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)))) …)

14. HOW VISION WORKS

15. HOW VISION WORKS

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 Surfaces Shapes Solids Motion

22. Colors Points and edges Surfaces Shapes Solids Motion Tool Animal Fruit Face

23. Colors Points and edges Surfaces Shapes Solids Motion Tool Animal Fruit Face Tactile sen. Hearing 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 Surfaces Shapes Solids 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

33. Colors Points and edges Surfaces Shapes Solids 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

37. Colors Points and edges Surfaces Shapes Solids Motion Tool Animal Fruit Face Tactile sen. Hearing 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

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

43. Colors Points and edges Surfaces Shapes Solids Motion Tools Animals Fruits Faces Tactile sen. Hearing 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

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 Surfaces Shapes Solids Motion words Tools Animals Fruits Faces Tactile sen. Hearing 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… it’s a… lock”  (Pt. handed key)  “It’s a key!”