Preparatory suppression of the human primary motor cortex induced by repetition of simple and choice reaction time tasks: a transcranial magnetic stimulation study.

Transcranial magnetic stimulation was performed to investigate preparatory suppression of activity in the human primary motor cortex (M1) in relation to trial repetition of simple (SRT) and Go/NoGo choice RT (CRT) tasks. These tasks were performed in such a way that after a warning signal, the subjects (N=16) maintained 5% MVC isometric finger force against the force sensor to secure a facilitated state of M1. A response signal to generate pulsed force came at 2 s after the warning signal.

Temporal modulations of agonist and antagonist muscle activities accompanying improved performance of ballistic movements.

Although many studies have examined performance improvements of ballistic movement through practice, it is still unclear how performance advances while maintaining maximum velocity, and how the accompanying triphasic electromyographic (EMG) activity is modified. The present study focused on the changes in triphasic EMG activity, i.e.

Effects of motor imagery are dependent on motor strategies.

To investigate whether the facilitatory effects of motor imagery (MI) are dependent on motor strategies that vary with posture, we used transcranial magnetic stimulation to examine the effects of two forearm positions on motor-evoked potentials during an MI of index-finger abduction. MI-enhanced motor-evoked potentials of the first dorsal interosseous (prime mover) muscle in the forearm prone position were larger than those in the forearm neutral position. The opposite effects were seen in the extensor carpi radialis (synergist) muscle.

Effects of intermanual transfer induced by repetitive precision grip on input-output properties of untrained contralateral limb muscles.

Although there were many reports relating to intermanual transfer of behavioral motor tasks in humans, it is still not well-known whether the transfer phenomenon between the trained and untrained hand is accompanied by corresponding changes in motor system. In the present study we applied transcranial magnetic stimulation to investigate the practice effects of unilateral fingertip precision grip on corticospinal excitability, regarding both the trained and untrained hand muscles. The results showed that after practice fingertip grip force became steady and safety margin dramatically decreased not only in the trained hand, but also in the untrained hand.

Motor strategies and excitability changes of human hand motor area are dependent on different voluntary drives.

The present study examined whether there were different voluntary drives between intended and non-intended muscle contractions. In experiment 1, during intended and non-intended muscle contractions, electromyograms (EMGs) were recorded from the first dorsal interosseous (FDI) and extensor carpi radialis (ECR) muscles when force levels were varied from 10% to 50% maximal voluntary contraction (MVC) in 10% MVC steps. In experiment 2, using transcranial magnetic stimulation, motor-evoked potentials (MEPs) were recorded from the FDI muscle when EMGs were varied from 10% to 40% EMGmax (EMG activities during MVC) in 10% EMGmax steps during intended and non-intended muscle contractions.

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.

Functional demanded excitability changes of human hand motor area.

The present study was performed to examine if there are functional differences between the first dorsal interosseous (FDI) and the abductor digit minimi (ADM) muscles during different muscle contractions, namely dynamic and static contractions of the index and little finger abductions. It was also examined whether these functional differences occur at the cortical level. The motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) and force curves, during the muscle contractions, were simultaneously recorded.

Differential modulations of intracortical neural circuits between two intrinsic hand muscles.

Objective: To investigate whether the intracortical inhibitory (ICI) and facilitatory (ICF) circuits in the primary motor cortex between the first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles are modulated differently.

Methods: We conducted paired-pulse transcranial magnetic stimulation in combination with different current directions (anterior-medially: AM, and posterior-laterally: PL) under relaxed and active muscle conditions with interstimulus intervals (ISIs) between 2 and 16 ms.

Results: In both muscle conditions, the conditioned motor-evoked potential (MEP) responses obtained with the AM current direction (preferentially eliciting early I-waves) were similar between the two muscles at all ISIs, but the MEP responses obtained with the PL current direction (preferentially eliciting late I-waves) were different between FDI and ADM muscles, in that the conditioned MEP responses in FDI muscle were inhibited at all ISIs under both muscle conditions, whereas those in ADM muscle were suppressed at only short ISIs (2-4 ms).

Physical practice induces excitability changes in human hand motor area during motor imagery.

The present study was undertaken to investigate the effects of physical practice on excitability changes in human primary motor cortex (M1) during motor imagery (MI). Using different intensities of transcranial magnetic stimulation (TMS), we examined changes in the motor evoked potential (MEP) of the first dorsal interosseous (FDI) muscle with and without MI, and before and after physical practice. On comparing results for MEPs recorded before and after physical practice, the difference between the MEP amplitudes observed at rest and during MI only increased at higher TMS intensities.

Modulations of input-output properties of corticospinal tract neurons by repetitive dynamic index finger abductions.

The goal of this study was to investigate how corticospinal tract neurons (CTNs) are modulated after repetitive dynamic muscle contractions. To address this question, changes of motor evoked potentials (MEPs) to transcranial magnetic stimulation and background EMG (B.EMG) activities were examined.

Excitability changes of motor evoked potentials dependent on muscle properties and contraction modes.

Using transcranial magnetic stimulation (TMS), differences in the excitability changes of motor evoked potentials (MEPs) between isometric (force task) and isotonic (movement task) muscle contractions in a distal (first dorsal interosseous; FDI) and a proximal (middle deltoid; MD) muscle were studied. In the FDI muscle, the active threshold of MEP recruitment was significantly lower in the isotonic than that in the isometric muscle contraction in spite of identical background EMG activity levels. Additionally, the dependence of the MEP amplitude on background EMG activity was significantly greater in the isotonic than in the isometric muscle contraction at low EMG activity levels, but the difference disappeared beyond middle EMG activity levels.

Effect of vibration-induced postural illusion on anticipatory postural adjustment of voluntary arm movement in standing humans.

We investigated the contribution of sensory signals arising from muscle proprioceptive receptors to anticipatory postural adjustments (APAs). During vibration applied to ankle (tibialis anterior; TA, soleus; Sol) or neck muscles, subjects generally describe having illusory sensations of whole-body movement, namely, whole-body movement in a backward and forward direction induced by vibration of the Sol or TA, respectively, and the front or back surface of the neck muscles, respectively. Preceding electromyographic (EMG) activity of the ipsilateral biceps femoris (BFi) muscle induced by rapid voluntary arm movement and the typical phenomenon of APA were changed dependent on these illusory whole-body movements, with preceding EMG activities of BFi appearing earlier in vibration applied to TA and later in vibration applied to Sol muscle.