bricks.

Also an important factor to notice is that peripheral vision gives a higher rate of distortion of the

image. It has been noted that several other factors come in change of this illusion. The café wall

distortion feels stronger at mid range luminance, and the mortar between the bricks should be

thin for a good effect. In their findings, Gregory & Heard (1979) considered that the mortar lines

tilt as a result of the striate cortex, more exactly by the orientation cells in there. Figure 10 shows

the Original Café in Bristol

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VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL

Figure 10

Invisibility Illusions

Viewers failing to perceive objects from the physical world encounter the phenomenon of

invisibility illusion. As Goldstein (2010, pp. 1080) mentions, the illusion is usually motion

induced, staying focused at one point while other parts move to create the invisibility.

Jeremy L Hilton is the creator of one invisibility illusion called the “Lilac chaser”, where a small

black point in the center of the image is the point of vision focus. Circling the point, purple

spheres disappear one at the time, giving the feeling of clockwise movement. Hilton’s work was

posted firstly by Michael Bach in 2005, on his website, as the result of a personal communication

between them. Here Bach (2005) comes in help with an explanation this invisibility illusion.

Negative retinal afterimage refers to hues that stay for some time in the same eye position. The

afterimage is built as the viewer maintains eye focus, and for this the purple circles become

firstly green and then invisible. A green disk is forced as a single moving disk.

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VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL

Figure 11

Illusory Motion illusions

Repetitive visual patters, displayed in a specific way, combined with the movement of the eye

can create illusory motion, Goldstein (2010,pp 1080) . The reasons why this happens will be

argued using the following example

The rotating Snakes illusion is a representation of Op-Art. Akiyoshi Kitaoka’s (2003) illusion is

an art form, where circles of different measurements have been inter positioned to create the

feeling of rotation , motion. It’s important to notice that circles have been designed at high

details, the center being a black dot for each one. The effect is maintained with a grey scale

version of the illusion, but the color one accentuates it. Kitaoka and Ashida(2003) include the

snake illusion under the category of peripheral drift illusions. Here saw teeth patterns combined

with attention on the outsides of the circles confer the perception of illusory motion.

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VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL

Figure 12

Conclusion

Definitely, visual illusions are a way to study brain activity in relation to visual context

manipulation. As seen in the examples before visual illusions can arise from visual stimuli

inheritance, or visual patter that react it specific ways, but they can also be crafted for research

purposes. The aim of the paper was to get accustomed with the concept of visual illusions, how

do they arise at anatomical level, but also at cognitive level in the brain. Exemplifying and

describing different categories of illusions, integrates the knowledge gathered in the field and

leaves place for further investigation. As E.B. Goldstein (2010) said in the Encyclopedia of

perception, intentionally developing visual illusions represent only a small part of the total of

material existent out there, but by working in this direction we could understand more how we

could use this in everyday life, not only tricking our brains into an illusory perception.

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References:

Aglioti, S., DeSouza, J. F. X., Goodale, M.A.(1995): Size-contrast illusions deceive the eye but

not the hand, ScienceDirect, retrived from :

http://www.sciencedirect.com/science/article/pii/S0960982295001333

Bach, M. (2005, June 22). Hinton's "Lilac Chaser" Retrieved February 15, 2015, from

http://www.michaelbach.de/ot/col-lilacChaser/index.html

Brown, J.L. (1965). Flicker and Intermittent Stimulation. In C. H. Graham, ed., Vision and

Visual Perception (pp. 251-320), New York: Wiley.

Checker shadow description. (n.d.). Retrieved February 15, 2015, from

http://web.mit.edu/persci/people/adelson/checkershadow_description.html

Goldstein, E. (2010). Encyclopedia of perception (pp. 1077-1081). Thousand Oaks, Calif.: Sage

Publications.

Gregory, R., & Heard, P. (1979). Border locking and the Café Wall illusion. Perception,pp. 365-

380.

Howe, C., & Purves, D. (2005). Perceiving geometry geometrical illusions explained by natural

scene statistics. New York, NY: Springer.

Kitaoka, A., & Ashida, H. (2003).Phenomenal characteristics of the peripheral drift illusion (4th

ed., Vol. 15, pp. 261-262).

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Luckiesh, M. (1965). Visual illusions: Their causes, characteristics, and applications. New

York: Dover Publications.

Oppel JJ (1855) ‘Uber geometrisch-optische T¨auschungen’. Jahresbericht des Frankfurter

Vereins 1854– 1855, 55:37–47.

Shepard, R. (1990). Mind sights: Original visual illusions, ambiguities, and other anomalies,

with a commentary on the play of mind in perception and art. New York: W.H. Freeman and.

Figures:

Figure1.The T illusion,, retrieved from :

http://www.indiana.edu/~ensiweb/lessons/percep.html

Figure 2.The Muller Lyer Illusion, retrieved from :

http://www.wpclipart.com/signs_symbol/optical_illusions/Muller_Lyer_illusion.png.html

Figure 3 Herman Grid, retrieved from:

http://www.michaelbach.de/ot/lum_herGrid/hermannGrid.gif

Figure 4 Edward H. Adelson chessboard, retrieved from :

http://web.mit.edu/persci/people/adelson/checkershadow_proof.html

Figure 5 Benham’s Top, retrieved from:

http://www.psy.ritsumei.ac.jp/~akitaoka/subjectivecolor-e.html

Figure 6 Beau Lotto’s Rubik cube, retrieved from:

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VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL

http://www.bbc.co.uk/news/magazine-11553099

Figure 7 Beau Lotto’s Rubik cube in different light, retrieved from:

http://news.bbcimg.co.uk/nol/shared/bsp/hi/dhtml_slides/10/illusion3/img/

illusion_dhtml_7_v2.gif

Figure 8 Ebbinghaus illusion, retrieved from:

http://www.kodaklens.co.uk/kidz/learnAndPlay/ebbinghaus/answer/

Figure 9 Shepard’s Tables, retrieved from:

http://www-psych.stanford.edu/~lera/psych115s/notes/lecture8/figures3.html

Figure 10 Oirginal Café Wall Illusion in St Michael’s Hill, Bristol, England, retrieved from:

http://www.exploratorium.edu/seeing/exhibits/caffe.html

Figure 11 Lilac Chaser Illusions, retrieved from:

http://www.tissy.it/le-illusioni-ottiche-piu-belle-come-funzionano/

Figure 12 The rotating snakes illusion, retrieved from :

http://www.ritsumei.ac.jp/~akitaoka/index-e.html

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