Using voluntary motor commands to inhibit involuntary arm movements

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 When this voluntary effort was withdrawn, the involuntary arm lift resumed.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

Tonic and Rhythmic Spinal Activity Underlying Locomotion

“…The Kohnstamm generator is assumed to reflect a property of the central "tonogenic" neuronal structures to maintain long-lasting activation states though the opinions about the exact physiological mechanisms remain controversial (for review see [91,92]. Even though the "residual" sensory activity after the cessation of voluntary effort may take place [93,94], purely peripheral factors such as thixotropic properties of muscle proprioceptors [94,95] can unlikely explain continuous rhythmic movements since the aftereffect would be expected to disappear after one or two cycling leg movements during air-stepping (as thixotropy of both extrafusal and intrafusal muscle fibers substantially declines following muscle lengthening or shortening, [96]).…”

“…It has even been argued that the Kohnstamm command may also originate upstream of the motor cortex, since the silent period in EMG activity is present during both involuntary (Kohnstamm-evoked) and voluntary muscle contraction in response to transcranial magnetic stimulation of the human motor cortex. [92]. The neurophysiological substrates may comprise spinal circuitry (interneurons and motoneurons), increased input from peripheral sensory nerves and supraspinal structures [55,69,92,93].…”

“…[92]. The neurophysiological substrates may comprise spinal circuitry (interneurons and motoneurons), increased input from peripheral sensory nerves and supraspinal structures [55,69,92,93].…”

Experimental investigations of control principles of involuntary movement: a comprehensive review of the Kohnstamm phenomenon

“…Mean muscle silent period duration following application of TMS did not differ across aftercontraction and voluntary movement conditions ( e ). Muscular contractions made a full recovery after the silent period for both Kohnstamm aftercontractions and voluntary movements ( f ).Adapted from Ghosh et al (2014) …”

“…Ghosh et al (2014) examined the subjective experience of participants as they lowered their arms during an aftercontraction, and compared this to the feeling of lowering the arm without an aftercontraction. In the latter condition the arm was first held in the abducted position at shoulder level for 1 min.…”