Visual Pathways Review: Dorsal vs. Ventral Functions and Lesions

1. Test is Friday Nov 20 Review session today after class

2. Dorsal and Ventral Pathways • V4 and V5 are key parts of two larger functional pathways: • Dorsal or “Where” pathway • Ventral or “What” pathway • Ungerleider and Mishkin (1982) • Magno and Parvo dichotomy arose at the retina and gives rise to two distinct cortical pathways

3. Dorsal and Ventral Pathways • Pohl (1973) Early dissociations of Temporal and Parietal functions • Landmark task: • Monkeys trained to find reward in well near a landmark • once they get the task the contingency is switched – monkey must find well opposite to the landmark • #errors until relearning indicates ability to use the spatial relationship information to perform task

4. Dorsal and Ventral Pathways • Pohl (1973) Early dissociations of Temporal and Parietal functions • Landmark task: • Dissociates Parietal and Temporal lobes • Parietal lesions impair relearning of landmark task

5. Dorsal and Ventral Pathways • Pohl (1973) Early dissociations of Temporal and Parietal functions • Object task: • Reward location is indicated by one of two objects • contingency is switched – monkey must use other object • # errors to relearn indicates ability to use object distinction to perform task

6. Dorsal and Ventral Pathways • Pohl (1973) Early dissociations of Temporal and Parietal functions • Object task: • Adding this task doubly dissociates Parietal and Temporal lesions • Temporal lesions impair object task

7. Dorsal and Ventral Pathways • Different kinds of information are represented in the two visual pathways • do both of these pathways equally contribute their “contents” to visual awareness? V5 V4

8. Agnosia • Lesions (especially in the left hemisphere) of the inferior temporal cortex lead to disorders of memory for people and things • recognition and identification are impaired • prosopagnosia is a specific kind of agnosia: inability to recognize faces • explicit (conscious) decisions about object features are disrupted

9. Agnosia • Goodale and Milner – Patient DF • Patient could not indicate the orientation of a slot using conscious information • Patient could move her hand appropriately to interact with the slot

10. Agnosia • Single dissociation of action from conscious perception • Dorsal pathway remained intact while ventral pathway was impaired • Dorsal Pathway seems to guide motor actions, at least for ones that need spatial information • Activity within the Dorsal Pathway seems not to be sufficient for consciousness

11. Blindsight

12. Lesions of Retinostriate Pathway • Lesions (usually due to stroke) cause a region of blindness called a scotoma • Identified using perimetry • note macular sparing X

13. Retinocollicular Pathway independently mediates orienting • Rafal et al. (1990) • subjects move eyes to fixate a peripheral target in two different conditions: • target alone

14. Retinocollicular Pathway independently mediates orienting • Rafal et al. (1990) • subjects move eyes to fixate a peripheral target in two different conditions: • target alone • accompanied by distractor

15. Retinocollicular Pathway independently mediates orienting • Rafal et al. (1990) result • Subjects were slower when presented with a distracting stimulus in the scotoma (359 ms vs. 500 ms)

16. Retinocollicular Pathway independently mediates orienting • Blindsight patients have been shown to posses a surprising range of “residual” visual abilities • better than chance at detection and discrimination of some visual features such as direction of motion • These go beyond simple orienting - how can this be?