How does alien hand syndrome work?

Question

How does alien hand syndrome work?

Answer

Alien hand syndrome is a rare neurological condition, in which one hand (almost always the left) conducts movements without conscious control of the person affected (it can also affect the left leg). Sufferers frequently remark that someone else is moving their hand, or that it is moving on its own. They may also be unaware of the alien hand’s actions until something draws their attention to it.

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The most common cause of alien hand syndrome is damage to the corpus callosum – the part of the brain that serves as the only connection between the two hemispheres. While this can be the result of strokes or other brain damage, severance of the corpus callosum has been a common treatment for severe epilepsy. When the two hemispheres cannot communicate, they each act as separate entities. Each hemisphere controls the hand on the opposite side of the body, and when no longer connected each can form differing opinions, and can act upon them independently. However, when such a person speaks, only one hemisphere is really speaking, as the speech centres are located in the left hemisphere. This leads to the impression that the actions undertaken by the left hand are not under control – the entity controlling them is simply mute, and cannot justify its actions.
Some other conditions can also result in alien hand syndrome.
One-sided damage to mesial parts of the frontal lobe can cause exploratory reaching movements in the corresponding hand, where stretching towards and grasping nearby objects is reinforced through a tactile positive feedback loop (sometimes called magnetic apraxia). It can be difficult to extract the object from the affected hand once this has happened.

Damage to posterior sections of the brain (particularly the inferior parietal lobule) can cause another distinct variant of alien hand syndrome. The affected hand avoids touching objects, and can withdraw in anticipation of things coming into contact with it.

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