Changes in interhemispheric inhibition from active to resting primary motor cortex during a fine-motor manipulation task

Abstract

Morishita T, Uehara K, Funase K. Changes in interhemispheric inhibition from active to resting primary motor cortex during a fine-motor manipulation task. J Neurophysiol 107: 3086 -3094, 2012. First published March 14, 2012 doi:10.1152/jn.00888.2011.-The effect of performance of a sensorimotor task on the interhemispheric inhibition (IHI) induced from the active primary motor cortex (M1) to the resting M1 was examined in 10 right-handed subjects. Transcranial magnetic stimulation (TMS) was performed to produce motor evoked potentials (MEP) in the resting right (Rt)-first dorsal interosseous (FDI). For the paired-TMS paradigm, a conditioning stimulus (CS) was delivered to the Rt-M1, and its intensity was adjusted from 0.6 to 1.4 times the resting motor threshold of the MEP in the left (Lt)-FDI in 0.2 steps. The test stimulus was delivered to the Lt-M1, and its intensity was adjusted to evoke similar MEP amplitudes in the Rt-FDI among the task conditions. The interstimulus interval was fixed at 10 ms. As a sensorimotor task, a fine-motor manipulation (FM) task (using chopsticks to pick up, transport, and release glass balls) was adopted. In addition, an isometric abduction (IA) task was also performed as a control task. These tasks were carried out with the left hand. The IHI from the active to the resting M1 observed during the FM task was markedly increased compared with that induced during the IA task, and this effect was not dependent on the MEP amplitude evoked in the active Lt-FDI by the CS. The present findings suggest that the increased IHI from the active to the resting M1 observed during the FM task was linked to reductions in the activity of the ipsilateral intracortical inhibitory circuit, as we reported previously.

Change in the Ipsilateral Motor Cortex Excitability Is Independent from a Muscle Contraction Phase during Unilateral Repetitive Isometric Contractions

“…In the present study, enhancement of ipsi-M1 excitability induced during the unilateral repetitive muscle contractions was observed as compared with that seen at resting condition, which was consistent with previous studies [1]–[3], [10]–[12]. However, Liepert et al [24], who reported that a unilateral force generation task using pinch grip at exceedingly-low force output (1–2% of MVC) can provoke an inhibitory effect of ipsi-M1.…”

“…Likewise, a human TMS study performed during various hand motor tasks found not only changes in contralateral M1 (contra-M1) excitability, but also changes in ipsi-M1 excitability, confirming the complex mechanism involved in their activation. In particular, motor tasks requiring a high degree of dexterity, rhythmic repetitive hand movements or sustained isometric contraction at relative weak muscle activity have been demonstrated to markedly enhance ipsi-M1 excitability [1]–[3], [9]–[12]. These studies suggest that the effect of performing a unilateral hand motor task on change in the ipsi-M1 excitability can be attributed to the transcallosal pathway.…”

“…Recent transcranial magnetic stimulation (TMS) studies in human subjects, we found an increase in motor evoked potential (MEP) induced in the resting hand muscle contralateral to movement side, which reflects ipsilateral primary motor cortex excitability (ipsi-M1), while performing a unilateral rhythmic voluntary abduction of the index finger [1] or a fine-motor manipulation task [2], [3] with weak muscle contractions (around 10 to 15% of maximal voluntary contraction, MVC). Among them, we observed that MEP induced in the contralateral resting hand did not depend on the unilateral muscle contraction phase, as assessed by using analysis of correlation between the MEP induced in the contralateral resting hand and the electromyogram (EMG) activity in the active muscles for the 100 ms preceding TMS onset [1]–[3].…”

“…Among them, we observed that MEP induced in the contralateral resting hand did not depend on the unilateral muscle contraction phase, as assessed by using analysis of correlation between the MEP induced in the contralateral resting hand and the electromyogram (EMG) activity in the active muscles for the 100 ms preceding TMS onset [1]–[3]. However, further direct evidence on the effect of muscle contraction phase dependent on the modulation of ipsi-M1 excitability during a performance of unilateral movement should be investigated to establish the systematic method for evaluation of the ipsi-M1 excitability using the TMS paradigm.…”

Transcranial Magnetic Stimulation to Investigate Motor Cortical Circuitry and Plasticity in Spinal Cord Injury

“…Evidence suggests that IHI occurs at the cortical level since Hoffman reflexes are not changed as a result of the two TMS pulses and no change is seen when Transcranial electrical stimulation (which directly activates cortico spinal axons) is used [68]. IHI at these two latencies is mediated by GABA B receptors as inhibition is further increased after administration of baclofen, a GABA B agonist [62,69] IHI shares similar neuronal populations to LICI as both circuits inhibit SICI and are directly inhibited by LAI [61,62] Activity in the limb ipsilateral to the TS can increase the amount of IHI in the opposite limb [68,70] and the intensity of the conditioning stimulus can increase inhibition [70].…”