Comparing effects of 2 mA M1 a-tDCS and c-tDCS
In this study, it was also hypothesized that compared to 2 mA M1 c-tDCS, 2 mA M1 a-tDCS had more efficacy on motor learning. The findings of the current study confirmed this hypothesis and indicated although 2 mA M1 c-tDCS induced online and offline learning, more significant effects of 2 mA M1 a-tDCS as compared to 2 mA M1 c-tDCS were observed during online motor learning. However, lasting motor learning effects of 2 mA M1 c-tDCS were similar to 2 mA M1 a-tDCS. In the study conducted by Shilo et al. (2019), a comparison between M1 a-tDCS and c-tDCS with a current intensity of 2 mA during an SRTT task was performed (Shilo & Lavidor, 2019). According to the results of this study, during the initial stage of the SRTT task, execution speed was faster with anodal stimulation, while during the later stages, execution speed was faster with cathodal stimulation (Shilo & Lavidor, 2019). It appears that M1 c-tDCS with 2 mA current intensity leads to an increase in the intensity of motor-evoked potentials (MEP) and then inducing lasting motor learning (Batsikadze et al. , 2013). Accordingly, it seems that 2 mA M1 c-tDCS has a similar effect to 2 mA M1 a-tDCS, resulting in increased motor learning. In this regards, Bogaard et al. (2019) indicted that M1 c-tDCS with higher current intensity than 1 mA has a similar effect to a-tDCS, increasing the excitability of the targeted area (Bogaardet al. , 2019). Furthermore, Shilo et al. (2019) found that applying 2 mA M1 c-tDCS decreased the neural excitability for the first 13 minutes from 20 minute stimulation session, while the neural excitability increased and ultimately motor learning improved in final 7 minute of intervention (Shilo & Lavidor, 2019). In a study by Greeley et al. (2020), it was observed that applying both 2 mA M1 c-tDCS and a-tDCS improved long-term offline motor learning in individuals (Greeleyet al. , 2020). This study suggests that c-tDCS and a-tDCS do not always have opposite effects and can have similar mechanisms under certain conditions (Greeley et al. , 2020). In a study conducted by Hsu et al., c- and a-tDCS with an intensity of 4 mA compared with the sham stimulation on motor learning. It was shown that a-tDCS compared to c-tDCS and sham stimulation improved significantly motor learning (Hsuet al. , 2023). Leow et al. compared the effects of the synergism of levodopa and a-tDCS with different intensities of 1,2 and 4 mA on motor learning and indicated that the application of levodopa concurrent with 4 mA a-tDCS causes a decrease, while concurrent with 1 and 2 mA a-tDCS causes an increase in motor learning compared to sham stimulation (Leow et al. , 2023). The current study also indicated the lasting effects of both M1 a-tDCS (1& 2 mA) and c-tDCS (2 mA) on motor learning in healthy young participants.
It seems that 2 mA c-tDCS generates excitatory after-effects by modulating the excitability of the motor cortex (Batsikadze et al. , 2013). This effect is achieved by inducing hyperpolarization in neurons (Batsikadze et al. , 2013). Additionally, the dopaminergic system plays a vital role in modulating the enduring effects of c-tDCS (Nitsche et al. , 2006). Research has highlighted the significance of dopamine in prolonging the excitability-diminishing effects of c-tDCS for up to 24 hours after stimulation, suggesting that tDCS may hold therapeutic potential for conditions characterized by increased cortical excitability (Nitsche et al. , 2006). These findings emphasize the complex relationship between neurotransmitter systems and the neuroplasticity induced by transcranial direct current stimulation (tDCS) (Nitsche et al. , 2006).
One of the limitations of the current study was that only the young healthy adults participated in this study, which limit the generalizability of these findings to this group. Conducting future studies to assess the efficacy of different intensities of a-tDCS and c-tDCS on motor learning in the other aging individuals is suggested. Another limitation in the current study was the absence of evaluation of neural activity both during and after interventions, which hinders the ability to elucidate the mechanisms underlying cortical activity changes following tDCS intervention. It is recommended to conduct a study that evaluates the excitability of the M1 by employing TMS-EEG techniques after the application of a-tDCS and c-tDCS at varying intensities.