Press the backs of your hands against the inside of a door frame for 30 seconds—as if you’re trying to widen the frame—and then let your arms down; you’ll feel something odd. Your arms will float up from your sides, as if lifted by an external force. Scientists call this Kohnstamm phenomenon, but you may know it as the floating arm trick. - The science of the floating arm trick news.sciencemag.org/brain-behavior/2014/09/science-floating-arm-trick Press the backs of your hands against the inside of a door frame for 30 seconds—as if you’re trying to widen the frame—and then let your arms down; you’ll feel something odd. Your arms will float up from your sides, as if lifted by an external force. Scientists call this Kohnstamm phenomenon, but you may know it as the floating arm trick. Now, researchers have studied what happens in a person’s brain and nerve cells when they repress this involuntary movement, holding their arms tightly by their sides instead of letting them float up. Two theories existed as to how this repression worked: The brain could send a positive “push down” signal to the arm muscles at the same time as the involuntary “lift up” signal was being transmitted to cancel it out; or the brain could entirely block the involuntary signal at the root of the nerves. The new study, which analyzed brain scans and muscle activity recordings from 39 volunteers, found that the latter was true—when a person stifles Kohnstamm phenomenon, the involuntary “lift” signal is blocked before it reaches the muscle. The difference between the repression mechanisms may seem subtle, but understanding it could help people repress other involuntary movements—including the tremors associated with Parkinson’s disease and the tics associated with Tourette syndrome, the team reports online today in the Proceedings of the Royal Society B. (Video credit: Miura Hawkins, Magali Chytiris, Arko Ghosh/Institute of Neuroinformatics, University of Zurich, and ETH Zurich) Reference Using voluntary motor commands to inhibit involuntary arm movements Proc. R. Soc. B 7 November 2014 vol. 281 no. 1794 20141139 doi: 10.1098/rspb.2014.1139 rspb.royalsocietypublishing.org/content/281/1794/20141139 Abstract A hallmark of voluntary motor control is the ability to stop an ongoing movement. Is voluntary motor inhibition a general neural mechanism that can be focused on any movement, including involuntary movements, or is it mere termination of a positive voluntary motor command? The involuntary arm lift, or ‘floating arm trick’, is a distinctive long-lasting reflex of the deltoid muscle. We investigated how a voluntary motor network inhibits this form of involuntary motor control. Transcranial magnetic stimulation of the motor cortex during the floating arm trick produced a silent period in the reflexively contracting deltoid muscle, followed by a rebound of muscle activity. This pattern suggests a persistent generator of involuntary motor commands. Instructions to bring the arm down voluntarily reduced activity of deltoid muscle. When this voluntary effort was withdrawn, the involuntary arm lift resumed. Further, voluntary motor inhibition produced a strange illusion of physical resistance to bringing the arm down, as if ongoing involuntarily generated commands were located in a ‘sensory blind-spot’, inaccessible to conscious perception. Our results suggest that voluntary motor inhibition may be a specific neural function, distinct from absence of positive voluntary motor commands.
Posted on: Tue, 30 Sep 2014 00:02:36 +0000
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