Dorsal and Ventral Pathways and What They Do

Dorsal and Ventral Pathways

• visual information arrives at V1 via the retinostriate pathway

• it is already segregated into M and P pathways

• V1 acts like a switchboard routing different kinds of information to dorsal and ventral pathways

V4

V5

Dorsal and Ventral Pathways

• We discussed some consequences of lesions of these structures

• Another dichotomy: conscious vs unconscious– do both of these pathways

contribute their “contents” to visual awareness?

– Why might the answer be no?

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 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 disco-ordination 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

Attention

Controlling how information flows through the brain

Attention as Information Selection

– consider a simple visual scene:

Attention as Information Selection

– consider a simple visual scene:

Attention as Information Selection

– consider a simple visual scene:

– What happens in the brain when this scene appears?

Attention as Information Selection

– consider a slightly more complex scene

– What happens in the brain when this scene appears?

Attention as Information Selection

– consider a slightly more complex scene and a simple task:

– What has to happen in order for this task to be accomplished?

“point to the vertical line”

Attention as Information Selection

That might not seem complex because the visual target and the output “device” are represented by the same hemisphere

Attention as Information Selection

– What if the scene gets more complex?

– What has to happen in order for this task to be accomplished?

Attention as Information Selection

– What if the scene gets more complex?

– What has to happen in order for this task to be accomplished?

Attention as Information Selection

– What if the scene and task gets more complex: “Point to the red vertical line”?

– What has to happen in order for this task to be accomplished?

Attention as Information Selection

• problem: When those stimuli appear, activity begins simultaneously among many different neurons in the cortex. How does the rest of the brain (memory, motor planning, consciousness) know which is the target?– recall that the dorsal pathway doesn’t know anything about

orientation (or color or complex forms or identities)

– What if the scene is really complicated!?

Attention as Information Selection

Attention as Information Selection

• One conceptualization of attention is that it is the process by which irrelevant neural representations are disregarded (deemphasized? suppressed?)

• Another subtly different conceptualization is that attention is a process by which the neural representations of relevant stimuli are enhanced (emphasized? biased?)

Attention as Information Selection

• These ideas apply to other modalities– auditory “Cocktail Party”

problem

– somatosensory “I don’t feel my socks” problem

Attention as Information Selection

• There are a number of classic paradigms used to study attention– visual search

Attention as Information Selection

• There are a number of classic paradigms used to study attention– visual search

– shadowing

Attention as Information Selection

• There are a number of classic paradigms used to study attention– visual search

– shadowing

– cue-target

Attention as Information Selection

• There are a number of classic paradigms used to study attention– visual search

– shadowing

– cue-target

– Stroop Task

B L U E

B L U E