visual perception-for

Disorders of Object Recognition
AGNOSIA : a general term for a loss of ability
to recognize objects, people, sounds, shapes, or
Agnosias result from damage to cortical areas
of the visual system (retina and optic nerve are
not impaired, nor is visual acuity, color,
motion or depth perception impaired).
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DSM does not officially categorize agnosia
however, they are commonly divided into two
categories: Apperceive Agnosia and Associative
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Apperceptive Agnosias (Difficulty with
perceptual processes)
Have trouble recognizing, copying, or
discriminating between different visual
stimuli. Ex. may not be able to distinguish a
poker chip from a scrabble tile despite their
clear difference in shape and surface
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When patients are able
to identify objects,
they do so based on
inferences using color,
size, texture and/or
reflective cues to piece
it together.
Video (begin @ .33)
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Simultanagnosia refers to an inability to
recognize two or more things at the same
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- may have difficulty reading and counting
because these activities involve viewing more
than one thing at a time.
- may seem to be "blind" since they bump into
objects that are close together. Motion may
further impair their ability to perceive objects.
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Associative agnosias - Perceptual
processes are intact but patient is
unable to recognize visually
presented objects
-may be able to replicate a drawing
of the object but still
fail to recognize it.
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Object Recognition
Integrating or
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Face Recognition
Is it different than Object recognition? Yes.
More Holisitc - altering the appearance of one
facial region can strikingly affect the percept
of other regions and of the whole face.
Memory for a face is nore accurate
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Part-whole illusion.
Part-whole illusion. The only difference between the
two images is the mouth. Altering the mouth creates
illusions of alteration in regions of the rest of the
face (e.g., makes the nose appear shorter on left and
longer on right, makes the eyes appear more
interested on left and less interested on right). In the
inverted version, the difference in the mouth shape
can be easily seen but the illusory changes in the rest
of the face are not apparent.
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Composite Face Illusion
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Prosopagnosia - failure to
recognize faces
Video described a woman
who could not recognize
her family’s faces or her
own. She could recognize
people through voices, hair
color, eye color…
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Fusiform Face Area of temporal Lobe.
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Bruce and Young (1986) model
of face recognition.
(1) Face detection followed by
(2) processing of the face’s
structure which is then
matched to a memory
representation (face memory).
(3) The perceptual representation
of the face can also be used
for recognition of facial
expression and gender
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Evidence of separate processes
• Edward: impaired face recognition but
normal face detection.
• Some Prosopagnosia patients cannot
recognize faces but can recognize emotions.
• Young et. Al (1985) diary study –
Information about person before name
• Alexithymia - Emotion blindness
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Individual differences in Face
Recognition Ability
Developmental vs. Acquired Prosopagnosia
Criminal Justice Application
Want to test your own Face Recognition Ability?
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Visual Imagery and Visual Perception
Visual Perception relies heavily on Bottom-up
Visual Imagery is totally top-down.
Confusions between Imagery and Perception
Charles Bonnet Syndrome - form of
phantom vision.
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Visual Imagery resembles Visual Perception
Where are Visual images formed in the brain?
Primary Visual Cortex (V1 and V2) referred to as
the visual buffer. These areas are active in both
visual perception and visual imagery.
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Binocular rivalry occurs when different images
are presented, one to each eye, observers do not
perceive a blend of the two stimuli, but instead
experience irregular perceptual alternations
between the two images such that only one image
is typically perceived at a time.
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If one image is presented slightly
beforehand, it primes (increases the
likelihood of) the perception of that image
in the binocular rivalry situation.
What happens when the prime is imagined
rather that perceived? Same effect.
Imagery can facilitate perception!
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Baddley and Andrade (2002)
Participants rated the visual vividness of
images under three different conditions.
No other simultaneous task. – baseline.
Auditory simultaneous task – same as baseline
Visual simultaneous task – less vivid
Perceptual and Imagery tasks interfere with
each other.
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Brain Imaging
Extensive overlap (~ 2/3rds) in areas used for
visual perception and visual imagery.
Visual Imagery involves only some of the
processes involved in perception – more
memory areas and less lower level processes.
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Disorders of Visual Imagery
Damage to areas of the Left Temporal lobe
(thought to store knowledge of objects) can
cause impairment in ability to use imagery to
accomplish a task (draw an object from
memory) but does not interfere with perception.
Anton’s Syndrome – Blindness denial!
- cortical blindness (visual cortex destroyed)
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Mental Rotation
Shepard & Metzler (1971)
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Found that the farther you have to rotate an object
mentally, the longer the comparison takes. The
speed at which you can complete the tasks
provides a general measure of your spatial ability.
-takes time to mentally
rotate object
-greater the rotation,
longer the time
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Gender Differences in
Mental Imagery
Nature or Nurture??
Cross Cultural Studies
Hunter Gatherer Theory
Practice and Gender Differences
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Perception & Action
Which Line appears Longer?
Mϋller-Lyer Illusion
Two Visual Systems:
Perception and Action
Visual information for perception and for action
are processed separately through independent
The ventral stream, projecting from the
primary visual cortex to temporal lobe,
processes information relevant for object
recognition, while the dorsal stream, projecting
from the primary visual cortex to the parietal
lobe, processes information for action. (Goodale
& Milner)
Systems are Partially Independent.
When we ask people which
line is longer, they rely on the
“Where” or “How”
“What” system and they show
dorsal stream
evidence of the Illusion.
“What” or
ventral stream
When you ask people to
interact, (e.g. point) they use
the “Where” system and the
illusion is greatly reduced?
Ames Window Illusion
Action based performance
Pointing or grasping reduces
the illusion.
Patient DF
D.F. has a profound inability to recognize
objects, places and people, in large part because
of her inability to make perceptual
discriminations of size, shape or orientation
(agnosia), despite having good visual acuity.
When DF is asked to reach for an object, she can
do so well and she shows appropriate preshaping of her hand which scales correctly with
the size of the object. This suggests she has
intact visual perception-for-action.
Figure 4.29 Performance of D.F. and a person without brain damage for two tasks: (a) judging the
orientation of a slot; and (b) placing a card through the slot. See text for details. (From Milner & Goodale,
In sight But out of Mind
Inattentional Blindness - This is only a test
Factors Effecting Inattentional
Gorilla more likely to be detected (83%) when
counting black team throws than when
counting white team throws.
• Unattended object attracts more attention
when it is similar to task-relevant stimuli.
Change Blindness
Inattentional Blindness
Change Blindness
The Door Study - Video
Another example
Change Blindness Stimuli
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• Type changes are more likely detected than
token changes.
• Less likely when viewer is forewarned there
will be a change.
• Detection more likely if the changed object
received attention (fixated) before it changes.
Why does Change Blindness Occur?
Failure of Attention?
We initially form detailed and complete
representations but these representations fade
(decay) quickly or are overwritten by subsequent
Change Blindness Blindness: optimistic belief
about our ability to detect visual changes.
Does Perception Require Conscious
Subliminal Perception
Limen = subjective threshold.
Subjective vs. Objective threshold
Subliminal Perception: occurs when a
stimulus is too weak to be perceived yet a
person is influenced by it.
Subliminal Perception
Naccache et al. (2002)
Task: Decide as quickly as
possible if the letter
presented is larger or smaller
than 5.
Masked (Subliminal) Prime preceded the visible
Participants reported no conscious perception of the prime.
However, their performance on the task indicated that they
were effected by the subliminal prime.
Congruent condition - both prime and visible digit produce
same answer.
Incongruent condition - prime and visible digit produce
different answer.
The incongruent prime condition slower than congruent.
fMRI pictures of houses or faces either at objective threshold or
clearly visible:
Both conscious and unconscious faces activated Right
fusiform area (to some degree);
conscious words led to more extensive brain activity,
including the prefrontal cortex.
90% could be accurately identified as houses or faces .
How Long Does the Objective Effect Last
Lexical Priming Procedure
Stimulus flashed for a split second then quickly
masked with another stimulus.
e.g. Doctor
Then a target stimuli is shown either an unrelated
word (e.g., bread), a related word (e.g. nurse) or a
letter string (djotr).
Lexical Decision Task – (is target a word?)
If the priming stimulus bears a close relationship
to the target word, the subject can respond
slightly faster.
This is a subliminal effect, because the priming
stimulus cannot be seen consciously, and
subjects cannot report what it is.
However, the effect lasts only a tenth of a second
(Greenwald, Draine, & Abrams, 1996).
People can detect THAT a change is
happening, but can’t consciously identify
WHAT is changing.
Blind Sight
Video - begin at 15:40
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