September 24, 2014 report
Brain scans reveal true nature of muscle control in 'floating arm trick'
(Medical Xpress)—A trio of neuroscientists has learned more about what the brain does during the "floating arm trick" by enlisting volunteers who underwent brain scans while performing the trick. In their paper published in Proceedings of the Royal Society B: Biological Sciences, Arko Gosh, John Rothwell and Patrick Haggard, describe their ideas leading into the experiment and what they learned from the brain scans.
The "floating arm trick" (Kohnstamm phenomenon) has been around for awhile—a person stands next to a wall and pushes against the wall with the back of their hand for approximately one minute. After that the person steps away from the wall they find their arm rising without them telling it to do so. An alternative is to stand in a doorway pushing with both hands to get both arms to rise unbidden. Scientists have used brain scans to study the parts of the brain that are at play during the trick, but until now, none have tested the brain to see what happens if the person intentionally holds their arms down, instead of letting them float up. Besides an area of interest, learning more about how the brain resists an involuntary movement could help researchers trying to find therapies for such ailments as Parkinson's disease.
The researchers enlisted the assistance of 39 male and female volunteers with ages ranging from 18 to 60. Each was fitted with electrodes to record brain activity. In one set of tests electromyography was used to monitor electrical activity in the brain, in another transcranial magnetic stimulation was used to look into the relationship between the neural centers generating the Kohnstamm response and the primary motor cortex. Most specifically, the researchers wanted to know if the muscles that control arm movement were getting two contradictory commands at the same time (one consciously, one unconsciously) with one being overridden by the other, or if the brain was heading off the unconscious signal before it was sent to the muscle—after studying the data, the researchers found that it was the latter—instead of sending conflicting messages, the brain figures out a way to shut down the Kohnstamm response before it can even get started. The end result is a feeling of resisting pressure, the team reports, as if there were helium balloons tied to the arms.
More information: Using voluntary motor commands to inhibit involuntary arm movements, Proceedings of the Royal Society B, Published 24 September 2014 DOI: 10.1098/rspb.2014.1139
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.
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