Download - Test is Friday Nov 20
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
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
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
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
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
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?
V4
V5
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
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
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
Lesions of Retinostriate Pathway
• Lesions (usually due to stroke) cause a region of blindness called a scotoma
• Identified using perimetry• note macular sparing
X
Retinocollicular Pathway independently mediates orienting
• Rafal et al. (1990)
• subjects move eyes to fixate a peripheral target in two different conditions:– target alone
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
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)
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?