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The Ventral Stream and Visual Agnosia

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The Ventral Stream and Visual Agnosia David Glenn Clark, MD Department of Neurology, UAB and BVAMC Outline What does it mean to see? Neural organization of visual ... – PowerPoint PPT presentation

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Title: The Ventral Stream and Visual Agnosia


1
The Ventral Stream and Visual Agnosia
  • David Glenn Clark, MD
  • Department of Neurology, UAB and BVAMC

2
Outline
  • What does it mean to see?
  • Neural organization of visual processing
  • Examination of ventral stream functions
  • Brain lesions

3
Main Points
  • The ventral stream refers to the flow of visual
    information from striate cortex toward the
    temporal poles
  • Lesions of the ventral stream induce disorders of
    complex visual processing
  • Receptive fields of neurons in the temporal lobe
    may be specific for certain semantic categories

4
What does it mean to see?
  • To learn what is where by looking. (Aristotle)
  • Marr, 1982 Vision is the process of discovering
    from images what is present in the world, and
    where it is.

5
What does it mean to see?
  • To learn what is where by looking. (Aristotle)
  • Marr, 1982 Vision is the process of discovering
    from images what is present in the world, and
    where it is.

6
where
what
7
Sources of Information
  • Artificial Intelligence
  • How would you build a robot that brings you a
    coke from the fridge?
  • Patients
  • Lesion-symptom mapping
  • Functional imaging, EEG, MEG
  • Non-human primate studies

8
Why See?
  • If we want a robot to retrieve cokes or other
    beverages, it might help if it can see
  • Seeing (like all senses) appears to be useful
    only for guiding movements
  • Seeing helps us (and other animals) to
  • Identify tigers, cokes, enemies, potential mates
  • Use this information to guide fleeing, drinking,
    attacking, and mating calls

9
What Our Robot Needs To Accomplish
  • Process images from its environment
  • Lines, borders, shapes, solids, colors
  • Identify objects from processed images
  • Maintain a representation of the environment
  • Multiple objects, spatial relationships among
    them
  • Represent itself within its environment
  • Compute movements to manipulate objects based on
    these representations

10
given a graphic scene, return true if an
object is present and false if no object is
present (defun find_object (scene) ) given
a location and a graphic scene, find the nearest
90 degree angle and return its location.
Return false if there is no corner (defun
find_corner (x y scene) ) given a scene, use
find_corner to identify the locations of all
corners and ensure that they are connected by
lines (defun find_4corners (scene) (let ((corner
(find_corner (0 0 scene)))) )
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14
HOW VISION WORKS
15
HOW VISION WORKS
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18
Advantages of Neurons
  • Parallel processing
  • Fault tolerant
  • Fuzzy reasoning
  • Form generalizations
  • Permits cascading neural events
  • Top-down processing

19
Outline
  • What does it mean to see?
  • Neural organization of visual processing
  • Examination of ventral stream functions
  • Brain lesions

20
Colors
Points and edges
Surfaces
Motion
21
Colors
Points and edges
Shapes
Solids
Surfaces
Motion
22
Colors
Points and edges
Shapes
Solids
Surfaces
Motion
Tool
Face
Animal
Fruit
23
Colors
Points and edges
Shapes
Solids
Surfaces
Motion
Tool
Face
Animal
Fruit
Hearing
Tactile sen.
Gustation
Emotion
24
Outline
  • What does it mean to see?
  • Neural organization of visual processing
  • Examination of ventral stream functions
  • Brain lesions

25
Examining Ventral Stream Function
  • Ensure that basic visual perception is normal
  • Visual acuity
  • Visual fields
  • Brightness discrimination, edge detection, number
    of stimuli, depth perception
  • Also assess
  • Color perception
  • Motion processing

26
Examining Ventral Stream Function
  • Evaluate naming
  • Visual confrontational naming
  • Line drawings, photographs, real objects, moving
    stimuli
  • Various categories faces, animals, artifacts,
    plants
  • Naming in other sensory modalities (tactile,
    auditory)
  • Verbal fluency
  • Naming to definition
  • Color naming

27
Examining Ventral Stream Function
  • Nonverbal evaluation of complex visual perception
  • Matching
  • Copying
  • Verbal description of visual percepts
  • Semantic knowledge pertaining to percepts
  • Can the patient recognize an object but not name
    it?
  • Can the patient answer conceptual questions about
    visual percepts or questions about concrete
    entities in the world?

28
Outline
  • What does it mean to see?
  • Neural organization of visual processing
  • Examination of ventral stream functions
  • Brain lesions

29
Colors
Points and edges
Shapes
Solids
Surfaces
Motion
30
Central Achromatopsia
  • A defect in color perception caused by an
    acquired cerebral lesion
  • Reduced hue discrimination
  • Deficient color constancy
  • Cannot match colored plates
  • Ishihara plates may help diagnosis
  • Lesion is in ventro-medial occipital lobe(s)
  • Colors are either all gray, or washed out,
    dirty, or faded
  • Some subjects report defective color imagery
  • Also known as color agnosia

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Colors
Points and edges
Shapes
Solids
Surfaces
Motion
34
Apperceptive Agnosia
  • Disruption of early image processing
  • Cannot be explained by defects of visual fields,
    color vision, brightness detection or other
    elementary visual processes
  • Patients cannot
  • Recognize visually presented objects
  • Accurately describe shapes or features of
    visually presented items
  • Copy figures
  • Match figures
  • Most common with diffuse brain injury CO or Hg
    poisoning
  • At least one case after focal brain injury

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Colors
Points and edges
Shapes
Solids
Surfaces
Motion
Tool
Face
Animal
Fruit
Hearing
Tactile sen.
Gustation
Emotion
38
Associative Agnosia
  • A normal percept stripped of its meanings
  • Disrupted activation of conceptual knowledge
    after visual form is processed
  • Patients CANNOT
  • Recognize visually presented objects
  • Patients CAN
  • Recognize and name objects in other modalities
  • Copy pictures of objects
  • Match one picture to another

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41
Associative Agnosia
  • Lesions have various descriptions, but are
    predominantly in ventral stream
  • Bilateral temporo-occipital with underlying white
    matter
  • Perhaps more common with right hemisphere lesions
    when naming is unimpaired
  • Etiologies stroke (PCA), AD, SD, DLB

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43
Colors
Points and edges
Shapes
Solids
Surfaces
Motion
Tools
Faces
Animals
Fruits
Hearing
Tactile sen.
Gustation
Emotion
44
Prosopagnosia
  • A deficit of face processing and recognition
  • Lesion always temporo-occipital, probably always
    right hemispheric
  • Fusiform face area (R fusiform gyrus)
  • Right temporal pole
  • Left temporal pole seems to be necessary for
    accurate face naming

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46
Faces Are Special
  • One prosopagnosic patient could identify specific
    sheep better than specific people
  • Farah studied a patient who performed normally
    recalling pictures of objects (e.g., eyeglass
    frames) but not faces
  • Same subject showed better recognition memory of
    inverted faces relative to controls

47
Conscious vs. Emotional Face Processing
  • Patients with prosopagnosia may still exhibit
    autonomic (GSR) response to familiar faces
  • Patients with intact facial recognition may lose
    autonomic responses to familiar or angry faces

48
Neuropsychiatric Syndromes
  • Misidentification
  • Capgras - Invasion of the Body Snatchers
  • Fregoli - Fallen (with Denzel Washington)
  • Intermetamorphosis - Lost Highway, Mulholland
    Drive
  • Visual Hallucinations
  • DLB well-formed, often animate, associated with
    more Lewy bodies in temporal lobe

49
Colors
Points and edges
Shapes
Solids
Surfaces
Motion
words
Tools
Faces
Animals
Fruits
Hearing
Tactile sen.
Gustation
Emotion
50
Optic Aphasia
  • Lesion in ventral stream of language-dominant
    hemisphere
  • Patients show intact visual recognition but
    naming defect only in response to visual stimuli
  • (Pt. shown a key) ? You open a door with it
    its a lock ? (Pt. handed key) ? Its a key!

51
Colors
Points and edges
Shapes
Solids
Surfaces
Motion
words
52
Pure Alexia
  • Classically a L PCA infarction with R hemianopsia
    and damage to splenium of corpus callosum
  • AKA Pure Word Blindness
  • Disorder of reading with generally intact visual
    naming and other language functions

53
Colors
Points and edges
Shapes
Solids
Surfaces
Motion
Tool
Face
Animal
Fruit
Hearing
Tactile sen.
Gustation
Emotion
54
Category Specific Deficits
  • Numerous patients described with defective
    recognition or naming of concrete entities
  • Per Caramazza, always animals, plant matter, or
    conspecifics
  • Several competing hypotheses for explaining this
  • My personal favorite is Damasios Convergence
    Zone hypothesis

55
Convergence Zones
  • Entities within a semantic category have
    overlapping features
  • Association cortices capture statistical
    regularities in other cortical regions
  • Neurons with similar receptive fields tend to
    group together in associative maps
  • Leads to clustering of neurons that have relative
    specificity for a given category

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58
Main Points
  • The ventral stream refers to the flow of visual
    information from striate cortex toward the
    temporal poles
  • Lesions of the ventral stream induce disorders of
    complex visual processing
  • Receptive fields of neurons in the temporal lobe
    may be specific for certain semantic categories

59
Recapitulation
  • Achromatopsia
  • Apperceptive agnosia
  • Associative agnosia
  • Optic aphasia
  • Pure alexia
  • Category-specific semantic or lexical defects

60
Recommended Reading
  • Visual Agnosia (2004) - Martha Farah
  • Vision (1982) - David Marr
  • Neural systems behind word and concept retrieval
    (2004) - Damasio, Cognition (92) pp. 179-229
  • Two hierarchically organized neural systems for
    object information in human visual cortex (2008)
    - Konen, Nature Neuroscience (11) pp. 224-231
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