Shikako Hayashi

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. This finding indicates a physical practice-dependent increase of the higher threshold recruitment of corticospinal tract neurons (CTNs), consistent with synchronization for efficient movement, and provides evidence that neural mechanisms of MI depend not only on the type of movement but also on the extent of the motor adaptation (the physical practice). These present findings also show the benefit of MI and highlight beneficial neural mechanisms related to the activation of M1 during MI. In other words, MI may reflect functional changes of M1 that are similar to the changes observed after physical practice.

Transcranial Magnetic Stimulation Study of Plastic Changes of Human Motor Cortex after Repetitive Simple Muscle Contractions

Studies of use-dependent changes in neural activation have recently focused on the primary motor cortex. To detect the excitability changes in the primary motor cortex after practice in human subjects, motor-evoked potentials by transcranial magnetic stimulation during motor imagery after just 10 sessions of simple index finger abduction were examined. The present results indicate that width of the output map and amplitudes of motor-evoked potential became progressively larger until practice ended. These flexible short-term modulations of human primary motor cortex seem important and could lead to structural changes in the intracortical networks as the skill becomes more learned and automatic, i.e., ‘adaptation’ as one of the neural mechanisms related to motor learning.

Rapid plastic changes of human primary motor cortex with repetitive motor practice and transcranial magnetic stimulation

Excitability changes of human primary motor cortex are assumed to be associated with motor learning processes. To examine motor behavioral and neural mechanisms in these processes, the adaptive motor learning processes of the index finger abduction were investigated using motor evoked potential (MEP) elicited from the first dorsal interosseous and extensor carpi radialis muscles. Practice effects were examined on changes of MEP amplitudes elicited from these muscles during motor imagery. Given general consensus that the MEP amplitude change during motor imagery is a useful parameter reflecting changes in excitability of the human primary motor cortex, the present results, that MEP amplitudes of both muscles increased with repeated practice by the index finger abduction and that magnitudes of MEP amplitudes of both muscles (motor learning curves) were clearly different, suggested that participation of the muscles performing the index finger abduction gradually changed with practice. Short-term plastic changes of human primary motor cortex occur with repetitive practice and such adaptive change in human primary motor cortex is expressed in human voluntary movement that becomes more automatic.

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.

Modulations of use-dependent excitability changes of human motor cortex (M1) by practice condition.

Effects of repetitive index finger abductions on excitability changes in the human primary motor cortex (M1) are assumed to be dependent on practice conditions of the task. To address how different effects occur dependent on various practice conditions, motor evoked potentials (MEPs) elicited from the first dorsal interosseous (FDI) muscle were investigated. Practice effects on the index finger abduction were examined for changes in excitability of first dorsal interosseous muscle under three forearm position changes (neutral vs prone) and two muscle contraction modes (isometric vs isotonic). Analysis showed that after practice MEP amplitude increased in the prone position but not in the neutral position and MEP increases in the isotonic contraction were larger than those in the isometric mode. These results suggest that use-dependent excitability changes are largely dependent on practice conditions because the amount of afferent input depends on the practice conditions.