Blindsight
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- Created by: Yasmetron
- Created on: 08-03-23 21:21
Define blindsight
a phenomenon seen in cases of cortical blindness. Typically damage to visual cortex (striate cortex or V1) in one hemisphere. Results in blindness for opposite visual field (hemianopia). Animals and some humans can perform visual tasks outside of consciou
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explain what happened to patient TN
• Strokes caused damage to visual cortex in both hemispheres
• TN reports being totally ‘blind’
• But he shows the ability to detect things in the environment without being aware of them.
• TN reports being totally ‘blind’
• But he shows the ability to detect things in the environment without being aware of them.
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What happens when there is damage to the occipital cortex?
leaves the person blind and would report not seeing anything
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What happens at the primary visual cortex?
The first cortical destination of projections and input from the retina
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What are the functions of the visual cortex?
• It has been proposed that animals’ visual abilities can survive damage to visual cortex while humans’ visual abilities cannot
• Studies of blindsight arose primarily from comparing the effects of damage to visual cortex in man and monkey
• Studies of blindsight arose primarily from comparing the effects of damage to visual cortex in man and monkey
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When was blindsight first identified?
Riddoch(1917)
- Working with brain damaged
soldiers in WWI
• Noticed patients’ ability to detect motion in an otherwise blind visual field
- Working with brain damaged
soldiers in WWI
• Noticed patients’ ability to detect motion in an otherwise blind visual field
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How can you measure blindsight?
• Detection of stimuli in blind hemifield – Pointing and eye movements
• Discrimination of stimuli in blind hemifield – Simple shapes
• Implicit influences of stimuli in blind hemifield
– On reaction times
– On galvanic skin conductance
– On pupil dilatio
• Discrimination of stimuli in blind hemifield – Simple shapes
• Implicit influences of stimuli in blind hemifield
– On reaction times
– On galvanic skin conductance
– On pupil dilatio
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What was the first experimental demonstration of blindsight in humans?
Pöppel, Held and Frost (1973)
• Four patients with visual field defects following unilateral brain damage
• Visual stimuli projected at different locations in their blind visual field
• Patients asked to move their eyes to the position of the stimuli (aud
• Four patients with visual field defects following unilateral brain damage
• Visual stimuli projected at different locations in their blind visual field
• Patients asked to move their eyes to the position of the stimuli (aud
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What happened in the case study of DB?
1 Light flashed at one of 7 locations in blind hemi-field, followed by an auditory tone
2 target location not illuminated
3 Both conditions: DB asked to move his eyes to the target location on hearing the tone, or to point to the location
Much higher spa
2 target location not illuminated
3 Both conditions: DB asked to move his eyes to the target location on hearing the tone, or to point to the location
Much higher spa
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how does multiple targets impact reaction time?
Reaction times are faster when two targets are present than when only one is present
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What is the galvanic skin response?
Skin conductance
Reflects physiological arousal to a stimulus
Not under conscious control modulated by sympathetic nervous system
Reflects physiological arousal to a stimulus
Not under conscious control modulated by sympathetic nervous system
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What is the pupil response?
- Constriction and dilation
- Projections from optic nerve to midbrain (subcortical)
Depends on light level and indicates person’s interest and arousal
- Projections from optic nerve to midbrain (subcortical)
Depends on light level and indicates person’s interest and arousal
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define visual acuity
a measure of the ability of the eye to distinguish shapes and the details of objects at a given distance
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What did they find with patient SJ about grasping the non-conscious?
Grasping 4 wooden objects differed in width (20, 35, 50, 65mm)
Reach and grasp in good and blind visual field
Grip size adjusted for size of block she had not seen
Reach and grasp in good and blind visual field
Grip size adjusted for size of block she had not seen
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what is affective blindsight?
the uncanny ability of such patients to respond correctly, or above chance level, to visual emotional expressions presented to their blind fields.
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what is residual vision as an explanation of blindsight?
Some remaining visual functioning in the main (geniculostriate) visual pathway
Blindsight cases, like DB, may have small regions of intact (residual) vision
• Light from stimulus may have reached these small regions of spared vision, enabling the observed
Blindsight cases, like DB, may have small regions of intact (residual) vision
• Light from stimulus may have reached these small regions of spared vision, enabling the observed
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What case study shows cortical plasticity?
GY (may be a special case)
Diffusion-weighted MRI – controls ipsilateral pathway LGN to V5/MT
Evidence of novel pathways in GY:
- From right LGN to left MT/V5 (motion area)
- Cortico-cortico pathway from MT/V5 both hemispheres
- meaning they use other vis
Diffusion-weighted MRI – controls ipsilateral pathway LGN to V5/MT
Evidence of novel pathways in GY:
- From right LGN to left MT/V5 (motion area)
- Cortico-cortico pathway from MT/V5 both hemispheres
- meaning they use other vis
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what is subcortical pathways as an explanation of blindsight?
Two visual pathways:
What pathway (ventral)required for object- recognition and awareness
Where pathway (dorsal) without awareness
What pathway (ventral)required for object- recognition and awareness
Where pathway (dorsal) without awareness
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What did they find about GY?
– Blinking significantly better than pointing – Pointing sig. better than verbal report
May reflect weak activity in dorsal stream that can
influence arbitrary responses
May reflect weak activity in dorsal stream that can
influence arbitrary responses
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How can blindsight patients discriminate
Discrimination thought to be a function of ventral pathway which does not receive input after V1 lesion
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Why might the V1 be required for awareness?
• Hierarchical models propose that damage to V1 disrupts the flow of information to other higher order regions that are crucial for awareness
• Interactive models propose that recurrent connections between V1 and higher areas form functional circuits that
• Interactive models propose that recurrent connections between V1 and higher areas form functional circuits that
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Other cards in this set
Card 2
Front
explain what happened to patient TN
Back
• Strokes caused damage to visual cortex in both hemispheres
• TN reports being totally ‘blind’
• But he shows the ability to detect things in the environment without being aware of them.
• TN reports being totally ‘blind’
• But he shows the ability to detect things in the environment without being aware of them.
Card 3
Front
What happens when there is damage to the occipital cortex?
Back
Card 4
Front
What happens at the primary visual cortex?
Back
Card 5
Front
What are the functions of the visual cortex?
Back
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