Study sheds light on stroke patients' 'forgotten' limbs
26 November 2008
Patients with motor neglect tend not to use one arm, seemingly forgetting that they can use it even though it is not weak or impaired - by a loss of touch or muscle tremor, for example.
The condition affects about a third of stroke patients and a form may also be present in patients with Parkinson’s disease. However, little is known about the underlying causes of the condition.
Now, Professor Masud Husain and colleagues from the Institute of Neurology at University College London have found that the mechanism underlying motor neglect is related to the way the brain prepares us to move, and a malfunction in the inhibitory signals that help to regulate those actions.
Just looking at an object can 'prime' the brain, preparing it to react to that object, even if it doesn’t actually go through with the action. If we then decide to ignore the object, an inhibitory signal is sent to stop the action before the body executes the command.
In the study, seven stroke patients with motor neglect of their left hand undertook a test, pressing buttons with their left or right hand in response to target arrows on a screen. Unbeknown to the patients, the target arrow was actually preceded by a brief, subliminal 'priming' arrow - sometimes matching the target arrow, sometimes not. These priming stimuli subconsciously prompted the brain to prepare a movement.
The researchers found a significant delay in patients with left-sided motor neglect when rightward priming arrows were shown, indicating that priming signals that prepare movement of the right hand interfere with the legitimate actions the patient wanted to carry out with their left hand.
"If we subconsciously prime the brains of right-hemisphere stroke patients to make a rightward movement but then actually ask them to use their impaired left hand - their neglected limb - they are slowed in responding," says Husain, a Wellcome Trust Senior Research Fellow.
The researchers say this may indicate a malfunction with the inhibitory signal to cancel out the priming signal of the 'good' right hand. Brain scans revealed two areas of the brain - the putamen and nearby subcortical white matter - were more likely to be damaged in patients suffering the delay.
"We would need further studies to say exactly what the contribution of the putamen is, but we know that in these patients with right putamen damage, plans for moving their left hand are at a disadvantage to right hand movements," Husain says.
The findings may point to future treatment of motor neglect by attempting to restore the balance between the two sides of the brain using magnetic stimulation, drugs or even restraint therapy - temporarily putting the good arm in a sling so that it can't be used and thereby forcing the patient to make use of the neglected arm.
Husain adds that the finding of putamen involvement is intriguing, since this region is part of the basal ganglia - a group of structures deep within the brain affected in Parkinson’s disease. His group's investigations of motor neglect in stroke patients may also offer a potential avenue for better understanding of Parkinson’s disease.
Image: Morguefile/wesleyman
References
Coulthard E et al. Motor neglect associated with loss of action inhibition. J Neurol Neurosurg Psychiatry 2008;79:1401-1404; doi:10.1136/jnnp.2007.140715