Form Perception
Perceptual Organization
and
Perceptual Phenomena
Perceptual Organization
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From Gestalt psychology c1930
Law of Prägnanz (“good configuration”)
Wertheimer 1958 -- “laws of grouping”
Grouping laws are corollaries to Prägnanz
Not much explanation given for these -Gestalt theorists like to think they are innate
• Opponents like to argue that it’s all learned
The Grouping Laws
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Proximity
Similarity
Orientation
Closure
Good continuation
Common Fate
Familiarity
Proximity
• The visual system groups things
which are close together
Similarity
• Objects of similar shape, intensity, or
color are grouped together
OOVVOO
VVOOVV
OOVVOO
VVOOVV
OOVVOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
OOOOOOOOOO
Orientation
• The visual system
groups items with
common
orientation
Closure
• We tend to fill out
incomplete details to
form “good figures”
Good Continuation
• The visual system
prefers C1
continuity
Figure
Preferred
interpretation
Other possible
interpretation
Common Fate
• Items with
common motion
are readily
grouped
Example:
Hidden Birdie
Familiarity
• You will undoubtedly see more than random
specs in this picture below due to your
familiarity with the subject.
Familiarity
• …but this probably looks pretty random
to most of you
Familiarity (or lack thereof)
• The word “recognition” in Chinese
characters:
“Modern” Theories
• Fourier based
– Campbell & Robinson 1968
• Autocorrelation
– Uttal 1975
• Network models
– Grossberg 1976, Leake and Anninos 1976, Sejnowski 1976
• Texons
– Julesz 1965, 1975, 1981
From Caelli, Terry. Visual Perception: Theory and Practice, Pergamon Press, 1981
Form Phenomena
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Müller-Lyer
Horizontal-Vertical
Poggendorf
Titchener
Kanisza
Ouchi
Baxter-Harris??
Adaptation examples
Müller-Lyer (1889)
• Which line is longer?
Müller-Lyer Explained
• In first version, perspective is common
explanation:
• In second, we need a different explanation
– Feedback from eye scanning back and forth?
– Nope. Lines presented tachistoscopically (really
fast) still exhibit effect.
• Interpreting whitespace as part of line?
• Many other theories have been proposed...
Horizontal-Vertical
• Which line is longer?
Horizontal-Vertical Explained
• The bisection is important -- no illusion if
segments joined at endpoints. ( i.e. |_ )
• Some say illusion only works with bisector
horizontal (works either way for me, though)
• Largest effect is actually for a line 20° off
the vertical (Cormack & Cormack 1974)
• Perspective effect? (Kunnapas 1957)
• Greater length of horizontal FOV vs vertical?
Titchener
• Which red dot is bigger?
Art: Bob Ausbourne
Titchener Explained
• Brain accentuates contrasts
– Small dot among big dots becomes smaller
– Big dot among small dots becomes bigger
Poggendorf (1860)
• Do the segments line up?
Interactive Demo
Poggendorf Explained
• Blakemore (1973) attributes this to "tilt
contrast," caused by lateral inhibition
between neural signals of orientation, which
will expand the appearance of acute angles.
• Explains some other illusions
• But doesn’t explain Poggendorf adequately
– No effect with just acute angles
– but do get it with just obtuse angles
Kanisza (1979)
• How many
triangles?
• Illusory contours
a.k.a. subjective
contours
Kanisza Explained
• Gestalt dudes say simple:“law of Prägnanz”
– Simplest interpretation is a white triangle
occluding simple geometric figures.
Ouchi
The effect may be heightened if you read these
words while paying attention to what is happening
with the circle in your peripheral vision
Ouchi Explained
• Inset perceived as being at different depth?
• “a failure to integrate two motion signals
into a single motion vector which
characterises rigid motion” (Hine, Cook, and Rogers
1995 p. 3093)
• An artifact of “biased flow estimation”
(Fermüller, Pless and Aloimonos 1998)
http://www.cfar.umd.edu./ftp/TRs/CVL-Reports-1998/TR3917-fermuller.ps.gz
Baxter-Harris??
• Note the black rectangular outline of the
figure.
Baxter-Harris Explained
• Scientists are baffled by this latest discovery.
Adaptation to Intensity
Disappearing Blobs Explained
• Normally saccades keep edges “alive”
• With gentle intensity gradient, though,
saccades are not large enough to re-trigger
edge detectors
Adaptation to Frequency
Adaptation to Frequency
Response from frequency detectors for a pattern of particular frequency
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Looking at a high freq pattern
causes these detectors to fire with
the strength indicated
=
Normally looking at a medium freq pattern
would result in this response
But the high freq detectors are tired and
don’t fire as much as usual. So you
perceive a lower frequency than actual
Face Recognition
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Neonates of < 1yr can do it
Special “face” cells in cortex for it
Studies of prosopagnosic patients revealing
Identity, familiarity, expression, sex all
processed separately in parallel
• Faces seem to be stored as diffs off of a
prototype, but using what bases?
From Bruce (Ed.), Face Recognition: a special issue of the European Journal of Cognitive Psychology, 1991
Face Recognition
• You might recognize
this face…
• Often as few as 18
pixels is enough to
identify a face
• Monkey “face” cells
fire for both high- and
low-pass filtered faces
Bibliography
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IllusionWorks http://www.illusionworks.com
The Joy of Visual Perception http://www.yorku.ca/eye/
Sandlot Science http://www.lainet.com/~ausbourn/
UIUC Internet Psych Lab, Visual Perception
http://kahuna.cogsci.uiuc.edu/ipl/vis/level_2_vis.html
• MIT Encyclopedia of Cognitive Science
http://mitpress.mit.edu/MITECS/
• Wientraub & Walker, Perception, Wadsworth Publishing
Co, Belmont,CA. 1966
Bonus “rule of thumb”
• The fovea covers only about 2° of visual
angle: roughly the area of the thumbnail at
arm’s length.
From Churchland and Ramachandran, Perception (Atkins, Ed.), Oxford Univ. Press, 1996
Sean Ho is next with
more form perception...
Stay tuned!