Visual Extinction

Visual Extinction is a condition caused by damage to the parietal lobe, and is similar, although distinct from Visual Neglect.

It is characterised by the ability to see stimuli in the opposite visual field to the brain damage, but only when there is no competition from other stimuli in the visual field on the same side as the brain damage. If there are stimuli in both visual fields then only the one which is projected to the intact side of the brain will be seen.

It is diagnosed using confrontation testing:
– the experimenter wiggles their left/right fingers or both in the air while sitting opposite the patient
– patient can detect each finger when they are presented separately
– however, if both are presented then they can only detect the finger on the left (assuming the right parietal lobe is damaged)

The video below shows a variation of this technique:

However, there are some circumstances in which extinction can be reduced.

Riddoch (2003) presented patients with pairs of objects, which were correctly or incorrectly presented for action. For example, a corkscrew pointing towards the cork in a wine bottle (correct), or at the bottom of the bottle (incorrect).
The results showed that they were better at reporting both items when they were correctly presented for action. When one item was extinguished, they were more likely to report the active item, even if it is in the impaired visual field.
Therefore, which object they reported was influenced by the interaction between them.
This finding was important as it suggests that extinction occurs quite late, in higher-order visual areas and there is some unconscious processing of extinguished items.

Like Neglect, extinction can also occur in motor actions, not just vision.
For example, several case studies have shown that patients can use both arms equally well separately, but become much worse at using their bad arm when doing so at the same time as the good arm.

Hope you enjoyed this post – don’t forget to check back soon for more!

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Visual Neglect

This is a neurological condition that is caused by damage to one if the hemispheres of the brain. This damage causes the patient to be unable to pay attention to one half of their visual field – they just ignore everything in it.

This condition – sometimes known as unilateral neglect, is most common when the right hemisphere is damaged as this hemisphere is involved in distributing attention. Damage to the right hemisphere causes neglect in the left visual field.

The area of the brain thought to be damaged in most cases of neglect is the posterior parietal cortex – shown on the diagram of the brain below:

 

http://www.livescience.com/7110-store.html

The following examples are from a patient with unilateral neglect who has been asked to copy some images:

http://www.acbrown.com/neuro/Lectures/Assc/NrAsscPrtl.htm

As you can see, they ignore the left side of the drawings. Patients will also ignore the food on one half of their plate and can also neglect one side of their body.

Neglect syndrome can occur after a stroke which damages the right hemisphere – the good news is that with occupational therapy, the patient’s condition can improve over time. One method which has been shown to help patients attend to their neglected side is to use prism glasses, which direct their eyes to their left visual field.

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