Dorsal and Ventral Pathways

Dorsal and Ventral Pathways Different visual cortex regions contain cells with different tuning properties represent different features in the visual field V5/MT is selectively responsive to motion V4 is selectively responsive to color

Dorsal and Ventral Pathways V4 and V5 are doubly-dissociated in lesion literature: Achromatopsia and Akinetopsia, respectively

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 Why “What” and “Where”? monkey lesion experiments human lesions differences in tuning properties of cells neuroimaging

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 #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 # 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 Another dichotomy: conscious vs unconscious do both of these pathways necessarily contribute their “contents” to visual awareness? V4

Agnosia V5 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 V4

Agnosia Goodale and Milner – Patient DF Patient could not indicate the orientation of a slot using her awareness Patient could move her hand appropriately to interact with the slot whether visually guided or guided by an internal representation in memory

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

Ataxia Ataxia is a discoordination of motor behaviour a variety of different symptoms and causes patients with ataxia due to lesions of parietal lobe in the dorsal pathway have difficulty operating and interacting with objects but they can identify them doubly dissociates conscious perception in the ventral pathway from visually guided action in the dorsal pathway