Blindsight - PS2021

Stroke caused damaged to visual cortex in both hemispheres

Patient reports being totally blind but he shows ability to detect things/objects without being aware of seeing them

"Visual discrimination in the absence of acknowledged awareness" - Weiskrantz (1990)

Riddoch (1917) - First noticed blindsight

Working with brain damaged soldiers from WW1

These people aren't fully blind, it's either left or right VF but still blindsight exists

Noticed patients' ability to detect motion in an otherwise blind visual field

Term 'blindsight' first used by Sanders et al. (1974)

Weiskrantz: 'blindsight and hindsight'

Blindsight - refers to behavioural findings

Hindsight - implies role of thehindbrain and subcortical visual pathways

(Hindbrain is the oldest part of the brain in evolutionary terms)

His work:

we now know that functions are localised in cortex

main pathway from eye to visual cortex - one milion nerve fibres

smaller sub cortical pathway - 150,000 nerves

- it has been proposed that animals' visual abilities can survive damage to visual cortex while humans' visual abilites cannot

- studies of blindsight arose primarily from comparing the effects of damage to visual cortex in man and monkey

monkeys without primary visual cortex can:

- discriminate shapes

- maintain (reduced) acuity

- fixate and reach towards small and brief visual events

- detect movemens

>humans typically functionally blind

cortical blindness - loss of vision following damage to visual cortex (e.g. area V1, not the retina or optic tract)

hemianopia - loss of vision in one half of visual field following unilateral brain damage

'what' pathway (Ventral stream) - object recognition

'where' pathway (Dorsal steam) - spatial recognition

- detection of stimuli in blind hemifield

- discrimination of stimuli in blind hemifield

- implicit influences of stimuli in blind hemifield

          > on reaction times

          > on eye movements

Poppel, 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 (auditory 'go' signal)

Patients were puzzled by the task: "How can i move my eyes to something i haven't seen?"

A realtionship was observed between target location and the size of the eye movement (10-30deg)

34 year old male at time of brain damage - DB

Surgical removal of tumor in right occpitial lobe (severe migraine attacks)

Resulted in a left hemianopia

One of the most studies blindsight cases

Experimental Condition: Light flashed at one of 7 locations in blind hemi-field, followed by an auditory tone

Control Condition: Target location not illuminated (auditory tone is still presented)

Both Conditions: DB asked to move his eyes to the target location on hearing the tone, or to point to the location

Results

Replicates the results of Poppel et al, much higher spatial accuracy for pointing movements than eye movements

DB surprised by his results and claimed to have seen nothing at all in blind field

When blank trials inserted DB had a vague feeling stimuli not always presented

Letters X or O presented in blind field

DB asked to guess which stimulus presented (O or X) on each trial

Performance well above chance (70-90%) if stimuli were large

Performance much worse with line orientation discrimination task

Single target in good field - double targets in good field - double targets in good and bad

measure the effects of an unseen stimulus in the blind field on reaction times to targets in good field

Spatial Summation:

reaction times are faster when two targets are present than when only one is present

S = Single light in good hemifield

D1 = 2 lights in good hemifield

D2: lights in good and bad hemifield

A) not significant

B) Case 2 shows a spatial summation facilitation effect

task - move eyes to light in good field (monocular viewing)

eye movements (saccades) were slowed by the presence of distractors in the blind field

Zihl et al. 1980

GSR from one hemianopic case

S=stimulus

C=blank trials

Response to visual stimulus that patient did not report seeing

Pupil Response- constriction and dilation

depends on light level and indicated person's interest and arousal

Is also sensitive to:

- spatial frequency (measure of visual acuity)

-cognitive load

visual acuity - how well we see fine detail (spatial resolution) - measured with gratings

pupil response: still responds after cortical damage

gY - pupil response modulated by 'spatial frequency' in blindfield (Cowey, 2004)

- spared projections from eye to dorsal visual pathway via the smaller 'retinotectal' (sub cortical) projections

Can blindsight be attributed to residual vision?

Patients such as DB may have small regions of intact vision - light from stimulus may have reached these small regions of spared vision, enabling the observed blindsight behaviour

Fendrich, Wessinger and Gazzaniga (1992)

Patient diagnosed with complete hemianopia, finer testing methods identified regions of intact vision ('islands') - However other cases have reported complete loss of V1 as confirmed by MRI (Bridge et al. 2008).

Case of GY

Brain damage restricted to primary visual cortex (V1) aged 8, right hemianopia, Barbur et al 1988 -GY made highly accurate eye movements to stimuli in his blind field but was not aware of the stimuli - GY absence of V1 in left hemisphere was confirmed by MRI

Superior colliculus - hindbrain

functions: eye movements (saccades), head movements, pointing and blink reflex

projects to dorsal visual stream via pulvinar

two visual pathways (Milner and Goodale, 1995)

GY asked to detect stimulus onset by:

- blinking - pointing - verbal report

blinking significantly better than pointing and pointing is significantly better than verbal report (Marcel, 1993)

Hemianopic patient: verbal responses to shapes at chance level, reaching and grasping correlated with shape and orientation (Perenin and Rossetti, 1996)

Supports claims that superior colliculus may be involved in blindsight responses

accurate eye and pointing responses may be mimedated by indirect projections to superior colliculus and and/or dorsal visual pathway

ventral pathway may be required for awareness

Discrimination thought to be a func2on of ventral pathway which does not receive input arer V1 lesion  
DB could discriminate X or O, square/ diamond  
No motor component to task but a motor program may be generated in dorsal stream (via pulvinar) e.g. shaping of hand to grasp an object of certain shape (X)?

- blindsight reflects visual activities perormed by the doral visual pathway (where pathway) without awareness

- ventrall pathway (what pathway) is required for object recognition and awareness

- some blindsight patients are able to percieve motion (known as type 2 blindsight)

  >argued to be a special case of blindsight

  >achieved by projections between area V5(MT) and the ventral stream

what causes patient's lack of awareness of the stimuli?
- evidence of projections to visual cortex other than via primary visual cortex (V1)

-patients can make accurate behavioural responses

-integrity of V1 required for conscious perception?

    -feed forward of sensory input V1 to higher areas

    - feedback from higher areas to V1

Zeki 1993

- hierarchical models purpose 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 support awareness Tong 2003

Ffytch and Zeki 2011

GY may be a special case:

- brain damage at 8, plasticity - uses other visual pathways; repeated training over years may have resultsed in other visual pathways developing

3 new cases damage to V1 and hemianopia: had some awareness of motion stimuli (like GY) and could draw it depsite absence of V1

So

Primary visual cortex (V1) and back projections to it are not necessary for visual awareness