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Background The motor cortex in the human brain can be modulated by the application of transcranial static magnetic field stimulation (tSMS) through the scalp. However, the effect of tSMS on the excitability of the primary somatosensory cortex (S1) in humans has never been examined. Objective This study was performed to investigate the possibility of non-invasive modulation of S1 excitability by the application of tSMS in healthy humans. Methods tSMS and sham stimulation over the sensorimotor cortex were applied to 10 subjects for periods of 10 and 15 min. Somatosensory evoked potentials (SEPs) following right median nerve stimulation were recorded before and immediately after, 5 min after, and 10 min after tSMS from sites C3′ and F3 of the international 10-20 system of electrode placement. In another session, SEPs were recorded from 6 of the 10 subjects every 3 min during 15 min of tSMS. Results Amplitudes of the N20 component of SEPs at C3′ significantly decreased immediately after 10 and 15 min of tSMS by up to 20%, returning to baseline by 10 min after intervention. tSMS applied while recording SEPs every 3 min and sham stimulation had no effect on SEP. Conclusions tSMS is able to modulate cortical somatosensory processing in humans, and thus might be a useful tool for inducing plasticity in cortical somatosensory processing. Lack of change in the amplitude of SEPs with tSMS implies that use of peripheral nerve stimulation to cause SEPs antagonizes alteration of the function of membrane ion channels during exposure to static magnetic fields. [ABSTRACT FROM AUTHOR]

Abstract: Reciprocal inhibition of antagonist muscles is crucial for motor skill learning in humans. However, the changes in reciprocal inhibition function during the motor learning process are unknown. The aim of this study was to systematically observe the changes in reciprocal inhibition function. We investigated the optimal coil position for simultaneously eliciting motor evoked potentials (MEPs) of reciprocal muscles, and the reciprocal changes in input–output (IO) curves during motor skill training. From ten healthy volunteers, the IO curves of MEPs were measured for the midpoint between the center of gravity (CoG) of the extensor carpi radialis (ECR) and the flexor carpi radialis (FCR) muscles, for the CoG of ECR, and for the FCR muscles using transcranial magnetic stimulation (TMS). In addition, the IO curves of the ECR and the FCR muscles were measured before and after the motor skill training of rapid wrist extension. The IO curves measured at the midpoint between the CoGs of the ECR and the FCR muscles and the CoG of each muscle were homogenous. However, after training to perform rapid wrist extension, the IO curve of the agonist (ECR) muscle was increased, while the antagonist (FCR) muscle was decreased. The present findings validate the IO curves simultaneously measured for reciprocal muscles, and suggest that motor skill training could induce reciprocal change in corticospinal excitability. [Copyright &y& Elsevier]

Abstract: Objective: This study was performed to elucidate whether transcranial direct current stimulation (tDCS) over the motor association cortex modifies the excitability of primary motor (M1) and somatosensory (S1) cortices via neuronal connectivity. Methods: Anodal, cathodal, and sham tDCS (1 mA) over the left motor association cortex was applied to 10 subjects for 15min using electrodes of two sizes (9 and 18cm2). Both motor evoked potentials (MEPs) and somatosensory evoked potentials (SEPs) were recorded before, immediately after, and 15min after tDCS. Electrode positions were confirmed by overlaying them on MRI anatomical surface images of two individuals. Results: After applying anodal tDCS using the large electrode, amplitudes of MEP components significantly decreased, whereas those of early SEP components (N20 and P25) increase. Opposite effects were observed on MEPs and SEPs after cathodal tDCS. However, a small electrode did not significantly influence either MEPs or SEPs, irrespective of polarity. The small electrode covered mainly the dorsal premotor cortex (PMd) while the large electrode involved the supplementary motor area (SMA) in addition to PMd. Conclusions: These results suggest that anodal tDCS over PMd together with SMA enhanced the inhibitory input to M1 and excitatory input to S1, and that cathodal tDCS might lead to an opposite effect. Significance: The finding that only the large electrode modulated M1 and S1 implies that activation of PMd together with SMA by tDCS can induce plastic changes in primary sensorimotor areas. [Copyright &y& Elsevier]

Objective: Although the supplementary motor area (SMA) is thought to contribute to the generation of anticipatory postural adjustments (APAs), which act to stabilize supporting body segments prior to movement, its precise role remains unclear. This study was performed to elucidate whether cathodal transcranial direct current stimulation (tDCS) applied over the SMA modifies the function of APAs. Method: Cathodal tDCS and sham tDCS were applied over the SMA or left of the leg area of the M1 of 11 healthy subjects for 15min. Subjects performed the task of self-paced rapid shoulder flexion before, immediately after, and 15min after tDCS. Electromyographic (EMG) activity was recorded from the deltoid anterior (DEL_A), as the prime mover muscle, and biceps femoris (BF), as the postural muscle during tasks. Results: Following cathodal tDCS over the SMA, the ΔEMG onset between DEL_A and BF was significantly shortened compared to after sham tDCS, while tDCS over the M1 did not produce any significant effect. Conclusions: These results suggest that the SMA may play a critical role in the generation of voluntary movement accompanied by APAs. Application of cathodal tDCS over the SMA could modulate the function and timing of these APAs. [ABSTRACT FROM AUTHOR]

• Effect of transcranial static magnetic stimulation (tSMS) on DLPFC was examined. • Working memory was impaired by tSMS over the left and right DLPFC. • The degree of impairment was similar between the left and right stimulations. • TSMS over the left DLPFC increased beta-band event-related synchronization. • TSMS over the right DLPFC did not show such a change in brain oscillations. Transcranial static magnetic stimulation (tSMS) is known to influence behavioral and neural activities. However, although the left and right dorsolateral prefrontal cortex (DLPFC) are associated with different cognitive functions, there remains a lack of knowledge on a difference in the effects of tSMS on cognitive performance and related brain activity between left and right DLPFC stimulations. To address this knowledge gap, we examined how differently tSMS over the left and right DLPFC altered working memory performance and electroencephalographic oscillatory responses using a 2-back task, in which subjects monitor a sequence of stimuli and decide whether a presented stimulus matches the stimulus presented two trials previously. Fourteen healthy adults (five females) performed the 2-back task before, during (20 min after the start of stimulation), immediately after, and 15 min after three different stimulation conditions: tSMS over the left DLPFC, tSMS over the right DLPFC, and sham stimulation. Our preliminary results revealed that while tSMS over the left and right DLPFC impaired working memory performance to a similar extent, the impacts of tSMS on brain oscillatory responses were different between the left and right DLPFC stimulations. Specifically, tSMS over the left DLPFC increased the event-related synchronization in beta band whereas tSMS over the right DLPFC did not show such an effect. These findings support evidence that the left and right DLPFC play different roles in working memory and suggest that the neural mechanism underlying the impairment of working memory by tSMS can be different between left and right DLPFC stimulations. [ABSTRACT FROM AUTHOR]

Abstract: Magnetoencephalography (MEG) recordings were collected to investigate the effect of the number of mechanical pins and inter-pin distance on somatosensory evoked magnetic fields (SEFs) following mechanical stimulation (MS). We used a 306-ch whole-head MEG system. SEFs were elicited through tactile stimuli with 1-, 2-, 3-, 4- and 8-pins using healthy participants. Tactile stimuli were applied to the tip of the right index finger. SEF following electrical stimulation of the index finger was recorded in order to compare the activity in the primary somatosensory cortex (S1) following MS. Prominent SEFs were recorded from the contralateral hemisphere approximately 54ms (P50m) and 125ms (P100m) after MS regardless of the number of pins. Equivalent current dipoles were located in the S1. The source activities for P50m and P100m significantly increased in tandem with the number of pins for MS. However, the increased ratios for the source activities according to the increase in the number of pins were significantly smaller than that induced by electrical stimulation, and when the number of the pins doubled from 1-pin to 2-pins, from 2-pins to 4-pins, and from 4-pins to 8-pins, S1 activities increased by only 130%. Additionally, source activities significantly increased when the inter-pin distance increased from 2.4 to 7.2mm. The number of stimulated receptors was considered to have increased with an increase in the inter-pin distance as well as an increase in the number of pins. These findings clarified the effect of the number of pins and inter-pin distance for MS on SEFs. [Copyright &y& Elsevier]

Abstract: We investigated whether anodal transcranial direct current stimulation (tDCS) applied to the motor cortex during non-exhaustive active or passive movements enhances corticomotor excitability after tDCS or whether it reduces post-exercise depression (PED) after non-exhaustive active or passive movements if PED was observed without tDCS. Nine healthy subjects participated in this study. Anodal tDCS with a current of 2mA was applied to the left scalp over the primary motor area. All subjects underwent the following five interventions: tDCS delivered for 10min during relaxation (tDCS condition) and repetitive voluntary and passive finger abduction–adduction movements, each performed without and with tDCS for 10min (active condition, tDCS+active condition, passive condition, tDCS+passive condition). The active movements were performed at 10% maximum voluntary contraction. Motor evoked potentials (MEPs) were recorded from the right first dorsal interosseus muscle before the intervention (pre-intervention) and 2 and 10min after the intervention (post-2min and post-10min, respectively). Under the tDCS condition, the MEP amplitudes at post-2 and -10min were significantly increased compared with those before the intervention. Under the active, passive, and tDCS+active conditions, the MEP amplitudes at post-2min were significantly decreased compared with those before the interventions. Under the tDCS+passive condition, the MEP amplitude remained unchanged. These results demonstrated that anodal tDCS did not reduce PED after active movements but after passive movements and that the anodal tDCS effects were highly dependent on the state of the subject during stimulation. [Copyright &y& Elsevier]

Abstract: Objective: Magnetoencephalography (MEG) recordings were performed to investigate the cortical activation following tactile-on and tactile-off stimulation. Methods: We used a 306-ch whole-head MEG system and a tactile stimulator driven by a piezoelectric actuator. Tactile stimuli were applied to the tip of right index finger. The interstimulus interval was set at 2000ms, which included a constant stimulus of 1000ms duration. Results: Prominent somatosensory evoked magnetic fields were recorded from the contralateral hemisphere at 57.5ms and 133.0ms after the onset of tactile-on stimulation and at 58.2ms and 138.5ms after the onset of tactile-off stimulation. All corresponding equivalent current dipoles (ECDs) were located in the primary somatosensory cortex (SI). Moreover, long-latency responses (168.7ms after tactile-on stimulation, 169.8ms after tactile-off stimulation) were detected from the ipsilateral hemisphere. The ECDs of these signals were identified in the secondary somatosensory cortex (SII). Conclusions: The somatosensory evoked magnetic fields waveforms elicited by the two tactile stimuli (tactile-on and tactile-off stimuli) with a mechanical stimulator were strikingly similar. These mechanical stimuli elicited both contralateral SI and ipsilateral SII activities. Significance: Tactile stimulation with a mechanical stimulator provides new possibilities for experimental designs in studies of the human mechanoreceptor system. [Copyright &y& Elsevier]

• Transcranial static magnetic field stimulation (tSMS) can reduce cortical activity. • The effect of tSMS over the SMA on APAs was investigated. • After 20 min of tSMS, the APAs function was impaired. We investigated the influence of transcranial static magnetic field stimulation (tSMS) over the supplementary motor area (SMA) on anticipatory postural adjustments (APAs), in which the activation of the postural muscles of the legs and trunk that control standing posture precedes the activation of the prime mover muscles during rapid shoulder flexion movement. Eighteen subjects performed a self-paced rapid shoulder flexion task before, during, and after tSMS. Electromyogram (EMG) activity was recorded from the deltoid anterior (AD) as the prime mover muscle and the biceps femoris (BF) as the postural muscle during the task. The EMG latency difference (ΔEMG onset) between the two muscles was calculated by subtracting the EMG burst onset of the BF from that of the AD. The ΔEMG onset was significantly shortened, but center-of-pressure parameters were not affected after tSMS stimulation. These findings suggest that tSMS applied over the SMA could inhibitively modulate APAs function. [ABSTRACT FROM AUTHOR]