Corticospinal Modulations during Motor Imagery of Concentric, Eccentric, and Isometric Actions

Motor imagery from brain to muscle: a commentary on Bach et al., (2022)

In a recent article entitled "Why motor imagery is not really motoric: towards a re-conceptualization in terms of effect-based action control", Bach et al. nicely renewed the concept of motor equivalence between actual movement and motor imagery (MI), i.e. the mental simulation of an action without its corresponding motor output. Their approach is largely based on behavioral studies and, to a lesser extent, on the literature using cerebral imagery. However, the literature on cortico-spinal circuitry modulation during MI can provide further, interesting aspects. Indeed, when it comes to addressing the motor system, one should consider the whole path from brain region to muscle contraction, including sub-cortical structures such as the spinal circuitry. This commentary aims at bridging this gap by providing supplemental evidence and outlining a complementary approach.

Corticospinal Excitability Is Lower During Eccentric Than Concentric Cycling in Men

How corticospinal excitability changes during eccentric locomotor exercise is unknown. In the present study, 13 volunteers performed 30-min strenuous concentric and eccentric cycling bouts at the same power output (60% concentric peak power output). Transcranial magnetic and electrical femoral nerve stimulations were applied at exercise onset (3rd min) and end (25th min). Motor-evoked potentials (MEPs) amplitude was measured for the rectus femoris (RF) and vastus lateralis (VL) muscles with surface electromyography (EMG) and expressed as a percentage of maximal M-wave amplitude (M_MAX). EMG amplitude 100 ms prior to MEPs and the silent period duration were calculated. There was no change in any neural parameter during the exercises (all P > 0.24). VL and RF M_MAX were unaffected by exercise modality (all P > 0.38). VL MEP amplitude was greater (26 ± 11.4 vs. 15.2 ± 7.7% M_MAX; P = 0.008) during concentric than eccentric cycling whereas RF MEP amplitude was not different (24.4 ± 10.8 vs. 17.2 ± 9.8% M_MAX; P = 0.051). While VL EMG was higher during concentric than eccentric cycling (P = 0.03), RF EMG showed no significant difference (P = 0.07). Similar silent period durations were found (RF: 120 ± 30 ms; VL: 114 ± 27 ms; all P > 0.61), but the silent period/MEP ratio was higher during eccentric than concentric cycling for both muscles (all P < 0.02). In conclusion, corticospinal excitability to the knee extensors is lower and relative silent period longer during eccentric than concentric cycling, yet both remained unaltered with time.

Time course changes in corticospinal excitability during repetitive peripheral magnetic stimulation combined with motor imagery

Repetitive peripheral magnetic stimulation (rPMS) induces proprioceptive afferents and facilitates corticospinal excitability. Short-term sessions of rPMS combined with motor imagery (MI) enhance corticospinal excitability more than rPMS alone. However, it is not clear how long the intervention of rPMS combined with MI would be needed to facilitate corticospinal excitability. Therefore, we investigated the time course change in corticospinal excitability during the combination of rPMS and MI. Thirteen healthy volunteers participated in a 20-min intervention under the following three experimental conditions on different days: rPMS, MI, and rPMS combined with MI (rPMS + MI). In the rPMS and rPMS + MI, the participants were delivered rPMS, which was 25 Hz, 2 s/train at 1.5 × of the train intensity induced muscle contractions, through the wrist extensor muscles. In the MI and rPMS + MI, the participants repeatedly imagined wrist movements for 2 s. Motor evoked potentials (MEPs) were recorded from the extensor carpi radialis (ECR) and flexor carpi radialis (FCR) muscles every 5 min for each condition. The MEP amplitudes of the ECR after > 10 min of intermittent rPMS combined with MI were greater than baseline. The MEP amplitude of the ECR in rPMS + MI was greater than that in rPMS condition after 20 min of intervention. The present results suggest that over 10 min of intermittent rPMS combined with MI facilitates corticospinal excitability, and that the effect of rPMS combined with MI on corticospinal excitability might be greater than that of rPMS alone.

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.

Effect of repetitive peripheral magnetic stimulation combined with motor imagery on the corticospinal excitability of antagonist muscles

OBJECTIVE

Repetitive peripheral magnetic stimulation (rPMS) combined with motor imagery facilitates the corticospinal excitability of the agonist muscles. However, the effects of rPMS combined with motor imagery on the corticospinal excitability of the antagonist muscles are unclear. This is an important aspect for applying rPMS in neurorehabilitation for sensorimotor dysfunction. Therefore, we investigated the real-time changes of corticospinal excitability of antagonist muscles during rPMS combined with motor imagery.

METHODS

Fourteen healthy volunteers underwent four different experimental conditions: rest, rPMS, motor imagery, and rPMS combined with motor imagery (rPMS + motor imagery). In the rPMS and rPMS + motor imagery conditions, rPMS (25 Hz, 1600 ms/train, 1.5× of the motor threshold) was delivered to the dorsal side of the forearm. In motor imagery and rPMS + motor imagery, the participant imagined wrist extension movements. Transcranial magnetic stimulation was delivered to record motor-evoked potentials of the antagonist muscle during experimental interventions.

RESULTS

The motor-evoked potential (normalized by rest condition) values indicated no difference between rPMS, motor imagery, and rPMS + motor imagery.

CONCLUSION

These results suggest that rPMS combined with motor imagery has no effect on the corticospinal excitability of the antagonist muscles and highlight the importance of investigating the effects of rPMS combined with motor imagery at the spinal level.