Blindsight Flashcards
What is Blindsight
- Damage to the visual cortex in one hemisphere which results in opposite visual field.
- unconscious vision
- visual discrimination In the absence of acknowledged awareness
Patient TN
- stroke caused damage to visual cortex in both hemispheres
- he reports being totally blind
- however, he is able to detect things in the environment without being aware of them.
Localisation of vision to occipital cortex
- Ferrier (1875), wrongly concluded that the angular gyrus is the seat of vision (animal study)
- Munk (1878) showed in dogs and monkeys. vision is localised in occipital lobe. Showed each eye is connected to both hemispheres
- James (1890) damage to occipital cortex leaves the person blind and they would report not seeing anything
Functions of the visual cortex
- Animals visual abilities can survive damage to visual cortex but humans cannot
- monkeys without primary visual cortex can discriminate shapes and detect movement but humans typically function blind
Early reports in humans
Riddoch(1917)
- brain damaged soldiers
- noticed patients can detect motion in an otherwise blind visual field
First experimental demonstration of blindsight in humans
Poppel, Held and Frost, 1973
- 4 patients with visual field deficits
- visual stimuli projected at different locations in their blind visual field
- Patients asked to move eyes to the stimuli
- week relationship found between target locations and size of eye movement
Further demonstrations of blindsight
Weiskrantz (1974)
- 34 year old male with brain damage
- removal of tumour
- DB asked to move his eyes to the target location where the light flashed
- higher spatial accuracy for pointing movements than eye movements
- DB was shocked, he had claimed to see nothing in blind field
- In blank trials, DB did have a feeling stimuli was not presented
- Discrimination in blindsight, db asked to guess which stimulus present on each trial (letter O or X)
- performance well above chance at 70-90%
Implicit processing in blindsight
Marzi et al (1986)
- measure the effects of a unseen stimulus in the blind field on reaction times (RTs) to targets in good field.
- Reaction time faster with two targets rather than one
- this affect is seen for some people when additional affect appears in blind field
Implicit measures
Galvanic skin response
Zihl et al (1980)
- Skin conduction
- shows physiological arousal to stimulus
- not under conscious control
recorded GSR
found damage to visual field doesn’t abolish response to light in blindfold
visual processing still occurs and does not depend on recognition of the stimulus
GSR reflects registration of stimulus
Implicit measures
Pupil response
- sensitive to light
- cognitive load
- recognition
- spatial frequency
GY - pupil response modulated by spatial frequency in blindfold
Grasping the non-conscious
Perenin and Rossetti (1996)
- patient could grasp movement direction towards object presented in blind field
Grasping the non conscious
(Whitewell et al, 2011)
- patient SJ, reach and grasp wooden objects in good and blind visual field
- grip size adjusted for the size of block she had not seen
Blindsight for emotion
(de Gelder 1999)
- can respond correctly at above chance levels to visual emotional expressions presented to them
- performance better with video clip rather than still images
- ERP showed activity in damaged left hemisphere
Explanations of blindsight
Residual vision
- some remaining visual functioning in the main visual path way (geniculostraite)
- light from stimulus may have reached these small intact regions enabling observation of ‘blindsight’ behaviour
Weiskrantz argues that case DB could not detect or point to stimulus within nature blind spot region
The case of GY
- brain damage restricted to primary visual cortex at age 8
- right hemianopia
- accurate eye movements to stimuli in blind field
- but doesn’t report seeing something with motion
- Riddoch thought sparing of V1 could be explanation but MRI scans confirm that GY has no V1
- there was evidence of novel pathways in GY
