Blindsight

Question 1

What is Blindsight?

Answer 1

The ability of people who are cortically blind to respond to visual stimuli that they do not consciously see due to lesions in the primary visual cortex

-Typically damage to visual cortex in one hemisphere
-Results in blindness for opposite visual field (hemianopia)
-Animals and some humans can perform visual tasks outside of conscious awareness

Question 2

How does it occur?

Answer 2

People have blindness due to damage to the primary visual cortex, but the actual visual system (the eyes) are undamaged.

• Individuals can see with their eyes, but their primary visual cortex cannot translate to the brain
• They will not be able to consciously see anything because the ability to convert the stimuli into a mental image does not exist

Question 3

Measures of blindsight

Answer 3

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 dilation
• On reaching and grasping

Question 4

Implicit measures
Galvanic skin response (GSR) (Zihl et al., 1980)

Answer 4

• Recorded GSR from one hemianopia participant
• Light stimulus in blind field
• Blanks – no light

FINDINGS
- So, damage to visual cortex does not abolish response to light in blind field
- The response shows visual processing still occurs
- Does not depend on recognition of stimulus

Question 5

Grasping the non-conscious
(Perenin & Rossetti, 1996)

Answer 5

• Hemianopia patients could scale a grasping movement directed towards an ‘unseen’ object presented in blind field
• Slot of various orientations
• Verbal forced choice and orientation matching task’s chance level
• Motor – posting a card influenced by size and orientation

Question 6

Blindsight for emotion
(De Gelder, 1999; Celeghin et al. 2015)

Answer 6

Affective blindsight
- Blindsight cases can respond correctly, or above chance level, to visual emotional expressions presented to their blind visual fields

• Amygdala – subcortical structure with role in emotion perception
• Role of subcortical structures such as the superior colliculus, the pulvinar and the amygdala in mediating affective blindsight and nonconscious perception of emotion
• Main pathway is from visual cortex to amygdala but smaller route from superior colliculus to amygdala via pulvinar nucleus (thalamus)

Question 7

Explanations of blindsight

Answer 7

1. Residual vision
   Some remaining visual functioning in the main (geniculostriate) visual pathway
   Geniculostriate:
   – 90% fibres from eye
   – LGN – lateral geniculate nucleus
   – Primary visual cortex
   Some blindsight cases, may have small regions of intact residual vision
2. Subcortical pathways (retinotectal)

Superior colliculus (hindbrain)
Functions:
- Eye-movements (saccades)- Important for orienting attention
- Head movement
- Pointing
- Blink reflex
Projects to dorsal visual stream via pulvina

Milner and Goodale’s account of blindsight
- Blindsight reflects visual activities performed by the dorsal visual pathway (‘where’ pathway) without awareness
- Ventral pathway (‘what’ pathway) is required for object- recognition and awareness

• Some blindsight patients are able to perceive motion (known as Type II blindsight - Weiskrantz)
  – argued to be a special case of blindsight (some awareness)
  – achieved by projections between area V5 (MT) and the ventral stream