Thursday, 20 October 2011

Week 4 revision blog - Visual perception

Lesions to the parietal cortex results in a deficit to a person’s spatial perception abilities. Neglect after stroke appears to have high consequences, an example of this is when a patient has a right hemisphere stroke they lose awareness of any left side stimulus when there is a competing right stimulus. It is reported that patients are only aware of half of the world. This disorder causes difficulties in most everyday life events, e.g. difficulty dressing one side of the body, reading things on only one side of the page, and often only eating from one side of their plate. Some patients are unaware that they are neglecting information on one hemifield, this is called 'anosagnosia' this is obviously detrimental to the recovery of the patient. Spatial neglect can be simply tested using a task that involves copying a drawing. A person with spatial neglect will only copy one half of a drawing. Blindsight is another sensory impairment, this is caused by damage to the striate cortex and involves the ability to locate an unseen object, even though reporting they could not see the object itself. Riddoch (1917) reported that patients with blindsight could tell the direction an object was moving, but could not actually see the object. Research has hypothesised that this happens due to 'extraocular scatter' which is when light reflects off of the stimuli therefore allowing it to be picked up by the visual system. The subcortical pathway that carries information from the retina to the superior colliculus could help explain blindsight, as this neural pathway only provides rough information at a quick pace, therefore the patient picks up rough movement, but cannot distinguish any fine detail.

Revision material from the article (week 4) – ‘Implicit face perception in a patient with visual agnosia? Evidence from behavioural and eye-tracking analyses’.

Le. S & Raufaste. E etal (2002)
investigate whether a patient with a face perception deficit can recognise faces in the absence of their awareness. Prosopagnosia is the inability to recognise faces that the person previously knew. It has been suggested that although prosopagnosia patients report not feeling familiarity when viewing a face it has been shown that the processing of familiar faces can still happen without the patients awareness (this has been shown by skin conductance test).
It has been shown that
object recognition and face recognition activate different loci in the same cortical area therefore although one type of recognition may stay intact, the other becomes severely damaged. The article puts forward two hypotheses to explain why a patient with visual agnosia and prosopagnosia was still capable of some visual processing. The two hypothesis are; 'The spared hypothesis' this suggest that the patients abilities are due to spared modules of implicit face processing. The second hypothesis is the 'general strategy hypothesis' this suggests the patient’s ability was due to compensatory strategies. Experiment one saw the patient along with control participants having to distinguish if the picture being shown to them was a face or a vegetable and experiment two saw the patient and control participants having to state whether the picture they were being shown was a face or not. Results showed that the patient was not aware of faces, however it showed that he processed them differently to non-faces. Eye tracking shows that facial features were processed similarly to the control participants, therefore it was discussed that the results rejected the 'general compensatory strategy hypothesis' therefore supporting the 'spared module hypothesis'. These results suggest that a patient with severe deficits to visual perception can recognise faces without conscious awareness.

Important info from lecture

- Lesions of the parietal cortex lead to spatial deficits.
-When a patient has had a right hemisphere stroke they lose awareness of any left stimulus when there is a competing right stimulus.

Neglect is a directional bias modulated by competing stimuli not a lack of awareness of one half of space.

- Patients with right parietal lesions have impaired detection on their right side and often revisit locations on the right side because they cannot remember where they have looked before.
- Neglect patients also show deficits on non-spatial tasks, therefore sustained attention & selective attention are impaired.

- The optic nerves from each eye meet at the optic chiasm - Information from the left part of the visual field goes to the right side of the brain & information from the right part of the visual field goes to the left side of the brain.

Homonymous hemianopia

- This is a loss of vision on the corresponding area of visual field in both eyes.


This includes; a loss of half of the field of vision and can detect/discriminate visual stimuli in their blind field e.g. colour, motion etc.

- The visual pathway is fast and unconscious and provides only rough information about the location and identity of stimuli not fine detail - this could explain why the patient can detect colour and motion but not actually able to fully detect/see the stimuli.

- Some researchers found that after training homonymous hemianopia patients with visual search everyday for a month the patients improved and became faster at finding targets - eye tracking also revealed that patients more effectively made allocation of fixations after training, therefore suggesting that this visual problem can be improved with visual training.

-Milner & goodale (1995) suggest that there are two vision systems; visual perception and visuomotor control only the first is involved in conscious awareness. Blindsight has been suggested as dissociation between fast motor reactions and conscious perception.

What and where

What = Bilateral lesion of the temporal lobe leads to a deficit in the discrimination of objects.
Where = Bilateral lesion of the parietal lobe leads to a deficit in the discrimination of locations.

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