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
1
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.
2
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
3Figure 1
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)
4
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
5
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
6
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.
7
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.
8
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.
9
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
10
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.
11
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.
12
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.
13
VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL
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).
14
VISUAL ILLUSIONS – STEFAN VALENTIN APOSTOL
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:
15
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
16
Top Related