Understanding alien limbs

  • December 5, 2013
Understanding alien limbs

Noham Wolpe is leader of a study which sheds light on a condition which causes involuntary movements of limbs.

Scientists have moved one step closer to understanding a syndrome which causes people to progressively lose control of their limbs and can result in dementia.

Corticobasal syndrome is a rare condition, often expressed in a difficulty to control a limb. In most people, this is usually their hand or arm, but sometimes a person’s leg can be affected. Some people with the syndrome have reported that the affected limb does things against their ‘will’, for example, grasping and reaching for objects, and sometimes the limb does not even feel like their own any more. These are known as alien limb symptoms. As Corticobasal syndrome progresses, it often affects balance and speech, and many people go on to develop dementia.

In a study published in the journal Brain and led by Gates Cambridge Scholar Noham Wolpe [2010], researchers studied the mechanisms behind the abnormalities affecting patients’ ability to control the movement of their limbs.

They combined several key methods to investigate these mechanisms, including a quantitative measure of conscious awareness and control of action, evaluating the perceived time of movements, as well as structural and functional brain imaging methods. Patients showed a specific deficit in their perception of action in their affected hand, with a large delay in their perception of the time an action took place. The extent of this deficit correlated with the severity of their alien limb symptoms.

Complementing this finding, structural brain imaging analyses showed the behavioural abnormality in patients was related to changes in grey matter volume in a frontal brain area that is central for voluntary action: the pre-supplementary motor area. In addition, the integrity of this area’s underlying white matter and of more frontal white matter tracts were related to the deficit.

Moreover, changes in functional connectivity in a frontal network, centred on the pre-supplementary motor area, were strongly associated with abnormality in voluntary movement. The researchers say that by identifying a brain network responsible for the abnormalities in voluntary action, these results can be used to evaluate new therapies for impairments in voluntary action.

The paper is based on Noham’s PhD thesis, and on research conducted during his time as a Gates Cambridge Scholar.

He says: “The findings shed new light on the mechanisms behind the abnormalities in conscious awareness and control of voluntary action. Our findings have significance for the understanding of human volition, suggesting a frontal brain network is responsible for switching between externally triggered, spontaneous actions and internally triggered, non-habitual actions. By underscoring the importance of this frontal brain network and the role of its abnormality in alien limb and apraxia, two of the defining symptoms of Corticobasal syndrome, our study provides a candidate marker for future treatments in this debilitating condition that is currently untreatable.”

Picture credit: www.freedigitalphotos.net and dream designs

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