Psy280: Perception
Prof. Anderson
Department of Psychology
Week 5
1
Visual philosophers
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What’s the meaning of life? Not exactly
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Computational approach to vision
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David Marr (MIT)
What is the purpose of vision?
What are the problems it must solve?
2
Computational problems in
object recognition

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What is it?
Object constancy: Variability in sensory
information
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Retinal position
Viewing position
Occlusion
Lighting, colour
3
Computational problems in
object recognition
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Where is it? Where’s Waldo?
?
4
Two visual cortical pathways
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These problems are reflected in the
organization of the visual system
Ventral “What” pathway
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Inferior longitudinal fasciculus
Dorsal “Where” pathway
Superior longitudinal
fasciculus
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5
Dissociation of what and
where in the monkey
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Landmark and object discrimination task
(Pohl, 1973)
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Parietal lobe
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Landmark
Object discrimination
Where
Temporal lobe
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What
6
Neuroimaging evidence for
“what” and “where:
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Attend to change in objects or locations
Same objects
Objects
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Occipito-temporal
Different location
Locations
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Posterior parietal
7
Ventral “what” pathway
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V4: Isoluminant color
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Lingual, fusiform gyrus
MT (V5): Motion
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Middle temporal gyrus
8
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Neuropsychological
evidence:
Color (V4) and Motion (MT)
Fractionation of perception following
cortical lesions
Achromatopsia
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Loss of colour vision
Akinetopsia
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Loss of motion
9
Higher-order “what” pathway
characteristics
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Complex response profile
Dissimilar to V1
 Not simple orientation, colour, motion
 Selectivity
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Hands,
faces etc
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10
Disorders of higher-order
ventral visual pathway

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Agnosia: “without knowledge”
Visual agnosia: vision w/out knowledge
Modality specific: Restricted to vision
 Not a memory disorder

Item can be recognized
through other modalities
 Touch, sound, smell
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11
Higher-order cortex is highly
specialized: Prosopagnosia
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Largely specific to faces
Can distinguish between faces and objects
Difficulty in distinguishing between faces
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Facial identification
Across category
Within category
12
Is there a region of the brain
devoted to faces?
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Fusiform face area (FFA)
Right middle fusiform gyrus especially
responsive to faces relative to other
objects
FFA
13
Neural selectivity:
Evolution vs experience
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FFA and other objects
“Greebles”
 Train to recognize
individuals
 Experts but not novices
activate FFA
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Potentially not face
specific
Reflects both evolution
and experience?
14
Object recognition:
Invariance
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Recognize object despite
differences in:
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Size, orientation, viewpoint,
lighting, colour, location
Inferotemporal cortex (IT)
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Anterior ventral stream
Invariance in neural
response
15
Evidence for constancy:
Lateral occipital complex
(LOC)
 Likely locus of object constancy

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Reduction in fMRI response w/ repetition
Invariance
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Size, location,viewpoint, illumination, occlusion
No effect of occlusion
16
How does cortex represent all
objects?
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Specificity coding
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Grandmother cells
Every orientation
Every color
Distributed coding
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“Coarse” coding across
neurons
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Color
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Combinatorial
Geons
3 cone types
Form
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Shape columns in IT
Geons
17
Ventral stream neurons and
consciousness
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Binocular rivalry
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fMRI evidence: FFA
and awareness
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FFA turns on when
aware of faces
FFA turns off when
unaware of faces
Ventral stream
representations support
consciousness
FFA
PPA
18
Dorsal pathway: Action
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Double dissociation
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Agnosia vs. optic ataxia
Apperceptive Agnosia
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Ventral stream damage
Impaired perception
Intact action
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Appropriate reaching
grasping
Optic ataxia
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Dorsal stream damage
Intact perception
Impaired action
19
Break!
10 minutes please
20
How does it all come together?
The binding problem
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Division of labour:
Parallel processing
Colour, shape,
motion, depth,
location
 All in separate regions
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How bound together?
Unified perception
 Not separate features
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21
Integration (binding) across
feature maps
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Synthesis requires attention—allows
coherence across feature maps: Objects
22
Attention is mental glue
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Allows features to stick
together
Without which,
perception falls apart
No coherent perception
of world
23
What is attention?
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Everybody knows what attention is. It is taking
possession by the mind, in clear and vivid form,
of one out of what seem several simultaneously
possible objects or trains of thought.
Focalization, concentration of consciousness are
of its essence. It implies withdrawal from
somethings in order to deal effectively with
others, and is a condition which has a real
opposite in the confused, dazed scatterbrain
state ….
William James (1890)
24
What is attention?
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“taking possession of the mind”
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“one out of what seem several simultaneously
possible objects”
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Inability attending to multiple things at once
“It implies withdrawal from somethings in order
to deal effectively with others”
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Control of the focus of attention
“Paying” attention comes with a cost
It has limited capacities that must be shared
“has a real opposite in the confused, dazed
scatterbrain state”
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Attention is the glue that keeps perception together
25
Pay attention!
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
26
Visual experience: A grand
perceptual illusion
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Rich and complex?
Sorry folks, its an illusion
We don’t “see” as much as we believe
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Don’t notice big changes in our environment
Change blindness
We fill-in our experience to make it coherent
 Little persistence of experience from one
moment to the next
 Seems coherent
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27
Selective attention
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Definition
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Why do we need selective
attention?
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Process relevant and ignore irrelevant
Can’t remember/processes everything
Can be independent from eye
movements
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Helmholtz (1894)
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Fixate eyes
Brief flash
Can pick what to perceive
Perception is not fixed
Not just what your eyes do
Perceptual free will
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Can choose what to perceive
28
Attention: Feature Integration
Theory
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Attention needed to glue
features together
Feature vs. conjunction visual
search (Treisman)
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Cereal
Parallel (“Pop out”)
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Parallel
E.g., Color, orientation
Requires little attention
Serial
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Color and orientation
Requires focal attention
Need to move attention around
29
Attention is required for
binding across feature maps
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Need attentional beam
Without attention?
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Illusory conjunctions
Unbound features
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Recombined
Illusory perception
Report seeing Yellow Square
and purple triangle
Need attention to glue
features together
Otherwise fall apart
30
Perceptual primitives:
What makes a “feature”?
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Perceptual primitives
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Molecules of perception?
Building blocks of perception
Cortical feature maps
Luminance, orientation, color,
Motion, depth
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Higher-order objects
Synthesis of primitives
 Objects defined by
conjunctions of primitives
 Need attention to bond into
compounds
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Pop-out
Unique
primitives
No pop-out
Share
primitives
31
Constructivism vs Gestalt
approaches to perception
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Structuralism
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Perception is created by adding elements together
Perception from the bottom up
Like building a house
Gestalt approaches
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Top-down perceptual organization
Perceptual inferences: Best Guesses
“Different than sum of its parts”
Not like building a house
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Don’t want best guess, want correct answer
Don’t want illusory foundation or beams!
32
Constructivism vs Gestalt
approaches to perception
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Perception is much
more than what is
projected on retina
Structuralism
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Gestalt
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inputs determine
perception
Active role of
perceiver
Same input, many different kinds of perception
33
Gestalt principles of
perceptual organization
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Best guesses about world
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Heuristics
“Rules of thumb”
Visual system is problem
solving for us
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Likely
Visual intelligence
Takes into account
probability of occurrence
What is likely vs not
Unlikely
34
Some of the
principles/heuristics
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Good figure/simplicity
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Good continuation
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Similarity
All have multiple
interpretations but
we all tend to agree
on one
35
Visual “guessing”:
Shape from shading
Square
raised
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Square
recessed
Sensitivity to perceptual invariance
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Light from above, not from below
36
Figure-ground segregation
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What’s an object?
What’s background?
Mental imposition of
depth
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Occlusion
Infer continuation of
background
underneath object
Best guess
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Unlikely share same
contours
37
Figure-ground segregation
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Factors that influence it
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Symmetry
Black
White
Development
 Beauty
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Meaning
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See arrows?
Emotion
Perceptual autism
 Pair faces w/ shock
 Bias perception toward
vase
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38
Chicken or the egg?

Meaning processed before or after figure-ground
segregation?
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Top-down perceptual organization over-rides
initial processing
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Only aware of the final products of this process
39
Substantial feedback onto
early visual processing
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Higher-order cortex
influences lower-order
cortex
More feedback than
feedforward
connections
Has delayed influence
Neural processing is
dynamic

Greater # of
feedback
connections
applies to early
cortex as well
Need to revise classic
definition of receptive
field
40
Meaning and perceptual
primitives
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Common fate

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Can perceive objects
based on motion
Higher-order meaning
influences lower-order
perceptual primitives
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E.g. motion
41
Meaning influences
perceptual primitives
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Structure from motion
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Can perceive objects
based on motion
Higher-order meaning
influences lower-order
perceptual primitives

E.g. motion
42
The End
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