Sensation, Perception, and Pattern Recognition

1
Sensation, Perception, and Pattern Recognition

• Brandon Beltz
• Feb 9, 2005

2
Vision Basics
3
Visual Perception
http//www.artlex.com/ArtLex/b/images/
4
The Fovea

• Light from direct focus lands in the Fovea.
• Contains almost all cones.
• Acuity -- accurate, precise vision-- is best in
  the fovea.
• Rods are abundant at the sides (periphery) of the
  fovea.
• Good for seeing in dark.

5
Blind spot

• A blind spot appears in vision due to lack of
  rods or cones on the retina where the optic nerve
  leaves the back of the eye into the brain.

To test your blind spot, 1. Cover your left eye
and stare at the circle with your right eye. 2.
Move your head back and forth until the square
disappears. 3. Test your left eye by covering the
right eye and staring at the square.
6
Eye Anatomy

• Iris
• Aqueous Humor
• Pupil
• Lens
• Cornea
• Retina

• Vitreous Humor
• Fovea
• Blind Spot
• Optic Nerve
• Rods
• Cones

7
Gathering Visual Information

• Saccades
• Rapid eye movements.
• Last between 25 and 100 msec.
• Fixations
• Pauses between saccades.
• The eye takes in visual information during
  fixations.

8
Eye Tracking Equipment
http//www.mpi.nl/world/tg/eye-tracking/eye-tracki
ng.html
9
Eye Movement Example
Eye Movements During Reading
http//www.mpi.nl/world/tg/eye-tracking/eye-tracki
ng.html
10
Eye Movement Example

• Eye movements while looking at a face.
• (notice the high number of fixations on eyes and
  mouth)

http//www.personal.psu.edu/users/e/l/elm173/schlw
ork/semester3/psych
11
Sensation versus Perception

• Sensation
• The reception of energy from the environment, and
  its initial encoding into the nervous system.
• Perception
• The process of interpreting and understanding
  sensory information.

12
Just noticeable difference (jnd)

• The amount by which two stimuli must differ so
  that the differences can be perceived.

13
What Makes Visual Perception Difficult?

• Patterns of light falling on your retina can be
  consistent with with many different scenes.

For example, is the figure to the left a square
or a cube face on? Maybe it is the base of a four
sided pyramid?
14
Visual Perception Difficulties

• Difficult problem of the visual system is the
  inverse projection problem
• The problem of recovering three-dimensional shape
  from a two-dimensional projection (image).
• A two dimensional projection may represent
  different three dimensional objects.

Can you see the Necker Cube on the Left as either
of the cubes below?
?
OR

• Thus, the visual system must deal with
  ambiguities in shape and orientation.

15
Visual Perception Difficulties

• The Second Important Problem - Surface Features
• The Visual System must deal with an objects
  surface features color, how dark or light it is,
  etc.
• There are ambiguities in
• Light Source
• Reflectance
• Shadow

Is the gray part a gray surface of a two part
object or is a shadow of the white part?
16
Visual Perception Difficulties

• The Third Problem - Object size and distance are
  ambiguous in a two dimensional representation.
• For example, the images of the sun and moon
  appear to be the same size. However, they are at
  different distances and are very different in
  size.
• Could you tell which is what by visual inspection
  alone?

http//www.astrosurf.com/re
17
Visual Perception Difficulties

• Inverse projection problem
• Surface features
• Object size and distance

18
2nd Half Outline

• Iconic Memory
• Sperlings Experiments
• Decay
• Interference/ backward masking
• Pattern Recognition
• Feature analysis
• Pandemonium model
• Context effects and top-down processing
• Object Recognition
• Recognition by Components (Biederman)
• Agnosia

19
Iconic Memory
20
Iconic Memory

• Icon a visual picture that represents an object.
• Think of a computer icon
• Iconic Memory also known as Visual Sensory
  Memory.
• Visual persistence The apparent persistence of a
  visual stimulus beyond its physical duration
• Example Rapidly wobble a pen between two fingers.

21
Sperlings Experiment (1960)

• Used a tachistoscope (t-scope) to rapidly present
  images to the eyes.
• Subjects saw a 3 x 4 grid of letters, presented
  very briefly (50 ms).
• Whole report condition
• Subjects recall any letters in the grid they
  could remember.
• Partial report condition
• Tone was sounded immediately after the grid
  disappeared.
• Subjects recalled one row of grid depending upon
  which tone they heard.

http//www.yorku.ca/ycom/gazette/past/archive/2000
22
Sperlings Experiment
23
Partial Report Condition
W. W. Norton
24
Sperling Task

• D T L K
• L K S F
• J S F H

25
Sperling Task

• S H D G
• L Z P V
• F G L Y

26
Sperlings Results

• Partial report condition more accurate than whole
  report with immediate recall.
• Partial report condition accuracy dropped quickly
  with time.
• After 1 sec, accuracy was same as whole report
  condition.

27
Sperlings Conclusions

• Why did Sperling argue that decay is the loss
  mechanism in iconic memory?
• How long does information remain in iconic
  memory?

28
Averbach and Coriell (1961)

• In a similar task as Sperlings, a visual circle
  cue was used after presentation of the letters.
• Circle cue used same sensory modality (e.g.
  vision) as letters.
• Sperling used auditory cue

29
Interference and Backward Masking

• S H D G L Z P V
• O S O I U G L Y

30
Interference and Backward Masking

• With visual circle cue, accuracy for letter
  recall was poor.
• Backward Masking When a later visual stimulus
  interferes with perception of an earlier one.

31
Why do images fade from iconic memory?

• Combination of image decay over time and
  interference from later images.
• Evidence from many studies show that iconic
  memory is an initial (and very fast) step in
  perception.

32
Ecological Validity and the Icon

• The icon is irrelevant to real-world perception.
• Haber (1983)
• Visual images are continuous
• Iconic memory data are useful only if reading in
  a thunderstorm.

33
Ecological Validity

• Degree to which our lab tasks resemble real
  world cognition.
• Also called External Validity.

34
Responses to Ecological Validity

• Later studies show that continuous sampling
  throughout fixations do not occur.
• Approximately first 50 ms of fixations gather
  information
• Studies show that people could detect the
  direction of moving icons very quickly
  (lt100 ms).
• Partial report conditions higher than whole
  report.

35
Pattern Recognition
36
Models of Pattern Recognition

• Feature analysis / Feature detection
• Recognition by components (object recognition)

37
Pandemonium Model
http//www.uic.edu/classes/psych/psych352jw/
38
Pandemonium Model
39
Pandemonium Model (1959)

• Types of Demons
• Data / Image
• Computational / Feature
• Cognitive
• Decision
• Demons represent mechanisms that interpret
  different levels of visual features.
• Parallel processing
• Bottom-up, data driven approach to processing

40
Pandemonium Model RT predictions

• List 1
• OQGGOEGO
• GOQOGGOQ
• GGOEGQOQ
• OEQQGOGO
• OQGGQOOG
• GOQOGOEO
• QOQOGOGG
• OOQGQOEO
• GOEOGGOQ

List 2 FHLLHFLH FLHLEHHF LLFHFHEF FLEHLFLH FLHEH
LFH HLHFFLHF HELFLHLF LHFLFHFL FLFHELHF
41
Context Effects

• Data Driven
• (bottom-up)
• Processing is driven by the stimulus.
• Conceptually Driven (top-down)
• Processing is driven by higher level knowledge.

stimulus
stimulus
42
Top-Down Processing
43
Top-Down Processing

• What is This and is this 15?
• What are these from?

Click on them!
44
Its a Face!
Click on the face to go back
45
Identifying Objects

• Key Question Supposedly objects require some
  type of memory representation. What are those
  memory representation like?
• Viewer-centered representations
• A mental representation of what an object looks
  like relative to the observer.
• Object- centered representation
• A mental representation of what an object looks
  like relative to the object itself.

46
Objects take different viewpoints
W. W. Norton
47
Objects take different viewpoints
W. W. Norton
48
Identifying Objects- Biederman (1987)
Which is easiest to recognize as a cup? The left
or right?
49
Recognition by ComponentsBiederman (1987)

• Geons building blocks of visual objects.

W. W. Norton
50
Recognition by Components- Limitations

• Over-reliance on data-driven processes?
• Evidence of context, top-down influences in
  recognition.
• Whole versus part perception?
• People recognize overall shape and pattern as
  quickly as components.
• Example. Recongize human body as quickly as an
  arm.
• Neuropsychological evidence?
• Two different processes in brain.
• Agnosia

51
Agnosia
52
Agnosia

• Failure or deficit in recognizing objects.

Patients can copy objects, but cannot recognize
them
http//www.cquest.utoronto.ca/psych/psy280f/
53
Agnosia
W. W. Norton
54
Apperceptive Agnosia

• Inability to integrate features of an object into
  an overall pattern.

Patients can not distinguish between objects,
despite clear differences in color and shape.
www.psych.ucalgary.ca/ pace/va-lab/
55
Associative Agnosia
Person can draw objects and distinguish shapes,
but cannot recognize what objects purpose is
W. W. Norton
56
Prosopagnosia

• Inability to recognize faces

http//www.radicalface.com/mediac/400_0/media/
57
Agnosia and Pattern Recognition

• Detecting features of a stimulus is different
  (and later) process than encoding stimulus.
• Detecting features is critical in constructing an
  overall pattern.
• Separate process exists in attaching a meaning
  or name to a pattern.

58
Lecture Summary

• Vision Basics
• Eye anatomy
• Eye movements
• Sensation and Perception
• Visual Perception Difficulties
• Inverse projection problem
• Surface features
• Object size and distance
• Iconic Memory
• Sperlings Experiments
• Decay
• Interference/ backward masking
• Pattern Recognition
• Feature analysis
• Pandemonium model
• Context effects and top-down processing
• Object Recognition
• Recognition by Components (Biederman)
• Agnosia