The main findings from this randomized, cross-over trial was that both unilateral and bilateral tDCS enhanced motor function of the non-dominant hand, which remained facilitated for 60 minutes post stimulation, and induced corticomotor plasticity for up to 60 minutes, but there were no differences in the responses between these two tDCS conditions. Furthermore, there was no association between the change in MEP amplitude and SICI, which are thought to reflect elements of corticomotor plasticity, and the magnitude of motor function for the peg board task in the non-dominant hand of healthy adults.
tDCS over M1 improves motor function of the non-dominant hand
Several previous studies have reported that tDCS, particularly unilateral stimulation (i.e. anode over non-dominant M1), can improve motor function [16, 17, 21, 32–35], but whether there is a differential effect between unilateral stimulation and bilateral stimulation improving motor function beyond 30 minutes of stimulation is not known. Several previous studies have shown that bilateral stimulation improves motor function immediately after stimulation when compared to unilateral and sham stimulation [13, 14], which has been attributed in part to reduction in IHI [14, 21]. In contrast to these findings, we found that there was a similar improvement in motor function following both unilateral and bilateral stimulation immediately after, which persisted for up to 60 minutes. This suggests that these two tDCS conditions do not differentially modulate motor function of the non-dominant hemisphere in healthy adults. In part support of these findings, several studies have reported that both unilateral and bilateral stimulation protocols can induce acute improvements in motor function in both upper and lower limb musculature of stroke patients [36–38] and in the elderly [12, 20].
The findings that motor function improved following both stimulation conditions is consistent with the results from previous studies, which reported gains in motor function between 9-11% [14, 17]. Interestingly, we observed that under both stimulation conditions, motor function was maintained at 30 minutes (11 and 6%) and at 60 minutes (19 and 10%) post stimulation. Furthermore, the time-course improvement of motor function was similar to the time-course effects on corticomotor excitability following tDCS (i.e. increased MEPs) (see 1 for a detailed review). Thus, motor function may have improved as a direct consequence of the effects of tDCS. For instance, an advantage of tDCS is that it modifies the transmembrane potential of neurons, which by default and in most cases, increases M1 excitability and primes a given motor area [14, 19, 21, 27]. Experimental data shows such an effect following unilateral and bilateral stimulation protocols in healthy adults , elderly  and stroke patients [37, 39]. However, the present findings show that motor function is not differentially modulated by the type of tDCS, indicating that the physiological mechanisms potentially regulating motor function are not different. In light of this, we cannot exclude the potential role of other cortical regions, such as the basal ganglia, somatosensory cortex and spinal cord underpinning the improvement in motor function.
Change in corticomotor excitability following unilateral and bilateral tDCS
An increase in MEP amplitude of the target muscle following unilateral and bilateral tDCS is thought to reflect cortical elements of plasticity  via intrinsic changes in excitability of corticospinal cells [41, 42]. A change in MEP amplitude that remains elevated for up to 60 minutes has been reported and confirmed by mathematical models, that show tDCS can modify the transmembrane potential [43, 44], which influences the excitability of individual neurons.
We hypothesized that unilateral and bilateral stimulation would differentially induce corticomotor excitability, with bilateral stimulation increasing MEP responses compared to unilateral stimulation. Given that unilateral and bilateral stimulation did not differentially induce corticomotor excitability at any time period shows that the same cortical circuits were stimulated to a similar magnitude and that the mechanisms involved in the after-effects for both stimulation conditions were similar. These findings are consistent with a previous unilateral stimulation study, whereby the physiological mechanisms appear to evolve during stimulation that most likely involves modification of the transmembrane potential .
There are several reports to show that corticomotor excitability is facilitated for up to 60 and even 90 minutes following unilateral stimulation , however there is only one report on the after-effects of bilateral stimulation . Our findings are in agreement with Mordillo-Mateos et al. ; however there are some important differences. First, they reported that MEP amplitudes returned to baseline by 20 minutes, whereas we have shown that unilateral and bilateral stimulation increases corticomotor excitability that remains elevated even at 60 minutes post stimulation. Second, we applied tDCS for 13 minutes in all conditions compared to only 5 minutes in their study. A series of studies examining the effects of different durations of unilateral stimulation on corticomotor excitability indicated a linear relationship between the duration of application and the increase in the duration of the after-effects [1, 4, 6, 7, 45–49]. Our results confirm these previous findings for unilateral stimulation; however, corticomotor excitability remained elevated at 60 minutes post bilateral stimulation, which shows that bilateral stimulation facilitates corticomotor excitability beyond 20 minutes and that the same after-effects occur as those for unilateral stimulation.
The mechanisms inducing the after-effects of unilateral stimulation are well described , however, the mechanism for modulating corticomotor excitability following bilateral stimulation remains uncertain. Given that IHI was not measured in the current study, the induction of corticomotor plasticity via bilateral stimulation may be related to the differences in the induced cortical electrical field current density. Certainly, current flow direction is different when using a motor cortex-contralateral supraorbital arrangement compared to motor cortex-motor cortex arrangement . Recent computational data suggests that the spatial focality of induced cortical electrical field current densities is greater when the active (anode) and return (cathode) electrodes are closer together. For example Faria et al.  demonstrated that decreasing the inter-electrode distance resulted in increased current density on the scalp under the edges of the electrode. It’s likely that the reduced inter-electrode distance in the bilateral stimulation configuration has increased the effectiveness of the anodal “target” electrode over the M1 hot spot for the non-dominant ECRL muscle. Therefore, the induction of corticomotor plasticity following bilateral stimulation is likely due to increased focality.
Although only speculative, it’s possible that the cathode decreased corticomotor excitability in the dominant M1 which reduced the inhibitory inputs onto the homologous non-dominant M1, which has subsequently increased corticomotor excitability by increasing the net effect of the anodal stimulation [13, 14, 21]. One caveat to this interpretation is that we did not measure MEPs for the dominant hemisphere.
Changes in intracortical inhibition following unilateral and bilateral tDCS
Adjustments in SICI have been reported to be critical in the selective activation of muscles, particularly hand muscles, suggesting that intracortical inhibition is important for motor function [51, 52]. No studies have compared the after-effects of unilateral and bilateral tDCS on modulating SICI. Similar to corticomotor excitability, unilateral and bilateral tDCS did not differentially induce SICI. Despite this, SICI was reduced for 30 minutes post stimulation, during both stimulation conditions it returned to baseline by 60 minutes. These findings are consistent with previous studies [7, 11], whereby SICI was reduced following unilateral stimulation. However, in contrast to our original hypothesis that bilateral stimulation would reduce SICI further compared to unilateral stimulation, there was no interaction between the type of stimulation and the modulation of GABAergic inhibition. While Nitsche et al.,  reported that the after-effects of SICI were predominantly induced by unilateral stimulation; a previous study reported no significant difference in SICI with bilateral stimulation .
The reduction in intracortical inhibition (for up to 30 minutes) could have contributed to the task-dependent MEP facilitation by unmasking existing excitatory connections to corticospinal neurons activated by TMS [53–55] and/or by enhancing synaptic plasticity at a cortical level [40, 41, 56]. Taken together, these results demonstrate a noticeable involvement of intracortical synaptic mechanisms that modulate indices of corticomotor plasticity.
We specifically investigated SICI as several plasticity inducing interventions, such as motor skill training, show that reduced SICI is an important mechanism for optimal motor skill learning and for inducing corticomotor plasticity . Additionally, tDCS studies have been advocated to act as a potential primer for improving motor function in healthy adults [16–18, 34] and stroke patients [11, 16, 36, 38]. However, the contribution of SICI to improved motor function remains unresolved.
Association between corticomotor plasticity and motor function from tDCS
There is a widespread belief that increased corticomotor excitability and reduced ICI are associated with the magnitude of behavioral improvement . However, in agreement with several published studies [23, 59], we found no association between the changes in corticomotor plasticity (increased MEP amplitude and reduced SICI) and improved motor function (pegboard task). Consistent with these findings, Rogaschet al.,  reported that increased MEP amplitude and reduced SICI were not associated with the degree of motor learning during a simple index finger motor task in young adults. Interestingly, Williams et al.,  reported a strong association between changes in MEP amplitude and motor performance, but no association between SICI and motor performance. These mixed findings between indices of corticomotor plasticity and motor function are likely to be the result of many factors that are known to influence the plasticity response, such as the extent of skilled hand use, prior history of synaptic activity, genetic factors (e.g. brain derived neurotrophic factor gene) , and gender, with the potential effect of the menstrual cycle . Nonetheless, motor function still improved following unilateral and bilateral stimulation, which provides further evidence that tDCS may be viable rehabilitation tool following neuromuscular injury or disease.
There are several limitations associated with this study. Indices of corticomotor plasticity were only measured from the non-dominant hemisphere, therefore, it is unclear whether cathodal stimulation over the dominant hemisphere would result in reduced corticomotor excitability as previous studies have suggested [1, 9]. However, there is evidence that bilateral stimulation can improve motor function of the non-dominant hand, with the reported mechanisms related to modifications in IHI following bilateral stimulation, without any TMS measures. Also, intracortical facilitation (ICF) was not measured, and as such the changes in SICI may have been as a result of increased ICF. Indeed, some evidence shows increased ICF following tDCS . Finally, it could be suggested that measuring MEPs and SICI from the ECRL muscle may lack specificity to the pegboard task that involves intrinsic muscles of the hand. However, the performance of the pegboard task requires the coordinated control and activation of the muscles of the entire arm, and not just the intrinsic muscles of the hand and fingers. Further, there are several kinematic phases involved, with the transportation of the pegs to be placed in the well, governed largely by the muscles proximal to the hand, and as such we don’t believe that recording from the ECR was a distinct limitation.