Transcript The Ventral Stream and Visual Agnosia

The Ventral Stream and Visual Agnosia
David Glenn Clark, MD
Department of Neurology, UAB and BVAMC
Outline
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What does it mean to see?
Neural organization of visual processing
Examination of ventral stream functions
Brain lesions
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
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.”
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.”
where
what
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
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
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
;; 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)))) …)
HOW VISION WORKS
HOW VISION WORKS
Advantages of Neurons
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Parallel processing
Fault tolerant
Fuzzy reasoning
Form generalizations
Permits cascading
neural events
 Top-down processing
Outline
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What does it mean to see?
Neural organization of visual processing
Examination of ventral stream functions
Brain lesions
Colors
Points
and edges
Surfaces
Motion
Colors
Points
and edges
Surfaces
Motion
Shapes
Solids
Colors
Points
and edges
Surfaces
Shapes
Solids
Motion
Tool
Animal
Fruit
Face
Colors
Points
and edges
Surfaces
Shapes
Solids
Motion
Tool
Tactile sen.
Animal
Hearing
Gustation
Fruit
Face
Emotion
Outline
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What does it mean to see?
Neural organization of visual processing
Examination of ventral stream functions
Brain lesions
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
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
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?
Outline
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What does it mean to see?
Neural organization of visual processing
Examination of ventral stream functions
Brain lesions
Colors
Points
and edges
Surfaces
Motion
Shapes
Solids
Central Achromatopsia
 A defect in color perception caused by an acquired cerebral
lesion
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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
Colors
Points
and edges
Surfaces
Motion
Shapes
Solids
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:
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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
Colors
Points
and edges
Surfaces
Shapes
Solids
Motion
Tool
Tactile sen.
Animal
Hearing
Gustation
Fruit
Face
Emotion
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
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
Colors
Points
and edges
Surfaces
Shapes
Solids
Motion
Tools
Tactile sen.
Animals
Hearing
Gustation
Fruits
Faces
Emotion
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
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
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
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
Colors
Points
and edges
Surfaces
Shapes
Solids
Motion
words
Tools
Tactile sen.
Animals
Hearing
Gustation
Fruits
Emotion
Faces
Optic Aphasia
 Lesion in ventral stream of languagedominant 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… it’s a… lock”  (Pt. handed key) 
“It’s a key!”
Colors
Points
and edges
Surfaces
Shapes
Motion
words
Solids
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
Colors
Points
and edges
Surfaces
Shapes
Solids
Motion
Tool
Tactile sen.
Animal
Hearing
Gustation
Fruit
Face
Emotion
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 Damasio’s
Convergence Zone hypothesis
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
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
Recapitulation
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Achromatopsia
Apperceptive agnosia
Associative agnosia
Optic aphasia
Pure alexia
Category-specific semantic or lexical
defects
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