Effects of anodal transcranial direct current stimulation over the leg motor area on lumbar spinal network excitability in healthy subjects

Transcranial Direct-Current Stimulation Over the Primary Motor Cortex and Cerebellum Improves Balance and Shooting Accuracy in Elite Ice Hockey Players

PURPOSE

To investigate the effects of transcranial direct-current stimulation (tDCS) applied over the primary motor cortex (M1) and cerebellum on balance control and shooting accuracy in elite ice hockey players.

METHODS

Twenty-one elite ice hockey players underwent anodal tDCS over the M1 (a-tDCSM1), anodal tDCS over the cerebellum (a-tDCSCB), concurrent dual-site anodal tDCS over the M1 and the cerebellum (a-tDCSM1+CB), and sham stimulation (tDCSSHAM). Before and after receiving tDCS (2 mA for 15 min), participants completed an ice hockey shooting-accuracy test, Pro-Kin balance test (includes stance test and proprioceptive assessment), and Y-balance test in randomized order.

RESULTS

For static balance performance, the ellipse area in the 2-legged stance with eyes open and the 1-legged stance with the dominant leg significantly improved following a-tDCSM1, a-tDCSCB, and concurrent dual-site a-tDCSM1+CB, compared with tDCSSHAM (all P < .05, Cohen d = 0.64-1.06). In dynamic balance performance, the average trace error of the proprioceptive assessment and the composite score of the Y-balance test with the dominant leg significantly improved following a-tDCSM1 and concurrent dual-site a-tDCSM1+CB (all P < .05, Cohen d = 0.77-1.00). For the ice hockey shooting-accuracy test, shooting-accuracy while standing on the unstable platform significantly increased following a-tDCSM1 (P = .010, Cohen d = 0.81) and a-tDCSCB (P = .010, Cohen d = 0.92) compared with tDCSSHAM.

CONCLUSION

tDCS could potentially be a valuable tool in enhancing static and dynamic balance and shooting accuracy on unstable platforms in elite ice hockey players.

Neuromodulation with transcranial direct current stimulation contributes to motor function recovery via microglia in spinal cord injury

Spinal cord injury (SCI) is damage or trauma to the spinal cord, which often results in loss of function, sensation, or mobility below the injury site. Transcranial direct current stimulation (tDCS) is a non-invasive and affordable brain stimulation technique used to modulate neuronal circuits, which changes the morphology and activity of microglia in the cerebral cortex. However, whether similar morphological changes can be observed in the spinal cord remains unclear. Therefore, we evaluated neuronal population activity in layer 5 (L5) of M1 following SCI and investigated whether changes in the activities of L5 neurons affect microglia-axon interactions using C57BL/6J mice. We discovered that L5 of the primary motor cortex (corticospinal neurons) exhibited reduced synchronized activity after SCI that correlates with microglial morphology, which was recovered using tDCS. This indicates that tDCS promotes changes in the morphological properties and recovery of microglia after SCI. Combining immunotherapy with tDCS may be effective in treating SCI.

Effects of Anodal Transcranial Direct Current Stimulation With Overground Gait Training on Lower Limb Performance in Individuals With Incomplete Spinal Cord Injury

OBJECTIVE

To determine the effects of anodal transcranial direct current stimulation (tDCS) combined with overground gait training on gait performance, dynamic balance, sit-to-stand performance, and quality of life in individuals with incomplete spinal cord injuries (iSCI).

DESIGN

Double-blind sham-controlled trial with a matched-pair design.

SETTING

Sirindhorn National Medical Rehabilitation Institute, Thailand.

PARTICIPANTS

Individuals with iSCI (n=34) were allocated to the anodal or sham groups.

INTERVENTION

Anodal tDCS was administered over the M1 lower-limb motor area at an intensity of 2 mA for 20 min in the anodal group, while the sham group received a 30-s stimulation. Both groups received 40 min of overground gait training after tDCS for 5 consecutive daily sessions.

MAIN OUTCOME MEASURES

The 10-meter walk test (10MWT) was the primary outcome, while spatiotemporal gait parameters, the timed Up and Go test, Five-Time Sit-to-Stand Test, and World Health Organization Quality of Life-BREF were secondary outcomes. Outcomes were assessed at baseline, post-intervention, and at 1-month (1M) and 2-month (2M) follow-ups.

RESULT

Improvements in walking speed measured using the 10MWT were observed in both groups. However, the anodal group showed a greater improvement than the sham group. For fast speed, the mean between-group differences were 0.10 m/s, 95% CI (0.02 to 0.17) (post-intervention), 0.11 m/s, (0.03 to 0.19) (1M), and 0.11 m/s, (0.03 to 0.20) (2M), while for self-selected speed, the median differences were 0.10 m/s, 95% CI (0.06 to 0.14) (post-intervention) and 0.09 m/s, (0.01 to 0.19) (2M). The anodal group also had a greater stride length difference post-intervention (median difference: 0.07 m, 95% CI (0.01 to 0.14)). No significant between-group differences were found for other outcomes.

CONCLUSION

Five-session of anodal tDCS with gait training slightly improved walking speed, sustained for 2 months post-intervention. However, effect on spatiotemporal gait parameters was limited and dynamic balance, functional tasks (ie, sit-to-stand), and quality of life were unaffected compared with overground gait training.

Horizontal foot orientation affects the distribution of neural drive between gastrocnemii during plantarflexion, without changing neural excitability

The distribution of activation among muscles from the same anatomical group can be affected by the mechanical constraints of the task, such as limb orientation. For example, the distribution of activation between the gastrocnemius medialis (GM) and lateralis (GL) muscles during submaximal plantarflexion depends on the orientation of the foot in the horizontal plane. The neural mechanisms behind these modulations are not known. The overall aim of this study was to determine whether the excitability of the two gastrocnemius muscles is differentially affected by changes in foot orientation. Nineteen males performed isometric plantarflexions with their foot internally (toes-in) or externally (toes-out) rotated. GM and GL motor unit discharge characteristics were estimated from high-density surface electromyography to estimate neural drive. GM and GL corticospinal excitability and intracortical activity were assessed using transcranial magnetic stimulation through motor-evoked potentials. The efficacy of synaptic transmission between Ia-afferent fibers and α-motoneurons of the GM and GL was evaluated through the Hoffmann reflex. We observed a differential change in neural drive between GM (toes-out > toes-in) and GL (toes-out < toes-in). However, there was no foot orientation-related modulation in corticospinal excitability of the GM or GL, either at the cortical level or through modulation of the efficacy of Ia-α-motoneuron transmission. These results demonstrate that change in the motor pathway excitability is not the mechanism controlling the different distribution of neural drive between GM and GL with foot orientation.NEW & NOTEWORTHY Horizontal foot orientation affects the distribution of neural drive between the gastrocnemii during plantarflexion. There is no foot orientation-related modulation in the corticospinal excitability of the gastrocnemii, either at the cortical level or through modulation of the efficacy of Ia-α-motoneuron transmission. Change in motor pathway excitability is not the mechanism controlling the different distribution of neural drive between gastrocnemius medialis and lateralis with foot orientation.

Transspinal Direct Current Electrical Stimulation Selectively Affects the Excitability of the Corticospinal System, Depending on the Intensity but Not Motor Skills

Transspinal direct current stimulation (tsDCS) is a non-invasive technique used to modulate spinal cord activity. However, the effects and mechanisms of this stimulation are currently not comprehensively known. This study aimed to estimate the effect of different intensities of tsDCS applied at the level of cervical enlargement of the spinal cord (C7-Th1 segments) on the excitability of the corticospinal system (CSS) and the correction of motor skills in healthy subjects. The effect of tsDCS was estimated by the motor-evoked potentials (MEP) elicited by transcranial magnetic stimulation (TMS) in the primary motor cortex (M1). The study involved 54 healthy adults aged 22 ± 4 years. The application of 11 min anodal tsDCS at the level of the cervical spine C7-Th1 with a current intensity of 2.5 mA did not change the MEP amplitude of the upper limb muscles, in contrast to the data that we previously obtained with a current intensity of 1.5 mA. We also found no difference in the effect of 2.5 mA stimulation on motor skill correction in healthy subjects in the nine-hole peg test (9-HPT) and the serial reaction time task (SRT) as with 1.5 mA stimulation. Our data show that an increase in the intensity of stimulation does not lead to an increase in the effects but rather reduces the effects of stimulation. These results provide information about the optimally appropriate stimulation current intensities to induce CSS excitability and the ability of tsDCS to influence motor skills in healthy adults.

Short-term effectiveness of transcranial direct current stimulation in the nociceptive behavior of neuropathic pain rats in development

Neuropathic pain (NP) is caused by a lesion that triggers pain chronification and central sensitization and it can develop in a different manner, dependent of age. Recent studies have demonstrated the efficacy of transcranial direct current stimulation (tDCS) for treating NP. Then, we aimed to investigate the effects of tDCS and BDNF levels in neuropathic pain rats in development, with 30 days old in the beginning of experiments. Eight-five male Wistar rats were subjected to chronic constriction injury. After establishment of NP, bimodal tDCS was applied to the rats for eight consecutive days, for 20 minutes each session. Subsequently, nociceptive behavior was assessed at baseline, 14 days after surgery, 1 day and 7 days after the end of tDCS. The rats were sacrificed 8 days after the last session of tDCS. An increase in the nociceptive threshold was observed in rats in development 1 day after the end of tDCS (short-term effect), but this effect was not maintained 7 days after the end of tDCS (long-term effect). Furthermore, brain derived neurotrophic factor (BDNF) levels were analyzed in the frontal cortex, spinal cord and serum using ELISA assays. The neuropathic pain model showed an effect of BDNF in the spinal cord of rats in development. There were no effects of BNDF levels of pain or tDCS in the frontal cortex or serum. In conclusion, tDCS is an effective technique to relieve nociceptive behavior at a short-term effect in neuropathic pain rats in development, and BDNF levels were not altered at long-term effect.

Participant attention on the intervention target during repetitive passive movement improved spinal reciprocal inhibition enhancement and joint movement function

This study aimed to evaluate the effects of the participant's attention target during repetitive passive movement (RPM) intervention on reciprocal inhibition (RI) and joint movement function. Twenty healthy adults participated in two experiments involving four attention conditions [control (forward attention with no RPM), forward attention (during RPM), monitor attention (monitor counting task during RPM), ankle joint attention (ankle movement counting task during RPM)] during 10-min RPM interventions on the ankle joint. Counting tasks were included to ensure the participant's attention remained on the target during the intervention. In Experiment 1, RI was measured before, immediately after, and 5, 10, 15, 20, and 30 min after the RPM intervention. In Experiment 2, we evaluated ankle joint movement function at the same time points before and after RPM intervention. The maximum ankle dorsiflexion movement (from 30° plantar flexion to 10° dorsiflexion) was measured, reflecting RI. In Experiment 1, the RI function reciprocal Ia inhibition was enhanced for 10 min after RPM under all attention conditions (excluding the control condition. D1 inhibition was enhanced for 20 min after RPM in the forward and monitor attention conditions and 30 min after RPM in the ankle joint attention condition. In Experiment 2, the joint movement function decreased under the forward and monitor attention conditions but improved under the ankle joint attention condition. This study is the first to demonstrate that the participant's attention target affected the intervention effect of the RI enhancement method, which has implications for improving the intervention effect of rehabilitation.

A Single Application of Cerebellar Transcranial Direct Current Stimulation Fails to Enhance Motor Skill Acquisition in Parkinson's Disease: A Pilot Study

Parkinson's disease (PD) is a progressive neurodegenerative disorder that leads to numerous impairments in motor function that compromise the ability to perform activities of daily living. Practical and effective adjunct therapies are needed to complement current treatment approaches in PD. Transcranial direct current stimulation applied to the cerebellum (c-tDCS) can increase motor skill in young and older adults. Because the cerebellum is involved in PD pathology, c-tDCS application during motor practice could potentially enhance motor skill in PD. The primary purpose was to examine the influence of c-tDCS on motor skill acquisition in a complex, visuomotor isometric precision grip task (PGT) in PD in the OFF-medication state. The secondary purpose was to determine the influence of c-tDCS on transfer of motor skill in PD. The study utilized a double-blind, SHAM-controlled, within-subjects design. A total of 16 participants completed a c-tDCS condition and a SHAM condition in two experimental sessions separated by a 7-day washout period. Each session involved practice of the PGT concurrent with either c-tDCS or SHAM. Additionally, motor transfer tasks were quantified before and after the practice and stimulation period. The force error in the PGT was not significantly different between the c-tDCS and SHAM conditions. Similarly, transfer task performance was not significantly different between the c-tDCS and SHAM conditions. These findings indicate that a single session of c-tDCS does not elicit acute improvements in motor skill acquisition or transfer in hand and arm tasks in PD while participants are off medications.

Effects of active and sham tDCS on the soleus H-reflex during standing

Weak transcranial direct current stimulation (tDCS) is known to affect corticospinal excitability and enhance motor skill acquisition, whereas its effects on spinal reflexes in actively contracting muscles are yet to be established. Thus, in this study, we examined the acute effects of Active and Sham tDCS on the soleus H-reflex during standing. In fourteen adults without known neurological conditions, the soleus H-reflex was repeatedly elicited at just above M-wave threshold throughout 30 min of Active (N = 7) or Sham (N = 7) 2-mA tDCS over the primary motor cortex in standing. The maximum H-reflex (Hmax) and M-wave (Mmax) were also measured before and immediately after 30 min of tDCS. The soleus H-reflex amplitudes became significantly larger (by 6%) ≈1 min into Active or Sham tDCS and gradually returned toward the pre-tDCS values, on average, within 15 min. With Active tDCS, the amplitude reduction from the initial increase appeared to occur more swiftly than with Sham tDCS. An acute temporary increase in the soleus H-reflex amplitude within the first minute of Active and Sham tDCS found in this study indicates a previously unreported effect of tDCS on the H-reflex excitability. The present study suggests that neurophysiological characterization of Sham tDCS effects is just as important as investigating Active tDCS effects in understanding and defining acute effects of tDCS on the excitability of spinal reflex pathways.

F-waves induced by motor point stimulation are facilitated during handgrip and motor imagery tasks

The F-wave is a motor response elicited via the antidromic firings of motor nerves by the electrical stimulation of peripheral nerves, which reflects the motoneuron pool excitability. However, the F-wave generally has low robustness i.e., low persistence and small amplitude. We recently found that motor point stimulation (MPS), which provides the muscle belly with electrical stimulation, shows different neural responses compared to nerve stimulation, e.g., MPS elicits F-waves more robustly than nerve stimulation. Here, we investigated whether F-waves induced by MPS can identify changes in motoneuron pool excitability during handgrip and motor imagery. Twelve participants participated in the present study. We applied MPS on their soleus muscle and recorded F-waves during eyes-open (EO), eyes-closed (EC), handgrip (HG), and motor imagery (MI) conditions. In the EO and EC conditions, participants relaxed with their eyes open and closed, respectively. In the HG, participants matched the handgrip force level to 30% of the maximum voluntary force with visual feedback. In the MI, they performed kinesthetic MI of plantarflexion at the maximal strength with closed eyes. In the HG and MI, the amplitudes of the F-waves induced by MPS were increased compared with those in the EO and EC, respectively. These results indicate that the motoneuron pool excitability was facilitated during the HG and MI conditions, consistent with findings in previous studies. Our findings suggest that F-waves elicited by MPS can be a good tool in human neurophysiology to assess the motoneuron pool excitability during cognitive and motor tasks.

Motor point stimulation induces more robust F-waves than peripheral nerve stimulation

The F-wave is a motor response induced by electrical stimulation of peripheral nerves via the antidromic firing of motor nerves, which reflects the motoneuron excitability. To induce F-waves, transcutaneous peripheral nerve stimulation (PNS) is used, which activates nerve branches via transcutaneous electrodes over the nerve branches. An alternative method to activate peripheral nerves, i.e., motor point stimulation (MPS) which delivers electrical stimulation over the muscle belly, has not been used to induce F-waves. In our previous studies, we observed that MPS induced F-wave like responses, i.e., motor responses at the latency of F-waves at a supramaximal stimulation. Here we further investigated the F-wave like responses induced by MPS in comparison to PNS in the soleus muscle. Thirteen individuals participated in this study. We applied MPS and PNS on the participant's left soleus muscle. Using a monopolar double-pulse stimulation, the amplitude of the second H-reflex induced by PNS decreased, while the amplitude of the motor response at the F-wave latency induced by MPS did not decrease. These results suggest that the motor response at the F-wave latency induced by MPS was not an H-reflex but an F-wave. We also found that the F-wave induced by MPS had a greater amplitude, higher persistence, and caused less pain when compared to the F-waves induced using PNS. We conclude that MPS evokes antidromic firing inducing F-waves more consistently compared to PNS.

The effects of concurrent M1 anodal tDCS and physical therapy interventions on function of ankle muscles in patients with stroke: a randomized, double-blinded sham-controlled trial study

One of the most common symptoms in stroke patients is spasticity. The aims were to investigate the effects of anodal trans-cranial direct current stimulation (a-tDCS) over the affected primary motor cortex (M1) on ankle plantar flexor spasticity and dorsiflexor muscle activity in stroke patients. The design of this study was a randomized sham-controlled clinical trial. Thirty-two participants with stroke were randomly assigned to three groups (experimental, sham, control groups). Participants in the experimental and sham groups received 10-session 20-min M1 a-tDCS concurrent with physical therapy (PT), while the control group only received 10-session PT. All groups were instructed to perform home stretching exercises and balance training. Berg Balance Scale (BBS), Modified Ashworth Scale (MAS) of plantar flexors, and EMG activity of lateral gastrocnemius (LG) and tibialis anterior (TA) were recorded during active and passive ankle dorsiflexion immediately and 1 month after interventions. A significant reduction was shown in MAS and EMG activity of LG during dorsiflexion, immediately and 1 month after intervention in the M1 a-tDCS group (p <0.001). BBS also significantly increased only in the M1 a-tDCS group (p <0.001). In addition, EMG activity of TA during active dorsiflexion increased immediately and 1 month after intervention in the M1 a-tDCS group (p <0.001). However, in the sham and control groups, EMG activity of TA increased immediately (p<0.001), while this was not maintained 1 month after intervention (p >0.05). PT concurrent with M1 a-tDCS can significantly prime lasting effects of decreasing LG spasticity, increasing TA muscle activity, and also balance in stroke patients.

Effect of transcranial direct current stimulation on the psychomotor, cognitive, and motor performances of power athletes

In sports science, transcranial direct current stimulation (tDCS) has many unknown effects on neuromuscular, psychomotor and cognitive aspects. Particularly, its impact on power performances remains poorly investigated. Eighteen healthy young males, all trained in a jumping sport (parkour) performed three experimental sessions: anodal tDCS applied either on the left dorsolateral prefrontal cortex (dlPFC, cathode in supraorbital area) or on the primary motor cortex (M1, cathode on contralateral shoulder), and a placebo condition (SHAM), each applied for 20 min at 2 mA. Pre and post, maximal vertical and horizontal jumps were performed, associated to leg neuromuscular assessment through electromyography and peripheral nerve stimulations. Actual and imagined pointing tasks were also performed to evaluate fine motor skills, and a full battery of cognitive and psychomotor tests was administered. M1 tDCS improved jump performance accompanied by an increase in supraspinal and spinal excitabilities. dlPFC stimulation only impacted the pointing tasks. No effect on cognitive tests was found for any of the tDCS conditions. To conclude, the type of performance (maximal versus accurate) affected depended upon the tDCS montage. Finally, athletes responded well to tDCS for motor performance while results to cognitive tests seemed unaffected, at least when implemented with the present rationale.

Breaking the ice to improve motor outcomes in patients with chronic stroke: a retrospective clinical study on neuromodulation plus robotics

BACKGROUND

Stroke is one of the main causes of impairment affecting daily activities and quality of life. There is a growing effort to potentiate the recovery of functional gait and to enable stroke patients to walk independently.

AIM

To estimate the effects of dual-site transcranial direct current stimulation (dstDCS) on gait recovery in chronic stroke patients provided with robot-aided gait training (RAGT).

METHODS

Thirty-seven patients were included in this retrospective clinical study. Nine patients were provided with dstDCS during the first 10 min of RAGT by using Lokomat®Pro (on-RAGT), 15 patients immediately after RAGT (post-RAGT), and 13 patients immediately before RAGT (pre-RAGT).

RESULTS

Each group improved over time concerning disability burden and lower limb strength. on-RAGT and post-RAGT experienced better improvement in balance (p < 0.001) and, moderately, gait endurance (p = 0.04) as compared to pre-RAGT. Furthermore, all treatments decreased the facilitation of the unaffected hemisphere (p < 0.001) and the inhibition of the affected hemisphere (p < 0.001). The duration of such aftereffects was found to be greater for post-RAGT.

DISCUSSION AND CONCLUSION

This is the first trial with dstDCS coupled with RAGT in chronic stroke patients with gait impairment. When timely coupled with RAGT, dstDCS may be considered an effective tool for the recovery of lower limb function in patients with first unilateral stroke in the chronic phase. Moreover, our data suggest the ductility of dstDCS concerning RAGT timing, thus making this intervention suitable in a neurorehabilitation setting and well adaptable to patients' needs.

Does anodal trans-cranial direct current stimulation of the damaged primary motor cortex affects wrist flexor muscle spasticity and also activity of the wrist flexor and extensor muscles in patients with stroke?: a Randomized Clinical Trial

Spasticity is a common symptom in stroke survivors. This study is double-blinded, sham-controlled randomized, clinical trial with three parallel arms. The aim of the study was to investigate the effects of anodal trans-cranial direct current stimulation (a-tDCS) over the damaged primary motor cortex (M1) on spasticity of the wrist flexor and also the activity of wrist flexor and extensor muscles in sub-acute stroke patients. This study was performed on 32 stroke patients. The patients are assigned to three groups (intervention, sham, and control). All participants in the first two groups received 20-min concurrent M1 a-tDCS or sham tDCS and functional electrical stimulation (FES) for 10 sessions (5 sessions per week), while participants in control group were given only 20-min FES for 10 sessions. Modified Ashworth scale of wrist flexors and also electromyography (EMG) activity of flexor carpi radialis (FCR) and extensor carpi radialis (ECR) were recorded before, immediately, and 1 month after the interventions. A significant reduction was shown in the MAS and EMG activity of FCR muscle at passive rest position of the wrist, immediately and 1 month after the intervention in M1 a-tDCS compared to sham and control groups (p < 0.001). Also, the EMG activity of FCR and ECR muscles during active wrist flexion and extension increased immediately and 1 month after intervention in M1 a-tDCS compared to the other groups, respectively (p < 0.001). M1 a-tDCS can significantly decrease the spasticity of wrist flexor muscle and also increase the wrist flexor and extensor muscles activity in stroke patients during active flexion and extension.

Transcranial Direct Current Stimulation (tDCS) Can Alter Cortical Excitability of the Lower Extremity in Healthy Participants: A Review and Methodological Study

OBJECTIVE

Transcranial direct current stimulation (tDCS) has been used to alter cortical excitability of the lower extremity (LE) and to influence performance on LE tasks like ankle tracking accuracy; but no study, to our knowledge, ever reported a significant change in cortical excitability relative to sham-tDCS. Additionally, because several different electrode montages were used in previous studies, it is difficult to know how stimulation should be applied to achieve this effect. Our objective was to determine whether active-tDCS alters cortical excitability of the LE and ankle tracking accuracy relative to sham-tDCS in healthy participants. The efficacy of two electrode montages and two conductance mediums were compared.

METHODS

A triple-blind, fully randomized, within-subjects study was conducted with healthy participants (N=18, 24.2 (6.6) years). Cortical recruitment curves and measures of ankle tracking accuracy for the dominant lower extremity were obtained before and after participants received active-tDCS at 2 milliamps for 20 minutes using montage-medium combinations of M₁-SO:Saline, M₁-SO:Gel, C1-C2:Saline, and C1-C2:Gel and a sham-tDCS condition (M1-SO: Saline).

RESULTS

The motor evoked potential maximum of the recruitment curve was significantly lower for active than sham-tDCS, but only for the M₁-SO:Saline combination. No other significant differences in the recruitment curve parameters or in ankle tracking were found.

CONCLUSIONS

This is the first study to our knowledge to demonstrate a significant difference in cortical excitability of the LE between active and sham-tDCS conditions. Given the order in which the experimental procedures occurred, the result is consistent with the concept of a homeostatic plasticity response.

Activation of the Supplementary Motor Areas Enhances Spinal Reciprocal Inhibition in Healthy Individuals

The supplementary motor area (SMA) may modulate spinal reciprocal inhibition (RI) because the descending input from the SMA is coupled to interneurons in the spinal cord via the reticulospinal tract. Our study aimed to verify whether the anodal transcranial direct current stimulation (anodal-tDCS) of the SMA enhances RI. Two tDCS conditions were used: the anodal stimulation (anodal-tDCS) and sham stimulation (sham-tDCS) conditions. To measure RI, there were two conditions: one with the test stimulus (alone) and the other with the conditioning-test stimulation intervals (CTIs), including 2 ms and 20 ms. RI was calculated at multiple time points: before the tDCS intervention (Pre); at 5 (Int 5) and 10 min; and immediately after (Post 0); and at 5, 10 (Post 10), 15, and 20 min after the intervention. In anodal-tDCS, the amplitude values of H-reflex were significantly reduced for a CTI of 2 ms at Int 5 to Post 0, and a CTI of 20 ms at Int 5 to Pot 10 compared with Pre. Stimulation of the SMA with anodal-tDCS for 15 min activated inhibitory interneurons in RIs by descending input from the reticulospinal tract via cortico–reticulospinal projections. The results showed that 15 min of anodal-tDCS in the SMA enhanced and sustained RI in healthy individuals.

Anodal Transcranial Direct Current Stimulation Enhances Retention of Visuomotor Stepping Skills in Healthy Adults

Transcranial direct current stimulation (tDCS) paired with exercise training can enhance learning and retention of hand tasks; however, there have been few investigations of the effects of tDCS on leg skill improvements. The purpose of this study was to investigate whether tDCS paired with visuomotor step training can promote skill learning and retention. We hypothesized that pairing step training with anodal tDCS would improve skill learning and retention, evidenced by decreased step reaction times (RTs), both immediately (online skill gains) and 30 min after training (offline skill gains). Twenty healthy adults were randomly assigned to one of two groups, in which 20-min anodal or sham tDCS was applied to the lower limb motor cortex and paired with visuomotor step training. Step RTs were determined across three time points: (1) before brain stimulation (baseline); (2) immediately after brain stimulation (P0); and (3) 30 min after brain stimulation (P3). A continuous decline in RT was observed in the anodal tDCS group at both P0 and P3, with a significant decrease in RT at P3; whereas there were no improvements in RT at P0 and P3 in the sham group. These findings do not support our hypothesis that anodal tDCS enhances online learning, as RT was not decreased significantly immediately after stimulation. Nevertheless, the results indicate that anodal tDCS enhances offline learning, as RT was significantly decreased 30 min after stimulation, likely because of tDCS-induced neural modulation of cortical and subcortical excitability, synaptic efficacy, and spinal neuronal activity.

Effects of anodal transcranial direct current stimulation on motor evoked potentials variability in humans

Motor evoked potentials (MEPs) obtained from transcranial magnetic stimulation (TMS) allow corticospinal excitability (CSE) to be measured in the human primary motor cortex (M1). CSE responses to transcranial direct current stimulation (tDCS) protocols are highly variable. Here, we tested the reproducibility and reliability of individual MEPs following a common anodal tDCS protocol. In this study, 32 healthy subjects received anodal tDCS stimulation over the left M1 for three durations (tDCS‐T5, tDCS‐T10, and tDCS‐T20 min) on separate days in a crossover‐randomized order. After the resting motor threshold (RMT) was determined for the contralateral first dorsal interosseous muscle, 15 single pulses 4–8 sec apart at an intensity of 120% RMT were delivered to the left M1 to determine the baseline MEP amplitude at T₀, T₅, T₁₀, T₂₀, T₃₀, T₄₀, T₅₀, and T₆₀ min after stimulation for each durations. During TMS delivery, 3D images of the participant's cortex and hot spot were visualized for obtaining MEPs from same position. Our findings revealed that there was a significant MEPs improvement at T₀ (P = 0.01) after 10 min of anodal stimulation. After the 20‐min stimulation duration, MEPs differed specifically at T_0, T_5, T₃₀ min (P < 0.05). This indicates that tDCS is a promising tool to improve MEPs. Our observed variability in response to the tDCS protocol is consistent with other noninvasive brain stimulation studies.

Looking at ancillary systems for verb recovery: Evidence from non-invasive brain stimulation

Several behavioural and neuroimaging studies have suggested that the language function is not restricted into the left areas but it involves regions not predicted by the classical language model. Accordingly, the Embodied Cognition theory postulates a close interaction between the language and the motor system. Indeed, it has been shown that non-invasive brain stimulation (NIBS) is effective for language recovery also when applied over sensorimotor regions, such as the motor cortex, the cerebellum and the spinal cord. We will review a series of NIBS studies in post-stroke aphasic people aimed to assess the impact of NIBS on verb recovery. We first present results which, following the classical assumption of the Broca's area as the key region for verb processing, have shown that the modulation over this area is efficacious for verb improvement. Then, we will present experiments which, according to Embodied Cognition, have directly investigated through NIBS the role of different sensorimotor regions in enhancing verb production. Since verbs play a crucial role for sentence construction which are most often impaired in the aphasic population, we believe that these results have important clinical implications. Indeed, they address the possibility that different structures might support verb processing.

Bilateral Transcranial Direct Stimulation Over the Primary Motor Cortex Alters Motor Modularity of Multiple Muscles

Transcranial direct current stimulation (tDCS) over the primary motor cortex (M1) has been demonstrated to modulate the motor performance of both healthy individuals and patients with neuromuscular disorders. However, the effect of tDCS on motor control of multiple muscles, which is a prerequisite to change in motor performance, is currently unknown. Using dimensionality reduction analysis, we investigated whether bilateral tDCS over M1 modulates the coordinated activity of 12 muscles. Fifteen healthy men participated in this randomized, double-blind crossover study. Each participant received a 20-min sham and 2-mA stimulation bilaterally over M1 (anode on the right M1 and cathode on the left M1), with a minimum washout period of 4 days. Muscle activation and end-point kinematics were evaluated during a task where participants reached out to a marked target with non-dominant hand as fast as possible, before and immediately after tDCS application. We found decreased similarity in motor modularity and significant changes in muscle activation in a specific motor module, particularly when reaching out to a target placed within arm's length and improved smoothness index of movement only following 2-mA stimulation. These findings indicate that clinicians and researchers need to consider the simultaneous effect of bilateral tDCS over M1 on multiple muscles when they establish tDCS protocol to change in motor performance of patients with neuromuscular deficits.

Combined Transcranial Direct Current Stimulation and Selective Motor Training Enhances Balance in Children With Autism Spectrum Disorder

Previous studies have shown that transcranial direct current stimulation (tDCS) over the primary motor cortex (M1) can facilitate the consolidation of motor skills in people who are typically developed, especially when tDCS is combined with goal-oriented exercises. Yet, the effect of tDCS on the motor skills of children with autism spectrum disorder is unknown. This study sought to investigate the effects of combined anodal tDCS and selective motor training on balance among 18 children with autism spectrum disorder (aged 6–14 years) who we randomly assigned to equal-sized experimental and control groups. The experimental group practiced motor training to improve balance after receiving 1.5 mA anodal tDCS over the left M1 for 20 minutes before each of 10 motor training sessions. The control (sham) group underwent a similar protocol with identical motor training, but tDCS was only artificially administered to them. We evaluated participants’ balance at baseline and after training. Data analysis showed that both anodal tDCS plus motor skill training and motor skill training with sham tDCS had significant positive impacts on balance, but tDCS participants who received both actual tDCS and motor skill training performed significantly better than those who received SHAM tDCS and motor skill training. These preliminary findings suggest that tDCS may enhance motor skill training for children with autism spectrum disorder, but replications with larger samples involving participants with varying levels of autistic symptoms and varied tDCS stimulation polarity are needed to affirm the practical use of this noninvasive brain stimulation.

Transcranial Direct Current Stimulation of the Leg Motor Area - is it partly somatosensory?

Non-invasive brain stimulation such as transcranial direct current stimulation (tDCS) involves passing low currents through the brain and is a promising tool for the modulation of cortical excitability. We computationally investigated the effects of the size of the anode in the conventional montage (contralateral supraorbital cathode) using finite element analysis (FEA) for the targeted leg area of the motor cortex where tDCS is challenging due to the depth and orientation of the leg motor area in the inter-hemispheric fissure. We used FEA to develop two anode sizes (same cathode size) with the same current density but different electric field magnitude at the targeted leg area of the motor cortex. Then, we evaluated the effects of the two anode sizes via neurophysiological testing on twelve healthy subjects, seven males and five females (age: 21-36 years, all right-leg dominant). Here, conventional anodal tDCS electrode montage for the leg area of the motor cortex used a large-anode (5cmx7cm, current strength 2mA) which was compared based on transcranial magnetic stimulation (TMS)-induced motor evoked potentials (MEP) with a small-anode (3.5cmx1cm at 0.2mA) montage of the same current density at the skin-electrode interface and identical contralateral supraorbital cathode placement. Small-anode decreased the electric field magnitude by almost one-tenth but still got a similar statistically significant $(\mathrm {P}<0.05)$ increase in the cortical excitability (MEP) at the targeted leg motor area when compared to sham tDCS. Since the electric field magnitude was similar at the scalp (skin-electrode interface) level but differed significantly at the leg motor area in the inter-hemispheric fissure, so a possible contribution of scalp sensory nerve responses to electrocutaneous stimulation is proposed.

Anodal Transcranial Direct Current Stimulation over the Vertex Enhances Leg Motor Cortex Excitability Bilaterally

In many studies, anodal transcranial Direct Current Stimulation (tDCS) is applied near the vertex to simultaneously facilitate leg motor cortex (M1) of both hemispheres and enhance recovery of gait and balance in neurological disorders. However, its effect on the excitability of leg M1 in either hemisphere is not well known. In this double-blind sham-controlled study, corticospinal excitability changes induced in leg M1 of both hemispheres by anodal (2 mA for 20 minutes) or sham tDCS (for 20 min) over the vertex were evaluated. Peak amplitudes of Transcranial Magnetic Stimulation (TMS) induced motor evoked potentials (MEPs) were measured over the contralateral Tibialis Anterior (TA) muscle before and up to 40 min after tDCS in 11 normal participants. Analysis of data from all participants found significant overall increase in the excitability of leg M1 after tDCS. However, in individual subjects there was variability in observed effects. In 4 participants, 20 min of tDCS increased mean MEPs of TAs on both sides; in 4 participants there was increased mean MEP only on one side and in 3 subjects there was no change. It’s not known if the benefits of tDCS in improving gait and balance are dependent on excitability changes induced in one or both leg M1; such information may be useful to predict treatment outcomes.

Voluntary contraction enhances spinal reciprocal inhibition induced by patterned electrical stimulation in patients with stroke

BACKGROUND

Reciprocal inhibition (RI) may be important for recovering locomotion after stroke. Patterned electrical stimulation (PES) can modulate RI in a manner that could be enhanced by voluntary muscle contraction (VC).

OBJECTIVE

To investigate whether VC enhances the PES-induced spinal RI in patients with stroke.

METHODS

Twelve patients with chronic stroke underwent three 20 min tasks, each on different days: (1) PES (10 pulses, 100 Hz every 2 s) applied to the common peroneal nerve; (2) VC consisting of isometric contraction of the affected-side tibialis anterior muscle; (3) PES combined with VC (PES + VC). RI from the tibialis anterior to the soleus muscle was assessed before, immediately after, and 10, 20, and 30 min after the task.

RESULTS

Compared to the baseline, PES + VC significantly increased the changes in reciprocal inhibition at immediately after and 10 min after the task. PES alone significantly increased this change immediately after the task, while VC alone showed no significant increase.

CONCLUSION

VC enhanced the PES-induced plastic changes in RI in patients with stroke. This effect can potentially increase the success rate of newer neurorehabilitative approaches in achieving functional recovery after stroke.

Priming With Intermittent Theta Burst Transcranial Magnetic Stimulation Promotes Spinal Plasticity Induced by Peripheral Patterned Electrical Stimulation

This study explored the effect of corticospinal activity on spinal plasticity by examining the interactions between intermittent theta burst transcranial magnetic stimulation (iTBS) of the motor cortex and peripheral patterned electrical stimulation (PES) of the common peroneal nerve (CPN). Healthy volunteers (n = 10) received iTBS to the tibialis anterior (TA) muscle zone of the motor cortex and PES of the CPN in three separate sessions: (1) iTBS-before-PES, (2) iTBS-after-PES, and (3) sham iTBS-before-PES. The PES protocol used 10 100-Hz pulses every 2 s for 20 min. Reciprocal inhibition (RI) from the TA to soleus muscle and motor cortical excitability of the TA and soleus muscles were assessed at baseline, before PES, and 0, 15, 30, and 45 min after PES. When compared to the other protocols, iTBS-before-PES significantly increased changes in disynaptic RI for 15 min and altered long-loop presynaptic inhibition immediately after PES. Moreover, the iTBS-induced cortical excitability changes in the TA before PES were correlated with the enhancement of disynaptic RI immediately after PES. These results demonstrate that spinal plasticity can be modified by altering cortical excitability. This study provides insight into the interactions between modulation of corticospinal excitability and spinal RI, which may help in developing new rehabilitation strategies.

Effects of Cathode Location and the Size of Anode on Anodal Transcranial Direct Current Stimulation Over the Leg Motor Area in Healthy Humans

Objective: Non-invasive brain stimulation such as transcranial direct current stimulation (tDCS) involves passing low currents through the brain and is a promising tool for the modulation of cortical excitability. In this study, we investigated the effects of cathode location and the size of anode for anodal tDCS of the right-leg area of the motor cortex, which is challenging due to its depth and orientation in the inter-hemispheric fissure. Methods: We first computationally investigated the effects of cathode location and the size of the anode to find the best montage for specificity of stimulation effects for the targeted leg motor area using finite element analysis (FEA). We then compared the best electrode montage found from FEA with the conventional montage (contralateral supraorbital cathode) via neurophysiological testing of both, the targeted as well as the contralateral leg motor area. Results: The conventional anodal tDCS electrode montage for leg motor cortex stimulation using a large-anode (5 cm × 7 cm, current strength 2 mA) affected the contralateral side more strongly in both the FEA and the neurophysiological testing when compared to other electrode montages. A small-anode (3.5 cm × 1 cm at 0.2 mA) with the same current density at the electrode surface and identical contralateral supraorbital cathode placement improved specificity. The best cathode location for the small-anode in terms of specificity for anodal tDCS of the right-leg motor area was T7 (10-10 EEG system). Conclusion: A small-anode (3.5 cm × 1 cm) with the same current density at the electrode surface as a large-anode (5 cm × 7 cm) resulted in similar cortical excitability alterations of the targeted leg motor cortex respresentation. In relation to the other stimulation conditions, the small-anode montage with the cathode positioned at T7 resulted in the best specificity.

Systems Analysis of Human Visuo-Myoelectric Control Facilitated by Anodal Transcranial Direct Current Stimulation in Healthy Humans

Induction of neuroplasticity by transcranial direct current stimulation (tDCS) applied to the primary motor cortex facilitates motor learning of the upper extremities in healthy humans. The impact of tDCS on lower limb functions has not been studied extensively so far. In this study, we applied a system identification approach to investigate the impact of anodal transcranial direct current stimulation of the leg area of the motor cortex via the human visuo-myoelectric controller. The visuo-myoelectric reaching task (VMT) involves ballistic muscle contraction after a visual cue. We applied a black box approach using a linear ARX (Auto-regressive with eXogenous input) model for a visuomotor myoelectric reaching task. We found that a 20th order finite impulse response (FIR) model captured the TARGET (single input)—CURSOR (single output) dynamics during a VMT. The 20th order FIR model was investigated based on gain/phase margin analysis, which showed a significant (p < 0.01) effect of anodal tDCS on the gain margin of the VMT system. Also, response latency and the corticomuscular coherence (CMC) time delay were affected (p < 0.05) by anodal tDCS when compared to sham tDCS. Furthermore, gray box simulation results from a Simplified Spinal-Like Controller (SSLC) model demonstrated that the input-output function for motor evoked potentials (MEP) played an essential role in increasing muscle activation levels and response time improvement post-tDCS when compared to pre-tDCS baseline performance. This computational approach can be used to simulate the behavior of the neuromuscular controller during VMT to elucidate the effects of adjuvant treatment with tDCS.

The effects of patterned electrical stimulation combined with voluntary contraction on spinal reciprocal inhibition in healthy individuals

The aim of this study was to examine the effects of voluntary contraction (VC) on the modulation of reciprocal inhibition induced by patterned electrical stimulation (PES) in healthy individuals. Twelve healthy volunteers participated in this study. PES was applied to the common peroneal nerve with a train of 10 pulses at 100 Hz every 2 s for 20 min. VC comprised repetitive ankle dorsiflexion at a frequency of 0.5 Hz for 20 min. All participants performed the following three tasks: (i) VC alone, (ii) PES alone, and (iii) PES combined with VC (PES+VC). Reciprocal inhibition was assessed using a soleus H-reflex conditioning-test paradigm at the time points of before, immediately after, 10 min after, 20 min after, and 30 min after the tasks. PES+VC increased the amount of reciprocal inhibition, with after-effects lasting up to 20 min. PES alone increased reciprocal inhibition and maintained the after-effects on reciprocal inhibition for 10 min, whereas VC alone increased only immediately after the task. VC could modulate the plastic changes in spinal reciprocal inhibition induced by PES in healthy individuals. PES combined with VC has a potential to modulate impaired reciprocal inhibition and it may facilitate functional recovery and improve locomotion after central nervous system lesions.

Transspinal Direct Current Stimulation Produces Persistent Plasticity in Human Motor Pathways

The spinal cord is an integration center for descending, ascending, and segmental neural signals. Noninvasive transspinal stimulation may thus constitute an effective method for concomitant modulation of local and distal neural circuits. In this study, we established changes in cortical excitability and input/output function of corticospinal and spinal neural circuits before, at 0–15 and at 30–45 minutes after cathodal, anodal, and sham transspinal direct current stimulation (tsDCS) to the thoracic region in healthy individuals. We found that intracortical inhibition was different among stimulation polarities, however remained unchanged over time. Intracortical facilitation increased after cathodal and anodal tsDCS delivered with subjects seated, and decreased after cathodal tsDCS delivered with subjects lying supine. Both cathodal and anodal tsDCS increased corticospinal excitability, yet facilitation was larger and persisted for 30 minutes post stimulation only when cathodal tsDCS was delivered with subjects lying supine. Spinal input/output reflex function was decreased by cathodal and not anodal tsDCS. These changes may be attributed to altered spontaneous neural activity and membrane potentials of corticomotoneuronal cells by tsDCS involving similar mechanisms to those mediating motor learning. Our findings indicate that thoracic tsDCS has the ability to concomitantly alter cortical, corticospinal, and spinal motor output in humans.

The effect of transcranial direct current stimulation (tDCS) on locomotion and balance in patients with chronic stroke: study protocol for a randomised controlled trial

BACKGROUND

Following stroke, patients are often left with hemiparesis that reduces balance and gait capacity. A recent, non-invasive technique, transcranial direct current stimulation, can be used to modify cortical excitability when used in an anodal configuration. It also increases the excitability of spinal neuronal circuits involved in movement in healthy subjects. Many studies in patients with stroke have shown that this technique can improve motor, sensory and cognitive function. For example, anodal tDCS has been shown to improve motor performance of the lower limbs in patients with stroke, such as voluntary quadriceps strength, toe-pinch force and reaction time. Nevertheless, studies of motor function have been limited to simple tasks. Surprisingly, the effects of tDCS on the locomotion and balance of patients with chronic stroke have never been evaluated. In this study, we hypothesise that anodal tDCS will improve balance and gait parameters in patients with chronic stroke-related hemiparesis through its effects at cortical and spinal level.

METHODS/DESIGN

This is a prospective, randomised, placebo-controlled, double-blinded, single-centre, cross-over study over 36 months. Forty patients with chronic stroke will be included. Each patient will participate in three visits: an inclusion visit, and two visits during which they will all undergo either one 30-min session of transcranial direct current stimulation or one 30-min session of placebo stimulation in a randomised order. Evaluations will be carried out before, during and twice after stimulation. The primary outcome is the variability of the displacement of the centre of mass during gait and a static-balance task. Secondary outcomes include clinical and functional measures before and after stimulation. A three-dimensional gait analysis, and evaluation of static balance on a force platform will be also conducted before, during and after stimulation.

DISCUSSION

These results should constitute a useful database to determine the aspects of complex motor function that are the most improved by transcranial direct current stimulation in patients with hemiparesis. It is the first essential step towards validating this technique as a treatment, coupled with task-oriented training.

TRIAL REGISTRATION

ClinicalTrials.gov, ID: NCT02134158 . First received on 18 December 2013; last updated on 14 September 2016. Other study ID numbers: P120135 / AOM12126, 2013-A00952-43.

Moving Beyond the Brain: Transcutaneous Spinal Direct Current Stimulation in Post-Stroke Aphasia

Over the last 20 years, major advances in cognitive neuroscience have clearly shown that the language function is not restricted into the classical language areas but it involves brain regions, which had never previously considered. Indeed, recent lines of evidence have suggested that the processing of words associated to motor schemata, such as action verbs, modulates the activity of the sensorimotor cortex, which, in turn, facilitates its retrieval. To date, no studies have investigated whether the spinal cord, which is functionally connected to the sensorimotor system, might also work as an auxiliary support for language processing. We explored the combined effect of transcutaneous spinal direct current stimulation (tsDCS) and language treatment in a randomized double-blind design for the recovery of verbs and nouns in 14 chronic aphasics. During each treatment, each subject received tsDCS (20 min, 2 mA) over the thoracic vertebrae (10th vertebra) in three different conditions: (1) anodic, (2) cathodic and (3) sham, while performing a verb and noun naming tasks. Each experimental condition was run in five consecutive daily sessions over 3 weeks. Overall, a significant greater improvement in verb naming was found during the anodic condition with respect to the other two conditions, which persisted at 1 week after the end of the treatment. No significant differences were present for noun naming among the three conditions. The hypothesis is advanced that anodic tsDCS might have influenced activity along the ascending somatosensory pathways, ultimately eliciting neurophysiological changes into the sensorimotor areas which, in turn, supported the retrieval of verbs. These results further support the evidence that action words, due to their sensorimotor semantic properties, are partly represented into the sensorimotor cortex. Moreover, they also document, for the first time, that tsDCS enhances verb recovery in chronic aphasia and it may represent a promising new tool for language treatment.

Polarity-dependent improvement of maximal-effort sprint cycling performance by direct current stimulation of the central nervous system

Sprint motor performance, such as in short-distance running or cycling, gradually decreases after reaching a maximum speed or cadence. This may be attributed to the central nervous system. Brain stimulation studies have recently revealed the plastic nature of the human brain and spinal cord, but it is unclear how direct current stimulation (DCS) affects sprint motor performance. To address this issue, we investigated DCS's effect on healthy volunteers' sprint cycling performance. DCS was applied to the lumbar spinal cord (3mA) or the leg area of the motor cortex (2mA) for 15min with 3 different polarities: anodal, cathodal, and sham. After DCS, the subjects performed maximal-effort sprint cycling for 30s under a constant load. Pooled mean power during the 30s was significantly greater after cathodal transcutaneous spinal DCS to the lumbar spinal cord (tsDCS) than anodal or sham tsDCS. The improvement with cathodal stimulation was notable both 0-5 and 20-25s after the performance onset. There were no significant inter-conditional differences in peak power. Pooled mean power was significantly greater after anodal transcranial DCS to the motor cortex (tDCS) than after cathodal tDCS, although mean powers of anodal and sham tDCS were not significantly different. The increase in mean power after cathodal tsDCS could result from a reduction in central fatigue. This stimulus method might improve sprint performance.

The effects of exercise training using transcranial direct current stimulation (tDCS) on breathing in patients with chronic stroke patients

[Purpose] This study was conducted to investigate the effect of exercise training using transcranial direct current stimulation (tDCS) on breathing in patients with chronic stroke patients. [Subjects and Methods] Thirty chronic stroke patients who do not show abnormal response to electric stimulation were enrolled in this study and each 15 subjects were randomized either into the study group and the sham-controlled group. The subjects performed diaphragmatic breathing exercise for 20 minutes while tDCS device was attached to them (for study group, the device was on while for the sham-controlled group, the device was turned off 30 seconds later) [Results] The results of FVC, FEF1 and FEV1/FVC in the study group and those of FVC and FEV1 in the sham-controlled group were significantly increased after the breathing exercise. The independent comparison result between the groups showed that the breathing performance of study group significantly improved based on the results of FVC and FEV1. [Conclusion] In conclusion, the results of this study confirmed that breathing exercise effectively improved FVC and FEV1 in chronic stroke patients. Also, the breathing exercise using tDCS was more effective in improving FVC and FEV1.

Bilateral tDCS on Primary Motor Cortex: Effects on Fast Arm Reaching Tasks

Background

The effects produced by transcranial direct current stimulation (tDCS) applied to the motor system have been widely studied in the past, chiefly focused on primary motor cortex (M1) excitability. However, the effects on functional tasks are less well documented.

Objective

This study aims to evaluate the effect of tDCS-M1 on goal-oriented actions (i.e., arm-reaching movements; ARM), in a reaction-time protocol.

Methods

13 healthy subjects executed dominant ARM as fast as possible to one of two targets in front of them while surface EMG was recorded. Participants performed three different sessions. In each session they first executed ARM (Pre), then received tDCS, and finally executed Post, similar to Pre. Subjects received three different types of tDCS, one per session: In one session the anode was on right-M1 (AR), and the cathode on the left-M1 (CL), thus termed AR-CL; AL-CR reversed the montage; and Sham session was applied likewise. Real stimulation was 1mA-10min while subjects at rest. Three different variables and their coefficients of variation (CV) were analyzed: Premotor times (PMT), reaction-times (RT) and movement-times (MT).

Results

triceps-PMT were significantly increased at Post-Sham, suggesting fatigue. Results obtained with real tDCS were not different depending on the montage used, in both cases PMT were significantly reduced in all recorded muscles. RT and MT did not change for real or sham stimulation. RT-CV and PMT-CV were reduced after all stimulation protocols.

Conclusion

tDCS reduces premotor time and fatigability during the execution of fast motor tasks. Possible underlying mechanisms are discussed.

Recurrence quantification analysis of surface electromyogram supports alterations in motor unit recruitment strategies by anodal transcranial direct current stimulation

PURPOSE

Recent evidence indicates that anodal transcranial direct current stimulation (tDCS) can selectively alter the EMG/force relationship of agonist arm muscles; however, the mechanisms mediating those changes are less clear. The purpose of this study was to evaluate the effect of anodal tDCS on motor unit synchronization by using a sophisticated non-linear EMG analysis called recurrence quantification analysis (RQA).

METHODS

Surface EMG signals were collected from the biceps brachii muscle of eighteen healthy young adults (9 tDCS and 9 control) at various force levels (12.5%, 25%, 37.5%, and 50% maximum) before and after the application of anodal tDCS over the primary motor cortex. RQA was employed to quantify the changes in percentage of determinism (% DET) and laminarity (% LAM) of the surface EMG signals, which are surrogate measures of motor unit synchronization.

RESULTS

RQA analyses indicated that the changes in % DET and % LAM scores were significantly higher in the tDCS group than in the control group (p < 0.05) and this effect was particularly pronounced at higher force levels.

CONCLUSION

The results of this study provide novel evidence supporting that anodal tDCS significantly alters motor unit firing strategies (i.e., the degree of synchronization) of the biceps brachii muscle.

Effects of a common transcranial direct current stimulation (tDCS) protocol on motor evoked potentials found to be highly variable within individuals over 9 testing sessions

Transcranial direct current stimulation (tDCS) uses a weak electric current to modulate neuronal activity. A neurophysiologic outcome measure to demonstrate reliable tDCS modulation at the group level is transcranial magnetic stimulation engendered motor evoked potentials (MEPs). Here, we conduct a study testing the reliability of individual MEP response patterns following a common tDCS protocol. Fourteen participants (7m/7f) each underwent nine randomized sessions of 1 mA, 10 min tDCS (3 anode; 3 cathode; 3 sham) delivered using an M1/orbito-frontal electrode montage (sessions separated by an average of ~5.5 days). Fifteen MEPs were obtained prior to, immediately following and in 5 min intervals for 30 min following tDCS. TMS was delivered at 130 % resting motor threshold using neuronavigation to ensure consistent coil localization. A number of non-experimental variables were collected during each session. At the individual level, considerable variability was seen among different testing sessions. No participant demonstrated an excitatory response ≥20 % to all three anodal sessions, and no participant demonstrated an inhibitory response ≥20 % to all three cathodal sessions. Intra-class correlation revealed poor anodal and cathodal test-retest reliability [anode: ICC(2,1) = 0.062; cathode: ICC(2,1) = 0.055] and moderate sham test-retest reliability [ICC(2,1) = 0.433]. Results also revealed no significant effect of tDCS at the group level. Using this common protocol, we found the effects of tDCS on MEP amplitudes to be highly variable at the individual level. In addition, no significant effects of tDCS on MEP amplitude were found at the group level. Future studies should consider utilizing a more strict experimental protocol to potentially account for intra-individual response variations.

The effects of anodal transcranial direct current stimulation and patterned electrical stimulation on spinal inhibitory interneurons and motor function in patients with spinal cord injury

Supraspinal excitability and sensory input may play an important role for the modulation of spinal inhibitory interneurons and functional recovery among patients with incomplete spinal cord injury (SCI). Here, we investigated the effects of anodal transcranial direct current stimulation (tDCS) combined with patterned electrical stimulation (PES) on spinal inhibitory interneurons in patients with chronic incomplete SCI and in healthy individuals. Eleven patients with incomplete SCI and ten healthy adults participated in a single-masked, sham-controlled crossover study. PES involved stimulating the common peroneal nerve with a train of ten 100 Hz pulses every 2 s for 20 min. Anodal tDCS (1 mA) was simultaneously applied to the primary motor cortex that controls the tibialis anterior muscle. We measured reciprocal inhibition and presynaptic inhibition of a soleus H-reflex by stimulating the common peroneal nerve prior to tibial nerve stimulation, which elicits the H-reflex. The inhibition was assessed before, immediately after, 10 min after and 20 min after the stimulation. Compared with baseline, simultaneous application of anodal tDCS with PES significantly increased changes in disynaptic reciprocal inhibition and long-latency presynaptic inhibition in both healthy and SCI groups for at least 20 min after the stimulation (all, p < 0.001). In patients with incomplete SCI, anodal tDCS with PES significantly increased the number of ankle movements in 10 s at 20 min after the stimulation (p = 0.004). In conclusion, anodal tDCS combined with PES could induce spinal plasticity and improve ankle movement in patients with incomplete SCI.