Kenichi Sugawara

Influence of mirror therapy on human motor cortex.

This article investigates whether or not mirror therapy alters the neural mechanisms in human motor cortex. Six healthy volunteers participated. The study investigated the effects of three main factors of mirror therapy (observation of hand movements in a mirror, motor imagery of an assumed affected hand, and assistance in exercising the assumed affected hand) on excitability changes in the human motor cortex to clarify the contribution of each factor. The increase in motor-evoked potential (MEP) amplitudes during motor imagery tended to be larger with a mirror than without one. Moreover, MEP amplitudes increased greatly when movements were assisted. Watching the movement of one hand in a mirror makes it easier to move the other hand in the same way. Moreover, the increase in MEP amplitudes is related to the synergic effects of afferent information and motor imagery.

Excitability changes in human hand motor area induced by voluntary teeth clenching are dependent on muscle properties

To investigate whether the early effects of voluntary teeth clenching (VTC) among the first dorsal interosseous (FDI), abductor digiti minimi (ADM), and abductor pollicis brevis (APB) muscles are differently modulated depending on their muscle properties, we examined the responses of motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation with selected current directions and by brainstem magnetic stimulation (BMS). Although MEP responses with anterior-medially current direction (preferentially elicited I1-waves) were facilitated in all three muscles, those responses with posterior-laterally current direction (preferentially elicited I3-waves) were different among FDI, ADM, and APB muscles. That is, MEP responses in FDI and APB muscles were significantly reduced, whereas those responses in ADM muscle were not significantly reduced. Further, inhibitory effects of VTC in FDI muscle were more potent than those in ADM or APB muscles. On the other hand, the responses to BMS were unchanged by VTC in all three muscles, suggesting that the modulations of MEP were attributed to the cortical origin. On the basis of our previous findings that the inhibitory connections in FDI muscle are more potent than those in ADM muscle (Takahashi et al. in Clin Neurophysiol 116:2757–2764, 2005), the cortical effects of VTC among three hand muscles are differently modulated, depending on muscle properties, presumably the extents of inhibitory connections to corticospinal tract neurons. Considering that the functional capacity in FDI muscle is higher than that in ADM or APB muscles, the cortical inhibitory effect of VTC might contribute to the sophisticated regulation of the motor outputs even during VTC.

Combined effect of motor imagery and peripheral nerve electrical stimulation on the motor cortex

Although motor imagery enhances the excitability of the corticospinal tract, there are no peripheral afferent inputs during motor imagery. In contrast, peripheral nerve electrical stimulation (ES) can induce peripheral afferent inputs; thus, a combination of motor imagery and ES may enhance the excitability of the corticospinal tract compared with motor imagery alone. Moreover, the level of stimulation intensity may also be related to the modulation of the excitability of the corticospinal tract during motor imagery. Here, we evaluated whether a combination of motor imagery and peripheral nerve ES influences the excitability of the corticospinal tract and measured the effect of ES intensity on the excitability induced during motor imagery. The imagined task was a movement that involved touching the thumb to the little finger, whereas ES involved simultaneous stimulation of the ulnar and median nerves at the wrist. Two different ES intensities were used, one above the motor threshold and another above the sensory threshold. Further, we evaluated whether actual movement with afferent input induced by ES modulates the excitability of the corticospinal tract as well as motor imagery. We found that a combination of motor imagery and ES enhanced the excitability of the motor cortex in the thenar muscle compared with the other condition. Furthermore, we established that the modulation of the corticospinal tract was related to ES intensity. However, we found that the excitability of the corticospinal tract induced by actual movement was enhanced by peripheral nerve ES above the sensory threshold.

Remote effects of voluntary teeth clenching on excitability changes of the human hand motor area

The aim of the present study was to investigate effects of voluntary teeth clenching (VTC) on motor evoked potentials (MEPs) from the first dorsal interosseous (FDI) muscle to transcranial magnetic stimulation (TMS) by different oriented currents (anterior-medially (AM), posterior-laterally (PL)) of the human motor cortex. In active FDI, VTC enhanced MEP responses induced by AM directed current but reduced these responses induced by PL. In relaxed FDI, VTC enhanced MEP responses by AM but had no significant effects on those by PL. Thus, the results suggest that any components produced by AM directed current were enhanced, whereas those by PL directed currents were not affected or reduced. The present evidence indicates that I-waves recorded at the same latency were not completely the same between those produced by AM and PL directed currents. Because VTC had no influence on responses to brainstem electrical stimulation (BES) or F-waves just after the onset of teeth clenching [T. Furubayashi, K. Sugawara, T. Kasai, A. Hayashi, R. Hanajima, Y. Shiio, N.K. Iwara, Y. Ugawa, Remote effects of self-paced teeth clenching on the excitability of hand motor area, Exp. Brain Res., 148 (2003) 261-265], these modulatory effects on MEPs to both AM and PL directed currents must be due to changes of the motor cortical excitability. Thus, we conclude that VTC affects the motor cortical circuits activated by PL and AM directed currents differentially; it facilitates the one and inhibits the other. This is the first demonstration of opposite effects of the same maneuver on MEP responses elicited by AM and PL directed currents.

Time Course of Corticospinal Excitability and Intracortical Inhibition Just before Muscle Relaxation

Using transcranial magnetic stimulation (TMS), we investigated how short-interval intracortical inhibition (SICI) was involved with transient motor cortex (M1) excitability changes observed just before the transition from muscle contraction to muscle relaxation. Ten healthy participants performed a simultaneous relaxation task of the ipsilateral finger and foot, relaxing from 10% of their maximal voluntary contraction (MVC) force after the go signal. In the simple reaction time (RT) paradigm, single or paired TMS pulses were randomly delivered after the go signal, and motor evoked potentials (MEPs) were recorded from the right first dorsal interosseous (FDI) muscle. We analyzed the time course prior to the estimated relaxation reaction time (RRT), defined here as the onset of voluntary relaxation. SICI decreased in the 80–100 ms before RRT, and MEPs were significantly greater in amplitude in the 60–80 ms period before RRT than in the other intervals in single-pulse trials. TMS pulses did not effectively increase RRT. These results show that cortical excitability in the early stage, before muscle relaxation, plays an important role in muscle relaxation control. SICI circuits may vary between decreased and increased activation to continuously maintain muscle relaxation during or after a relaxation response. With regard to M1 excitability dynamics, we suggest that SICI also dynamically changes throughout the muscle relaxation process.

Excitability changes in human hand motor area dependent on afferent inputs induced by different motor tasks

Using the technique of transcranial magnetic stimulation (TMS) with a figure-of-eight-shaped coil in 16 normal volunteers, we studied the extents of motor evoked potentials (MEPs) induced by remote facilitation of voluntary teeth clenching (VTC) and by motor imagery (MI). In particular, we examined whether different excitability changes in the primary motor cortex (M1) induced by both facilitation methods occur between early (I1 and I2) and late (I3 and I4) components of I-waves elicited from a first dorsal interosseous (FDI) muscle. Both components of I-waves were induced by anterior-medially (AM) directed currents or posterior-laterally (PL) directed currents. Our hypothesis was that facilitatory effects of VTC and MI on M1 differ because the neural pathways of these afferent inputs differ. The present results indicate that during MI MEP amplitudes of late components are significantly larger than those of early ones, although both MEP amplitudes are enhanced. On the other hand, during VTC MEP amplitudes of early components are significantly enhanced, but those of late ones are rather depressed. We conclude that recruitment of early and late components of I-waves differ depending on the afferent inputs to the motor cortex.

Excitability changes in primary motor cortex just prior to voluntary muscle relaxation

We postulated that primary motor cortex (M1) activity does not just decrease immediately prior to voluntary muscle relaxation; rather, it is dynamic and acts as an active cortical process. Thus we investigated the detailed time course of M1 excitability changes during muscle relaxation. Ten healthy participants performed a simple reaction time task. After the go signal, they rapidly terminated isometric abduction of the right index finger from a constant muscle force output of 20% of their maximal voluntary contraction force and performed voluntary muscle relaxation. Transcranial magnetic stimulation pulses were randomly delivered before and after the go signal, and motor evoked potentials (MEPs) were recorded from the right first dorsal interosseous muscle. We selected the time course relative to an appropriate reference point, the onset of voluntary relaxation, to detect excitability changes in M1. MEP amplitude from 80 to 60 ms before the estimated electromyographic offset was significantly greater than that in other intervals. Dynamic excitability changes in M1 just prior to quick voluntary muscle relaxation indicate that cortical control of muscle relaxation is established through active processing and not by simple cessation of activity. The cortical mechanisms underlying muscle relaxation need to be reconsidered in light of such dynamics.

Functional plasticity of surround inhibition in the motor cortex during single finger contraction training.

We investigated the functional changes in short intracortical inhibitory (SICI) circuits to determine whether surround inhibition is altered during a simple finger movement training. Using an electromyographic (EMG) feedback system linked to a computer monitor, participants practiced sustained index finger abduction by 40% maximum voluntary contraction of the first dorsal interosseous (FDI) while decreasing overflow EMG activity of the abductor digiti minimi (ADM) to less than 5% maximum voluntary contraction. Single transcranial magnetic stimuli (TMS) and paired-pulse TMS were applied to the left primary motor cortex to elicit motor-evoked potentials (MEPs) in the right FDI and ADM before/after training. In addition to recording MEPs from both muscles during voluntary FDI contraction, MEPs were recorded during motor imagery. MEPs from the FDI were not altered by training, indicating no functional changes in SICI circuits associated with the FDI field. In contrast, SICI circuits associated with ADM were significantly strengthened by training, as indicated by reduced baseline EMG activity during both actual FDI contraction and motor imagery and by reduced MEPs in response to post-training TMS. We propose that SICI circuits show functional plasticity during motor training and that surround circuit inhibition of nontarget muscle groups increases in proportion to the acquisition of motor skills.

Hemispheric asymmetry of ipsilateral motor cortex activation in motor skill learning.

In this study, we investigated how ipsilateral motor cortex (M1) activation during unimanual hand movements and hemispheric asymmetry changed after motor skill learning. Eleven right-handed participants preformed a two-ball-rotation motor task with the right and the left hand, separately, in all experimental sessions. Before and after exercise sessions, the degree of ipsilateral M1 activation during brief execution of the motor task was measured as changes in the size of motor-evoked potentials (MEPs) of the thenar and the first dorsal interosseous muscle of the nontask hand using transcranial magnetic stimulation. Before exercise, MEPs of the nontask hand were significantly facilitated on both sides during the motor task. After exercise, facilitation of MEPs of the nontask hand during the motor task was significantly reduced for the right hand (thenar: P=0.014, first dorsal interosseous: P=0.022) but not for the left hand. We conclude that ipsilateral M1 activation, associated with a complex motor task, is first symmetrical in both hemispheres. However, on exercise, ipsilateral activation is reduced only in left M1, indicating a stronger learning-dependent modification of motor networks within the left hemisphere.

The Effect of Visual and Auditory Enhancements on Excitability of the Primary Motor Cortex during Motor Imagery: A Pilot Study.

The effect of visual and auditory enhancements of finger movement on corticospinal excitability during motor imagery (MI) was investigated using the transcranial magnetic stimulation technique. Motor-evoked potentials were elicited from the abductor digit minimi muscle during MI with auditory, visual and, auditory and visual information, and no information. Ten healthy participants were instructed to imagine repetitive abduction and adduction of the fifth finger. After each condition, the extent of vividness of MI was rated using a visual analogue scale. The results showed that the mean visual analogue scale score and motor-evoked potentials amplitude for the auditory and visual condition were higher than those of other conditions, indicating an enhanced excitability of the primary motor cortex with a clearer image of motor action during MI.