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Perception
• Chris Rorden
• Lecture 8: Vision and perception
• Low level visual deficits:
• Visual field defects
• Blindsight
• Achromatopsia :: cortical colorblindness (V4)
• Akinetopsia :: motion perception (MT/V5)
• Agnosias :: apperceptive, associative, prosop- (FFA)
• ‘How’ versus ‘what’
www.mricro.com
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Vision
Human vision:
Lots of real-estate
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Visual Pathway
 Each eye sees both left
and right visual field.
 Ipsilateral information
crosses over at optic
chiasm.
 Some connections to
superior colliculi.
– Reflexive eye movments
 Others go to thalamus
(lateral geniculate nuclei)
and then cortex.
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Visual Defects
 Field defects reveal
anatomical injury
A. Monocular blindness
B.
C. Bitemporal hemianopia
D. Homonymous
hemianopia
E. Upper quadrantanopia
F. Lower quadrantanopia
G. Homonymous
hemianopia
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V1
Primary visual
cortex (V1) lies in
calcarine fissure.
Complete
damage leads to
Homonymous
hemianopia.
Partial damage
leads to scotomas
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V1 – retinotopic mapping
V1 is retinotopic:
distorted spatial map
of visual scene
Fovea has massively
over represented.
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V1 damage and blindsight
 People with damage to V1
fail to report objects
presented in their field
defect.
 However, when forced to
guess, they can accurately
point to location of unseen
visual stimulus!
 Can also accurately report
direction of motion.
Weiskrantz et al., 1974
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Implications of Blindsight
 V1 is crucial for conscious awareness.
 What explains blindsight? Why do only 20% of
V1 patients show blindsight?
– Incomplete damage to V1? Islands of spared tissue
(Gazzaniga, 1994).
– Typically seen in people who had injury while young
– neural plasticity?
– Small number of indirect connections to later cortical
visual centers?
– Visual connections to colliculi?
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The visual processing stream
 Three major streams of vision:
1. Subcortical
2. Dorsal
3. Ventral
Different streams do different things…
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Cortical visual processing
Dorsal system is fast, but color blind.
Helps with motor control (Where/How).
Parietal
MT
V5
M-ganglion cells
Magno
LGN
P-ganglion cells
Parvo
LGN
V1
V1
V2
V3
V2
V4
Ventral system is slow, but detailed.
Helps with object identification (What).
IT
cortex
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Achromatopsia :: V4
Achromatopsia is usually caused by
bilateral damage to V4 - lingual and
fusiform gyri (occipitotemporal junction)
and is characterized by an inability to
identify or discriminate colour
Still able to perceive form and motion
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Akinetopsia (Motion Blindness)
 Zilles reported first case of akinetopsia. Pure cases are rare,
as requires bilateral injury.
– Case LM - akinetopsia
 43 yr old. Sinus vein thrombosis
 V5 damaged bilaterally - V1 spared
 Could not see movement of objects but could see still objects. People
would suddenly appear
 Diagnosed as agoraphobic
 Can see movements/reach for/catch very slow moving objects (< 10°/s)
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V5 timecourse
 Beckers & Zeki (1995) examined brief V5
disruption using TMS.
 Motion perception disrupted most with V5
stimulation up to 30ms after visual stimulation
onset
 V1 stimulation also partially disrupts motion
perception, but later (60-70ms after VS onset).
 Takes 30-50ms for signals to go from V1 to V5
– Direct route to V5?
– Reafference to V1?
– May explain motion performance in blindsight?
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Agnosias
Three reasons why people might fail to
recognize objects:
– Perceptual Deficit: e.g. acuity, field cut, loss
of color vision
– Apperceptive agnosia: unable to perceive
full shape of object despite intact low level
processing.
– Associative agnosia: ability to perceive
shape, but unable to recognize it.
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Apperceptive agnosia
Intact low-level perception
– acuity
– brightness discrimination
– color vision
Unable to recognize objects
Unable to extract global
structure.
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Associative agnosia
Able to see whole
form of shapes
No problem
copying figures
However, unable to
recognize the
objects
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Associative agnosia
Theoretical explanations:
– Disconnection between visual representation
and language?
– Damage to visual memory representation?
– Slightly impaired perception?
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Anatomical considerations
Apperceptive agnosia:
– right inferior parietal lobe
(Middle Cerebral Artery)
Associative agnosia:
– left occipitotemporal
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Prosopagnosia
Wigan (1844), Quaglino & Borelli (1867),
Hughlings Jackson (1872), Charcot &
Bernard (1883), Wilbrand (1892)
Inability to visually recognize faces
Even a spouse’s face does not seem
familiar
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Prosopagnosia - specificity
Seems specific to faces. Patients can still
recognize others by:
– Silhouette
– Voice
– Clothing
Note: not like amnesia
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Prosopagnosia
Is face processing special?
Or, are faces simply the most difficult
objects we discriminate?
Most people withprosopagnosia have
difficulty recognizing differences within
categories:
– types of car
– porcelain fixtures
– breed of dog
Also, often suffer achromatopsia
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Faces are difficult
 Most objects are identified by unique
components
 However, faces have the same basic
components:nose, eyes,ears, hair
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Are faces special?
 Farah tried to find objects as difficult as faces:
– Spectacle frames
– Undergrads recognized 87% of faces, 67%
of eyeglass frames (faces easier)
– LH recognized only 64%
of faces, and 63% of
eyeglass frames
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Are faces special?
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Are faces special?
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Are faces special?
 Farah examined inversion effect
 Sequential matching task
 Undergrads:
– Upright: 94% correct
– Inverted: 82% correct
 Prosopagnosic LH
– Upright: 58%
– Inverted: 72%
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Double dissociations
Assal, Faure & Anderes (1984) report
zooagnosic farmer MX
– Lost ability to recognise cows
– Still recognises faces
Bruyer et al (1983) report reverse
– Fails to recognise faces
– Intact perception of cows
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Double Dissociation
Faces
RB
MX
RB
MX
Accuracy
 If faces are simply difficult, we should not find
patients with spared face recognition who are
impaired on other tasks.
Cows
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Prosopagnosia
 Selective to faces in a few patients
 Unable to recognize faces
 Able to discriminate equally difficult objects:
– cows
– office furniture
– spectacle frames
 Why are ‘pure’ prosopagnosics so rare?
– Lesions tend to be large?
– Overlap in processing in most patients?
 Functional imaging can resolve this question
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Anatomical considerations
 Fusiform gyrus.
 Usually bilateral,
occasionally right
hemisphere only (Landis
et al. 1986)
 Near V4 (color vision)
 Functional imaging
gives convergent
evidence (Sergent &
Signoret 1992)
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Vision in split brain patients
 Commissurotomy is
neurosurgical treatment
for intractable epilepsy
where the Corpus
callosum is completely
divided.
 Allows systematic
investigation of
hemispheric
specialization and
integration
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Split brain patients
 By using rapid
(tachistoscopic) stimuli
we can avoid eye
movements.
 Using chimeric faces,
Sperry projected
different images to each
hemisphere.
 Most able to return to
work within 2 years of
surgery.
 Typically, appear healthy
Language
‘man’
Left hand
woman
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Split brain patients
 Picture presented in
RVF (i.e. to LH)
– Patient could name or
reach for the object
correctly with right hand.
 Picture presented in LVF
(i.e. to RH)
– Patients could not
name/describe the object
– Subjects could reach for
the correct
– object with their left hand
 Likewise, unable to find
a object felt with one
hand by using the other
hand.
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Split brain patients
 Left hemisphere clearly specialized in language.
 Right hemisphere appears better at copying designs,
reading facial expressions, fitting forms in molds
 Similar effects can be seen in healthy people, e.g.
most think A and C look more similar than A and B
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Cortical visual processing
Dorsal system is fast, but color blind.
Helps with motor control (Where/How).
Parietal
MT
V5
M-ganglion cells
Magno
LGN
P-ganglion cells
Parvo
LGN
V1
V1
V2
V3
V2
V4
Ventral system is slow, but detailed.
Helps with object identification (What).
IT
cortex
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Visual Form Agnosia
 DF has ventral damage
– Profound agnosia :: can not even tell orientation of object
– Motor control accurate :: motor system functions
accurately.
Posting task
Patient DF
Perceptual
matching
Posting
Controls
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Ventral vs Dorsal damage (Goodale et al. [1994]
Curr Biol. 4:604-610)
When shown two
shapes (left), DF was
poor at saying if the
shapes were same or
different, RV was
good at this task.
chance
0%
DF RV
Control
25%
DF
RV
Frequency
When asked to grasp
an object, DF
grasped near the
centre (like healthy
people), RV was
poor at this task.
100%
0%
0
15 0
15 0
15
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Distance from centre (mm)