Scientists believe they have discovered the cause of a condition which prevents people seeing half the world.

The condition often follows injuries to the brain, such as stroke or brain damage.

Problems with vision on the left side is most common after a stroke or brain injury, and affects around 10% of those patients.

It can resolve itself, but some people are left with permanent sight difficulties.

It means people may only eat half a plate of food, or only draw half a picture.

However, when scientists have attempted to discover why people have the condition, they have found many areas of the brain which are responsible for visual processes are still working as they should.

'That's not my arm'

Now, researchers at Wake Forest University Baptist Medical Center have suggested the condition could be caused by faulty signals from cells in an area of the brain called the basal ganglia.

They suggest these signals may prevent affect how visual processes work.

They say it might be possible to alter the signals from the basal ganglia works so the visual areas can work effectively.

Dr John McHaffie, associate professor of neurobiology and anatomy at the Baptist Medical Center, who led the study, said: 'People have tried for a long time to understand why this happens.

'These people have very strange perceptions. They can go so far as to say, 'that's not my arm.''

'The position of the basal ganglia in the brain is very central, and that fits into the idea that it is coordinating a lot of activity for any particular behaviour.'

Dr Barry Stein, who also worked on the study, said: 'The potential site of treatment has now been identified. Once you know where the site is, you can speculate about cures and strategies for therapy.'

Signals

Dr Anthony Rudd, a stroke specialist from St Thomas's Hospital, London, told BBC News Online: 'This is a very common problem.

'Most people with brain stress will have an impairment of their visual faculties.

'It's not an eye problem. It's a problem with the brain and its control of vision.'

He said fibres connected the retinas in the eye to the occipital lobe at the back of the brain, which processes visual information.

'If there is any damage to the brain, these fibres, and the signals being carried by them, can be affected.'

Dr Rudd said damage to the basal ganglia could be involved in the condition.

The research is published in the magazine Nature.

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