Disorders of Visual Perception - 2 Flashcards
Which pathway gives the largest contribution to human visual perception?
-the retinal-geniculate-striatal pathway
What are the other visual pathways?
- 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.
What is unique about subcortical pathways?
- Evolutionarily more ancient
- Unconscious processing
What effect has evolution had on visual pathways?
- 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
What percentage of visual processing is carried out via subcortical processing?
-20
Outline sub-cortical pathway
Retina > Superior Colliculus > Pulvinar (thalamus) > secondary visual cortex pathway
Purpose of superior colliculus pathway
- Involved in the control of automatic reflexes
- Orienting movements of the head and eyes – especially when new stimuli appear in the visual field
What difference does superior colliculus pathway have with V1?
- much faster than V1 route
- so can alert of potentially dangerous stimuli
What does SC pathway suggest?
-how it is possible to unconsciously turn to look at something without realising its importance until after orienting
What are the two pathways in the occipital lobe?
- Dorsal stream
- Ventral stream
What does the ventral stream process?
- What the item is
What does the dorsal stream process?
- Where the item is
Why does the brain process where and what separately?
-For speed purposes, it is much quicker processing separately than sequentially
What significant contribution to vision does the sub-cortical pathway have?
-phenomenon of blindsight
What is blindsight also referred to as?
- Cortical blindness
What is cortical blindness?
- A condition in which a patient sustains damage to the primary visual cortex and loss of (at least part of) the visual field.
Describe the symptom of blindsight
- A symptom in which the patient reports not being able to consciously sees stimuli in a particular region but can nevertheless perform visual discriminations
Why is blindsight seen to be controversial?
-The idea of blindsight itself is not controversial but why it occurs is
What was theory proposed by Campion et al, 1983 for blindsight and why was it disproved?
- 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.
What is the stray light hypothesis?
- 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
What is issue with stray light hypothesis?
- 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
What is the most satisfactory explanation for blindsight?
-is that it reflects the operation of other visual routes from the eye to the brain
What evidence is there about visual systems?
-Supports the proposal that we have visual systems that operate both within and outside our conscious awareness
What evidence does blindsight provide?
-Provides evidence for the existence of unconscious (subcortical) routes to vision – but remains controversial.
