Intensity-Dependent Effects of 4-week Transcranial Direct Current Stimulation on Motor Learning in Healthy Young Adults

The present study aimed to investigate the effects of different intensities of tDCS-specifically 1 mA, 2 mA, and 3 mA-on motor learning in healthy adults. Both the acute effects following a single tDCS session and the cumulative effects after a 4-week intervention (10 sessions) were evaluated. Additionally, the study aimed to determine whether improvements in motor learning induced by 4 weeks of repeated tDCS would be sustained one week after completion of the intervention. We hypothesized that anodal tDCS at 2 mA would produce the most substantial improvement in motor sequence learning, both following a single session and after a 4-week intervention, compared to 1 mA, 3 mA, and sham stimulation. Furthermore, we expected that the motor learning gains observed in the 2 mA group would be sustained one week post-intervention, indicating the persistence of tDCS-induced effects. In contrast, we anticipated minimal improvement in the 1 mA group due to insufficient stimulation intensity, and no additional or even reduced benefits in the 3 mA group compared to the 2 mA group.

• 3 mA transcranial direct current stimulation (Device): The intervention will be conducted using a battery-powered stimulator (TCS-E2000, Shenzhen Yingchi Technology Co., Ltd) through a pair of saline-soaked sponge electrodes (5 cm × 7 cm).The stimulation intensity will be set at 3 mA. The stimulation will last for 20 minutes each time, with a 10 s fade-in period at the start and a 10-s fade-out period at the end of stimulation. The participants will receive intervention lasting for four weeks(10 sessions).The anodal electrode will be placed over the primary motor cortex (M1), and the reference electrode will be positioned at the center of the forehead.
• 2 mA transcranial direct current stimulation (Device): The intervention will be conducted using a battery-powered stimulator (TCS-E2000, Shenzhen Yingchi Technology Co., Ltd) through a pair of saline-soaked sponge electrodes (5 cm × 7 cm).The stimulation intensity will be set at 2 mA. The stimulation will last for 20 minutes each time, with a 10 s fade-in period at the start and a 10-s fade-out period at the end of stimulation. The participants will receive intervention lasting for four weeks(10 sessions).The anodal electrode will be placed over the primary motor cortex (M1), and the reference electrode will be positioned at the center of the forehead.
• 1 mA transcranial direct current stimulation (Device): The intervention will be conducted using a battery-powered stimulator (TCS-E2000, Shenzhen Yingchi Technology Co., Ltd) through a pair of saline-soaked sponge electrodes (5 cm × 7 cm).The stimulation intensity will be set at 1 mA. The stimulation will last for 20 minutes each time, with a 10 s fade-in period at the start and a 10-s fade-out period at the end of stimulation. The participants will receive intervention lasting for four weeks(10 sessions).The anodal electrode will be placed over the primary motor cortex (M1), and the reference electrode will be positioned at the center of the forehead.
• sham transcranial direct current stimulation (Device): The intervention will be conducted using a battery-powered stimulator (TCS-E2000, Shenzhen Yingchi Technology Co., Ltd) through a pair of saline-soaked sponge electrodes (5 cm × 7 cm).The intervention will last for 20 minutes each time, with a 10 s fade-in period at the start and a 10-s fade-out period at the end of stimulation，but there is not current in between. The participants will receive intervention lasting for four weeks(10 sessions).The anodal electrode will be placed over the primary motor cortex (M1), and the reference electrode will be positioned at the center of the forehead.

Primary outcomes

• Sequence Reaction Time Task [Assessments will conducted before the intervention (Baseline), immediately after the first intervention (T1), at the end of 4 weeks (T2), and 1 week after the end of intervention (T3).]: In this task, a white circle appears above one of four horizontally arranged white squares on the computer screen with black background. Participans were instructed to press the proper button on the response pad (buttons H, J, K, L) using the corresponding finger of the right hand (index finger for button H, middle finger for button J, ring finger for button K, and small finger for button L) when the white circle appeared at the left side, middle-left side, middle-right side, or right side of the computer screen, respectively. Each session consists of eight blocks, with each block comprising 120 trials.

Sex: All, ages: 18 Years to 25 Years, accepts healthy volunteers

Inclusion Criteria:

1. age between 18 and 25 years; (2) normal or corrected-to-normal vision; (3) no history of pregnancy, cardiovascular disease, other physical impairments or illnesses, and no personal or family history of mental illness; (4) no implantation of any internal or external medical devices; and (5) right-handedness.

Exclusion Criteria:

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