Disorders of Visual Perception - 2

Question 1

Which pathway gives the largest contribution to human visual perception?

Answer 1

-the retinal-geniculate-striatal pathway

Question 2

What are the other visual pathways?

Answer 2

• Around 10% of retinal ganglion cells branch away from the optic nerve before reaching the LGN
• These cells pass to subcortical regions, making up several (around 10) different pathways.

Question 3

What is unique about subcortical pathways?

Answer 3

• Evolutionarily more ancient

- Unconscious processing

Question 4

What effect has evolution had on visual pathways?

Answer 4

• Appears to have replaced these old routes with new (better/conscious) ones
• But have retained them and added new routes that enable finer levels of processing

Question 5

What percentage of visual processing is carried out via subcortical processing?

Answer 5

-20

Question 6

Outline sub-cortical pathway

Answer 6

Retina > Superior Colliculus > Pulvinar (thalamus) > secondary visual cortex pathway

Question 7

Purpose of superior colliculus pathway

Answer 7

• Involved in the control of automatic reflexes

- Orienting movements of the head and eyes – especially when new stimuli appear in the visual field

Question 8

What difference does superior colliculus pathway have with V1?

Answer 8

• much faster than V1 route

- so can alert of potentially dangerous stimuli

Question 9

What does SC pathway suggest?

Answer 9

-how it is possible to unconsciously turn to look at something without realising its importance until after orienting

Question 10

What are the two pathways in the occipital lobe?

Answer 10

• Dorsal stream

- Ventral stream

Question 11

What does the ventral stream process?

Answer 11

• What the item is

Question 12

What does the dorsal stream process?

Answer 12

• Where the item is

Question 13

Why does the brain process where and what separately?

Answer 13

-For speed purposes, it is much quicker processing separately than sequentially

Question 14

What significant contribution to vision does the sub-cortical pathway have?

Answer 14

-phenomenon of blindsight

Question 15

What is blindsight also referred to as?

Answer 15

• Cortical blindness

Question 16

What is cortical blindness?

Answer 16

• A condition in which a patient sustains damage to the primary visual cortex and loss of (at least part of) the visual field.

Question 17

Describe the symptom of blindsight

Answer 17

• A symptom in which the patient reports not being able to consciously sees stimuli in a particular region but can nevertheless perform visual discriminations

Question 18

Why is blindsight seen to be controversial?

Answer 18

-The idea of blindsight itself is not controversial but why it occurs is

Question 19

What was theory proposed by Campion et al, 1983 for blindsight and why was it disproved?

Answer 19

• That blindsight is the result of islands of spared cortex within the supposedly damaged region
• Many patients have undergone structural and functional MRI. These have established that no cortical activity remains in the region corresponding to the ‘blind’ field
• Evidence suggests that the spared striate cortex explanation is weak.

Question 20

What is the stray light hypothesis?

Answer 20

• Stray light from the stimulus is scattered onto intact parts of the visual field and is detected by intact parts of area V1
• In other words, if you don’t have total visual loss and have only lost partial visual field
• Brain would fill in missing segment

Question 21

What is issue with stray light hypothesis?

Answer 21

• Even though it can fill light in that region, cannot detect contrast in colour or movement
• Still able to make perceptual decisions in the presence of strong ambient light, which reduces the amount of stray

Question 22

What is the most satisfactory explanation for blindsight?

Answer 22

-is that it reflects the operation of other visual routes from the eye to the brain

Question 23

What evidence is there about visual systems?

Answer 23

-Supports the proposal that we have visual systems that operate both within and outside our conscious awareness

Question 24

What evidence does blindsight provide?

Answer 24

-Provides evidence for the existence of unconscious (subcortical) routes to vision – but remains controversial.