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

Outline  What does it mean to see?  Neural organization of visual processing  Examination of ventral stream functions  Brain lesions  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  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.”  “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.”  “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  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  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  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

Advantages of Neurons  Parallel processing  Fault tolerant  Fuzzy reasoning  Form generalizations  Permits cascading neural events  Top-down processing

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

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Points and edges Colors Shapes Motion SolidsSurfaces ToolFaceAnimalFruit HearingTactile sen.GustationEmotion

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

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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  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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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  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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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  “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  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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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  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  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  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  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

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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… it’s a… lock”  (Pt. handed key)  “It’s a key!”  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… it’s a… lock”  (Pt. handed key)  “It’s a key!”

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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  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

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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  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  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  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  Achromatopsia  Apperceptive agnosia  Associative agnosia  Optic aphasia  Pure alexia  Category-specific semantic or lexical defects  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  Two hierarchically organized neural systems for object information in human visual cortex (2008) - Konen, Nature Neuroscience (11) pp  Visual Agnosia (2004) - Martha Farah  Vision (1982) - David Marr  Neural systems behind word and concept retrieval (2004) - Damasio, Cognition (92) pp  Two hierarchically organized neural systems for object information in human visual cortex (2008) - Konen, Nature Neuroscience (11) pp