Introduction:
Engaging in repetitive performance practice has been associated with the enhancement of neuro-motor adaptations, functional abilities, and overall performance improvement, as well as motor learning (Jacksonet al. , 2019; Meek et al. , 2021; Parma et al. , 2021). Enhancing motor learning to facilitate skill acquisition and memory consolidation represents a current challenge in both skill acquisition and functional rehabilitation domains (Debarnot et al. , 2019).
Serial Reaction Time Tasks (SRTT) are widely utilized for evaluating implicit sequence motor learning (Trofimova et al. , 2020). This involves the observation of repeated sequences or random events during task performance and is recognized as the most commonly used test for this purpose (Debarnot et al. , 2019). Research has indicated neuroplastic changes during implicit motor learning across various brain regions in young, healthy adults (Debarnot et al. , 2019). Presently, non-invasive brain stimulation (NIBS) techniques are employed to modulate motor and cognitive functions, aiding the learning process in both healthy adults and individuals with neurological conditions (Buch et al. , 2017; Lefebvre et al. , 2017; Cole et al. , 2018; Santos et al. , 2020)
Research indicates that tDCS has the capacity to impact resting membrane potential, potentially leading to an increase or decrease in its levels (Nitsche & Paulus, 2000; Nitsche et al. , 2008). The direction of this influence depends on whether anodal or cathodal tDCS is administered (Lefebvre et al. , 2017; Cole et al. , 2018; Santos et al. , 2020). Notably, research indicates that combining M1 anodal tDCS with motor training can lead to improved motor performance and learning compared to cathodal tDCS and sham stimulation (Stagg et al. , 2009; Ostry & Gribble, 2016; Karok et al. , 2017; Spampinato & Celnik, 2018; Debarnot et al. , 2019; Talimkhani et al. , 2019; Iannone et al. , 2022). Conversely, a study found that applying cerebellar a-tDCS reduced motor learning during SRTT (Jongkees et al. , 2019). Some studies have demonstrated that c-tDCS can enhance motor learning in SRTT (Greeleyet al. , 2020; Pollok et al. , 2021). Furthermore, certain studies have highlighted the positive effects of both a-tDCS and c-tDCS over M1 compared to sham tDCS on motor learning (Ciechanski & Kirton, 2017; Shilo & Lavidor, 2019).
Evidence suggests that the intensity of tDCS, regardless of polarity, can significantly impact the outcomes of tDCS (Stagg et al. , 2011; Ciechanski & Kirton, 2017; Greeley et al. , 2020). Some studies have shown that both 1 mA and 2 mA tDCS can improve motor learning compared to sham conditions in healthy adults (Stagg et al. , 2009; Ciechanski & Kirton, 2017; Shilo & Lavidor, 2019). Conversely, other studies have indicated that 1 mA or 2 mA tDCS may interfere with the motor learning process compared to sham stimulation (Stagg et al. , 2011; Greeley et al. , 2020).
It appears that previous studies have presented conflicting findings regarding the efficacy of tDCS with different polarities and intensities on motor learning (Stagg et al. , 2011; Ciechanski & Kirton, 2017; Buchwald et al. , 2019; Shilo & Lavidor, 2019). However, it is essential to determine the most effective approach to optimize the use of tDCS for enhancing motor learning. Interestingly, there is a lack of research comparing the effects of different polarities and intensities of tDCS on motor learning in healthy young adults. This study aims to explore the impacts of tDCS on M1 at varying intensities and polarities on motor learning in healthy young adults using the SRTT. The hypothesis for this study is:
- Concurrent application of 1 and 2 mA M1 a-tDCS and SRTT would reduce RT and ER during and after application of interventions compared to concurrent sham a-tDCS and SRTT.
- Concurrent application of 1 and 2 mA M1 c-tDCS and SRTT would have no effect on reduction of RT and ER during and after completion of interventions compared to sham tDCS and SRTT.
- Concurrent application of 1 mA M1 a-tDCS and SRTT would reduce RT and ER during and after completion of interventions compared to 1 mA M1 c-tDCS and SRTT.
- Concurrent application of 2 mA M1 a-tDCS and SRTT would reduce RT and ER during and after completion of interventions compared to 2 mA M1 c-tDCS and SRTT.