Data from: Using voluntary motor commands to inhibit involuntarily arm movements

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 When this voluntary effort was withdrawn, the involuntary arm lift resumed.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.

自主运动控制的标志性特征之一，便是终止正在进行的动作的能力。自主运动抑制（voluntary motor inhibition）究竟是一种可聚焦于任意运动（包括非自主运动）的通用神经机制，还是仅为正向自主运动指令的终止？手臂不自主抬起，或称“浮臂现象（floating arm trick）”，是三角肌（deltoid muscle）一种独特且持续时间较长的反射活动。本研究探究了自主运动网络是如何抑制这类非自主运动控制的。在浮臂现象过程中对运动皮层施加经颅磁刺激（Transcranial magnetic stimulation），可在反射性收缩的三角肌中诱发一段静息期，随后出现肌肉活动的反弹现象。这一模式表明存在持续产生非自主运动指令的源头。要求自主放下手臂的指令会降低三角肌的活动水平。当这一自主努力被撤销后，手臂不自主抬起的现象会再次出现。此外，自主运动抑制还会引发一种怪异的体感错觉：仿佛在自主放下手臂时存在物理阻力，就如同正在进行的非自主运动指令处于“感觉盲区”之中，无法被意识感知。本研究结果表明，自主运动抑制或许是一种独立于“缺失正向自主运动指令”的特异性神经功能。