Vision 2 Flashcards
Lesions to V1
Can occur due to accident or stroke
Patients report complete or total blindness (single/both hemispheres)
When asked to detect objects-appear blind
Monkeys are able to do visual discrimination
When humans are forced to guess, or use vision, they do well. Called blindsight
Patient TN
Lost V1 in both hemispheres due to stroke
Ordinary vision tests indicated complete blindness
Researchers asked him to walk down a corridor where they had placed obstacles without his cane
Avoided obstacles smoothly
Is the patient blind
Yes, if vision=conscious sight
No, if vision=visual processing (neural & behavioral)
What does blindsight teach us
Primary visual cortex is critical for the conscious aspects of vision
From V1 to V2
Neurons in V2 have similar response properties to neurons in V1
But V2 neurons respond to visual illusions (V1 doesn’t)
V2 neurons will respond to the edge of a rectangle
V2 responses are closer to our visual perception
From V1:
- The Ventral “what stream”
2. The Dorsal “where stream”
The Ventral Stream Pathway
Parvocellular LGN Neurons—–V1—-V2—-V4—–INferior Temporal Lobe (IT)
Ventral Stream Characteristics
Responses to increasingly complex stimuli
V4 responds to complex geometric shapes
V4: first area in ventral stream to show attention modulation
IT responds to visual objects in a position-invariant and size-invariant manner
Neurons in the fusiform face area in IT
Only respond to faces
What is the controversy surround the FFA
Kanwisher argues that FFA is selective to faces
“Expert Hypothesis”: FFA is selective to identifying objects of expertise (Not necessarily faces)
Support: Car experts and bird experts show increased activity in FFA when shown their object of expertise
Visual Agnosia
Severe and permanent impairment in learning and remembering to recognize visual stimuli
Prosopagnosia
If lesion in FFA can cause this difficulty in recognizing faces, or face blindness
Case study of 41 year old male patient with a right occipitotemporal tumor and a left anterior temporoinsular tumor
First resection surgery caused no neurologic deficit, but tumor continued to spread
Second resection surgery caused a strong and permanent prosopagnosia
The Dorsal Stream Pathway
Magnocellular LGN Neurons—-V1—V2—-V5(MT)——Parietal Cortex
Dorsal Stream Characteristics
Spatial attention-guiding our view to points of interest
Using vision for our guidance of actions
Detecting and analyzing movements