Form Perception

1
Form Perception

• Perceptual Organization
• and
• Perceptual Phenomena

2
Perceptual Organization

• 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 its all learned

3
The Grouping Laws

• Proximity
• Similarity
• Orientation
• Closure
• Good continuation
• Common Fate
• Familiarity

4
Proximity

• The visual system groups things which are close
  together

5
Similarity

• Objects of similar shape, intensity, or color are
  grouped together

O O O O O O O O O O O O O O O O O O O O O O O O O
O O O O O O O O O O O O O O O O O O O O O O O O O
O O V V O O V V O O V V O O V V O O V V O O V V O
O V V O O
6
Orientation

• The visual system groups items with common
  orientation

7
Closure

• We tend to fill out incomplete details to form
  good figures

8
Good Continuation

• The visual system prefers C1 continuity

Figure
Preferred interpretation
Other possible interpretation
9
Common Fate

• Items with common motion are readily grouped

Example Hidden Birdie
10
Familiarity

• You will undoubtedly see more than random specs
  in this picture below due to your familiarity
  with the subject.

11
Familiarity

• but this probably looks pretty random to most of
  you

12
Familiarity (or lack thereof)

• The word recognition in Chinese characters

13
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
14
Form Phenomena

• Müller-Lyer
• Horizontal-Vertical
• Poggendorf
• Titchener
• Kanisza
• Ouchi
• Baxter-Harris??
• Adaptation examples

15
Müller-Lyer (1889)

• Which line is longer?

16
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...

17
Horizontal-Vertical

• Which line is longer?

18
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?

19
Titchener

• Which red dot is bigger?

Art Bob Ausbourne
20
Titchener Explained

• Brain accentuates contrasts
• Small dot among big dots becomes smaller
• Big dot among small dots becomes bigger

21
Poggendorf (1860)

• Do the segments line up?

Interactive Demo
22
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 doesnt explain Poggendorf adequately
• No effect with just acute angles
• but do get it with just obtuse angles

23
Kanisza (1979)

• How many triangles?
• Illusory contours a.k.a. subjective contours

24
Kanisza Explained

• Gestalt dudes say simplelaw of Prägnanz
• Simplest interpretation is a white triangle
  occluding simple geometric figures.

25
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
26
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

27
Baxter-Harris??

• Note the black rectangular outline of the figure.

28
Baxter-Harris Explained

• Scientists are baffled by this latest discovery.

29
Adaptation to Intensity
30
Disappearing Blobs Explained

• Normally saccades keep edges alive
• With gentle intensity gradient, though, saccades
  are not large enough to re-trigger edge detectors

31
Adaptation to Frequency
32
Adaptation to Frequency
Response from frequency detectors for a pattern
of particular frequency
-

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 dont
fire as much as usual. So you perceive a lower
frequency than actual
33
Face Recognition

• Neonates of lt 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
34
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

35
Bibliography

• IllusionWorks http//www.illusionworks.com
• The Joy of Visual Perception http//www.yorku.ca/e
  ye/
• 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

36
Bonus rule of thumb

• The fovea covers only about 2 of visual angle
  roughly the area of the thumbnail at arms length.

From Churchland and Ramachandran, Perception
(Atkins, Ed.), Oxford Univ. Press, 1996
37
Sean Ho is next with more form perception...
Stay tuned!