Cortical Pathways for Visual Perception

1. Cortical Pathways for Visual Perception • Output from occipital lobe follows two major fiber tracts • Superior longitudinal fasciculus to parietal lobe • Inferior longitudinal fasciculus to temporal lobe • Ungerleider & Mishkin propose that these pathways form functionally distinct processing systems • Dorsal (occipito-parietal) is the “where” system specialized for spatial analysis • Ventral (occipito-temporal) is the “what” system specialized for object perception and recognition

2. Dorsal and Ventral Functional Pathways • Pohl experiments reveal double dissociation • Landmark task: monkeys with bilateral parietal lesion have deficit, but monkeys with bilateral temporal lesion can learn task • Object discrimination task: monkeys with bilateral temporal lesion have deficit learning task, but monkeys with bilateral parietal lesion do not

3. Dorsal and Ventral Functional Pathways • Pohl experiments reveal double dissociation • Other bilateral lesion experiments show dissociation within temporal lobe • anterior temporal lesions disrupt visual memory • posterior temporal lesions disrupt visual discrimination

4. Dorsal and Ventral Functional Pathways • Pohl experiments reveal double dissociation • Other bilateral lesion experiments show dissociation within temporal lobe • Must lesions be bilateral to cause deficits? • Are dorsal and ventral pathways bilateral?

5. Dorsal and Ventral Functional Pathways • Ungerleiter and Mishkin combination lesion experiments • Ventral pathway is bilateral • Right striate + left inferior temporal lesions • No deficit in object discrimination • Severing corpus callosum creates deficit • Dorsal pathway is primarily unilateral • Right striate + left parietal lesions • Deficit in landmark task • Severing corpus callosum increases deficit

6. Dorsal and Ventral Functional Pathways • Neuron receptive field differences • Parietal lobe neurons • Large receptive fields • Specific to hemifield • More neurons have receptive fields outside the fovea than inside the fovea

7. Dorsal and Ventral Functional Pathways • Neuron receptive field differences • Parietal lobe neurons • Temporal lobe neurons • Large receptive fields • Not specific to hemifield • More neurons have receptive fields inside the fovea than outside the fovea • Majority of neurons respond selectively to complex stimuli

8. Dorsal and Ventral Functional Pathways • Kohler et al. PET study in humans supports "what/where" distinction • Position task: greater rCBF in right parietal lobe • Object task: greater rCBF biaterally at occipito-temporal areas

9. Dorsal and Ventral Functional Pathways • Kanwisher et al. PET study in humans fails to isolate cognitive functions during passive viewing of shapes

10. Computational Problems in Object Recognition • How to account for shape-based encoding? • Objects are more than the sum of their parts • Properties of object constancy • Viewing position • Illumination • Occlusion

11. Computational Problems in Object Recognition • Theoretical issues in object recognition • Frame of reference: object constancy across orientation • View-dependent • Separate representation of an object for each viewpoint • View-invariant • Critical properties used for object recognition • Major/minor axes, etc.

12. Computational Problems in Object Recognition • Theoretical issues in object recognition • Frame of reference • Shape encoding: hierarchical representations of increasing complexity • Salient features • Invariants: parallelism, symmetry, T-junctions, occlusion, etc

13. Computational Problems in Object Recognition • Theoretical issues in object recognition • Frame of reference • Shape encoding: hierarchical representations of increasing complexity • Salient features • Recognition by parts • Beiderman's geons : 24 fundamental 3-D shapes

14. Computational Problems in Object Recognition • Theoretical issues in object recognition • Frame of reference • Shape encoding • Neurophysiological representations • Hierarchical coding hypothesis • Object defined by Gnostic (or grandmother) cell: single neuron that represents"granny" activated by outputs from increasingly more complex detectors

15. Computational Problems in Object Recognition • Theoretical issues in object recognition • Frame of reference • Shape encoding • Neurophysiological representations • Hierarchical coding hypothesis • Ensemble coding hypothesis • Object defined by co-occurrence of complex feature detectors

16. Failures of Object Recognition • Visual Agnosia: Failure to recognize visual objects • Limited to visual modality • Not a sensory deficit • Not a memory deficit (anomia) • But deficits can co-occur • What can disorders of perception tell us about perceptual function?

17. Failures of Object Recognition • Patient GS • Normal visual acuity • Normal verbal memory

18. Failures of Object Recognition • Patient studies of visual perception deficits that support "what/where" distinction • Patient DF: bilateral occipital lobe lesions • Differentiates "perception for identification" from "perception for action"

19. Failures of Object Recognition • Subtypes of agnosia • Apperceptive agnosia: deficit in perceptual processing • Associative agnosia: "normal" perceptual processing, but deficit in linking percept to name

20. Failures of Object Recognition • Apperceptive agnosia • Deficits in perceptual processing may be subtle and may not be apparent in standard clinical tests

21. Failures of Object Recognition • Apperceptive agnosia • Warrington • RH patients had greater deficits in the Gollin Picture Test and Incomplete Letters Test than LH patients • RH patients with posterior damage have difficulty with object constancy • Unusual Views Test: can't recognize objects from unusual view • Shadows Test: can't recognize objects when illumination changes

22. Failures of Object Recognition • Associative agnosia • Warrington: Deficits are not at a perceptual level but at a semantic level • Patient FRA • LH occipito-temporal lesion • Could normally parse complex drawing into constituent objects, but could not name objects

23. Failures of Object Recognition • LH and RH patients both fail Matching-by-Function Test but for different reasons

24. Failures of Object Recognition • LH and RH patients both fail Matching-by-Function Test but for different reasons • Warrington's two-stage model of object recognition • Perceptual categorization occurs in the RH • Semantic categorization occurs in the LH • Perceptual categorization precedes semantic categorization • But, unilateral LH patients don't always have associative agnosia, usually requires bilateral lesions

25. Failures of Object Recognition • Integrative agnosia: some elements of apperceptive and associative agnosias • Patient HJA • Bilateral occipito-temporal lesion • Could recognize individual objects, but not line drawing of objects • Could perform Unusual Views Test • Could not recognize overlapping objects • Deficit in integrating and grouping features

26. Failures of Object Recognition • Integrative agnosia • May common problem in most agnosics • Other Integration failures • Patient CK • Closed head injury • Patient JR

27. Failures of Object Recognition • Category-specific agnosia: associative agnosia where deficit is specific to a semantic category • Patient JBR • Herpes simplex produced severe associative agnosia that was worse for living things than for inanimate objects

28. Failures of Object Recognition • Category-specific agnosia: associative agnosia where deficit is specific to a semantic category • Semantic knowledge is structured • Object categories • Tools, vehicles, etc. • Living vs nonliving

29. Failures of Object Recognition • Category-specific agnosia • Damasio suggests that nonliving activate kinesthetic associations that living things do not

30. Failures of Object Recognition • Category-specific agnosia • Gaffan and Heywood showed that normals made more errors to short latency exposures of living things than of nonliving things • Living things are more similar and share more features than nonliving things and are inherently more difficult to discriminate

31. Failures of Object Recognition • Farah and McClelland’s computational explanation for category-specific agnosia • Property-based organization of semantic system • Two main layers: semantic and input • Semantic has 3:1 ratio of visual and functional properties • Input is verbal and visual • Objects have visual and functional property codes • 7.7:1 for living things • 1.4:1 for nonliving things • Train model to discrimination 20 living and nonliving things • "Lesion" model and evaluate recognition accuracy

32. Prosopagnosia • Inability to recognize faces • Is there a separate face recognition system? • Evolutionary support • Phylogeny of face recognition • Primates show similar activation to faces • Development of face recognition • Neonates track faces longer than other stimuli • Cross-cultural interpretation of facial expression • Universal recognition of facial expressions

33. Prosopagnosia • Is there a separate face recognition system? • Evolutionary support • Neurophysiological support in primates • Baylis et al study on neuronal specificity to faces in the macaque • Farah's taxonomy of lesion foci

34. Prosopagnosia Majority of lesions in occipito-temporal area

35. Prosopagnosia • Is there a separate face recognition system? • Evolutionary support • Neurophysiological support in primates • Farah's taxonomy of lesion foci • Function neuroimaging in humans • Kanwisher et al. fusiform face area

36. Prosopagnosia • Can face and object perception be dissociated? • Dissociations between object and face perception • Right side up faces vs upside down faces • Patient CK • Could not identify objects in Arcimbaldo painting when right side up, but could identify face when painting was upside down • Patient WJ • Sheep farmer who had prosopagnosia for human faces but could still recognize sheep

37. Prosopagnosia • Can face and object perception be dissociated? • Dissociations between object and face perception • Tanaka and Farah: face recognition is more than the sum of parts

38. Prosopagnosia • Two systems for object recognition • Farah study of patterns of co-occurrence of prosopagnosia, visual agnosia, and alexia (acquired dyslexia) • Analytic processing • Holistic processing

39. Co-occurence of Prosopagnosia, Visual Agnosia, and Alexia Number of patients * possibly

40. Visual Imagery and Perception • Does visual imagery use the same neural mechanisms as visual perception?

41. Synesthesia • Involuntary experience of cross-modal association • 0.05% occurrence in population • More common in artists, novelists, poets, etc. • May be related to variation of fusiform area of temporal lobe