Motor imagery of voluntary muscle relaxation of the foot induces a temporal reduction of corticospinal excitability in the hand

Effects of Motor Imagery Tasks on Brain Functional Networks Based on EEG Mu/Beta Rhythm

Motor imagery (MI) refers to the mental rehearsal of movement in the absence of overt motor action, which can activate or inhibit cortical excitability. EEG mu/beta oscillations recorded over the human motor cortex have been shown to be consistently suppressed during both the imagination and performance of movements, although the specific effect on brain function remains to be confirmed. In this study, Granger causality (GC) was used to construct the brain functional network of subjects during motor imagery and resting state based on EEG in order to explore the effects of motor imagery on brain function. Parameters of the brain functional network were compared and analyzed, including degree, clustering coefficient, characteristic path length and global efficiency of EEG mu/beta rhythm in different states. The results showed that the clustering coefficient and efficiency of EEG mu/beta rhythm decreased significantly during motor imagery (p < 0.05), while degree distribution and characteristic path length increased significantly (p < 0.05), mainly concentrated in the frontal lobe and sensorimotor area. For the resting state after motor imagery, the changes of brain functional characteristics were roughly similar to those of the task state. Therefore, it is concluded that motor imagery plays an important role in activation of cortical excitability.

Time course effect of corticospinal excitability for motor imagery

This study examined the effect of temporal changes in corticospinal excitability in motor imagery (MI) and the effect of real-time guides for MI on excitability changes. The MI task involved wrist flexion and motor evoked potentials using transcranial magnetic stimulation were recorded and examined from the flexor carpi radialis. Ballistic (momentary MI) and tonic (continuous MI) conditions were used, and the duration of each MI was different. In Experiment 1, each MI task was performed using an acoustic trigger. In Experiment 2, a real-time guide was presented on a computer screen, which provided a visual indication of the onset and duration of the MI task through via moving dots on the screen. The results indicate that the corticospinal excitability changed differently, depending on the duration of MI. Additionally, with real-time guides, the change in corticospinal excitability became clearer. Thus, corticospinal excitability changes due to the temporal specificities of MI, as well as with actual motor output. Moreover, if MI is actively performed without a guide, it is likely to show an unintended change in corticospinal excitability. It is suggested that when MI is performed with visual guide, the excitatory changes of the corticospinal tract might be different from the actual motor output. Therefore, when using MI for mental practices, it is possible to improve the effect of a guide for MI, such as a visual indicator for motor output. Additionally, when examining neural activities in MI, it may be necessary to consider the characteristics of motion performed by MI.

MRCP as a biomarker of motor action with varying degree of central and peripheral contribution as defined by ultrasound imaging

Motor imagination is an alternative rehabilitation strategy for people who cannot execute real movements. However, it is still a matter of debate to which degree it involves activation of deeper muscle structures, which cannot be detected by surface electromyography (SEMG). Sixteen able-bodied participants performed cue based isometric ankle plantar flexion (active movement) followed by active relaxation under four conditions: executed movements with two levels of muscle contraction (fully executed and attempted movements, EM and AM) and motor imagination with and without detectable muscle twitches (IT and I). The most prominent peaks and distinctive phases of movement-related cortical potential (MRCP) were compared between conditions. Ultrasound imaging (USI) and SEMG were used to detect movements. IT showed spatially distinctive significant differences compared to both I and AM during active movement preparation and reafferentation phase; further widespread differences were found between IT and AM during active movement execution and posteriorly during preparation for active relaxation. EM and AM showed the largest differences frontally during active movement planning and posteriorly during execution of active relaxation. Movement preparation positivity P1 showed a significant difference in amplitude between IT and AM but not between IT and I. USI can detect subliminal movements (twitches) better than SEMG. MRCP is a biomarker sensitive to different levels of muscle contraction and relaxation. IT is a motor condition distinguishable from both I and AM. EEG biomarkers of movements could be used to identify pathological conditions, that manifest themselves during either active contraction or active relaxation.NEW & NOTEWORTHY Ultrasound imaging can detect subtle muscle movements (twitches) that are not detectable with electromyography. Almost a quarter of trials of imagined movements in able-bodied people are accompanied by twitches. Analysis of movement-related cortical potential showed that motor imagination with twitches is a condition distinguishable from motor imagination without twitches and from motor attempts.

Hypnosis-induced modulation of corticospinal excitability during motor imagery

Hypnosis can be considered an altered state of consciousness in which individuals produce movements under suggestion without apparent voluntary control. Despite its application in contexts implying motor control, evidence for the neurophysiological mechanisms underlying hypnosis is scarce. Inter-individual differences in hypnotic susceptibility suggest that sensorimotor strategies may manifest in a hypnotic state. We tested by means of transcranial magnetic stimulation applied over the primary motor cortex whether motor system activation during a motor imagery task differs in the awake and in the hypnotic state. To capture individual differences, 30 healthy volunteers were classified as high or low hypnotizable (Highs and Lows) according to ad-hoc validated scales measuring hypnotic susceptibility and personality questionnaires. Corticospinal activation during motor imagery in the hypnotic state was greater in the Highs than the Lows. Intrinsic motivation in task performance and level of persuasion modulated corticospinal activation in the Highs. Corticospinal system activation under hypnosis may have practical implications that merit research in areas where hypnosis can be applied to improve motor performance, such as loss of motor abilities and sports.

Brain Activity Underlying Muscle Relaxation

Fine motor control of not only muscle contraction but also muscle relaxation is required for appropriate movements in both daily life and sports. Movement disorders such as Parkinson’s disease and dystonia are often characterized by deficits of muscle relaxation. Neuroimaging and neurophysiological studies suggest that muscle relaxation is an active process requiring cortical activation, and not just the cessation of contraction. In this article, we review the neural mechanisms of muscle relaxation, primarily utilizing research involving transcranial magnetic stimulation (TMS). Several studies utilizing single-pulse TMS have demonstrated that, during the relaxation phase of a muscle, the excitability of the corticospinal tract controlling that particular muscle is more suppressed than in the resting condition. Other studies, utilizing paired-pulse TMS, have shown that the intracortical inhibition is activated just before muscle relaxation. Moreover, muscle relaxation of one body part suppresses cortical activities controlling other body parts in different limbs. Therefore, the cortical activity might not only be a trigger for muscle relaxation of the target muscles but could also bring about an inhibitory effect on other muscles. This spread of inhibition can hinder the appropriate contraction of muscles involved in multi-limb movements such as those used in sports and the play of musical instruments. This may also be the reason why muscle relaxation is so difficult for beginners, infants, elderly, and the cognitively impaired.