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

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

V4

V5

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