Imagining is Not Doing but Involves Specific Motor Commands: A Review of Experimental Data Related to Motor Inhibition

Abstract

There is now compelling evidence that motor imagery (MI) and actual movement share common neural substrate. However, the question of how MI inhibits the transmission of motor commands into the efferent pathways in order to prevent any movement is largely unresolved. Similarly, little is known about the nature of the electromyographic activity that is apparent during MI. In addressing these gaps in the literature, the present paper argues that MI includes motor execution commands for muscle contractions which are blocked at some level of the motor system by inhibitory mechanisms. We first assemble data from neuroimaging studies that demonstrate that the neural networks mediating MI and motor performance are not totally overlapping, thereby highlighting potential differences between MI and actual motor execution. We then review MI data indicating the presence of subliminal muscular activity reflecting the intrinsic characteristics of the motor command as well as increased corticomotor excitability. The third section not only considers the inhibitory mechanisms involved during MI but also examines how the brain resolves the problem of issuing the motor command for action while supervising motor inhibition when people engage in voluntary movement during MI. The last part of the paper draws on imagery research in clinical contexts to suggest that some patients move while imagining an action, although they are not aware of such movements. In particular, experimental data from amputees as well as from patients with Parkinson’s disease are discussed. We also review recent studies based on comparing brain activity in tetraplegic patients with that from healthy matched controls that provide insights into inhibitory processes during MI. We conclude by arguing that based on available evidence, a multifactorial explanation of motor inhibition during MI is warranted.

The Effect of Motor Imagery on Spinal Motor Neuron Excitability and Its Clinical Use in Physical Therapy

“…The activities of multiple brain regions as motor preparation and planning plausibly increased spinal motor neuron excitability via the corticospinal and/or extrapyramidal tract. Furthermore, MI is the mental rehearsal of a movement without any overt movement and muscle contraction [1]. Therefore, it is considered that motor inhibiting function was participated in simultaneously with motor preparation and planning.…”

“…Motor imagery (MI) is defined as a cognitive process in which the subjects imagine that they perform movements without actually performing movements and muscle contractions [1].…”

Incomplete inhibition of central postural commands during manual motor imagery

“…1), measurable postural adjustments during imagery would suggest that the (imagined) load was centrally factored into P, but P was not successfully inhibited. Thus, the failure to inhibit the postural component of imagined action could not be attributed to afference-influenced processes downstream of central command generation that are not accessible to a central inhibition command (inh-P) (e.g., Boulton and Mitra, 2013;Guillot et al, 2012;Grangeon et al, 2011).…”

“…This autonomy of response may arise in the way posture control encapsulates the use of efferent and afferent signals to balance the body while supporting the descending motor commands. The suggestion is that the inhibition command, if it is of a more central origin, may not have access to the postural synergies assembled further down the efferent pathway (Boulton and Mitra, 2013;Guillot et al, 2012;Grangeon et al, 2011).…”

New evidence of corticospinal network modulation induced by motor imagery

“…Today, the question of the release of a potential cortical output elaborated during MI remains open (Guillot et al 2012). Does this motor output reach the spinal level, therefore implying cortical and spinal interactions, or is it the product of supraspinal processes solely?…”