Theta-frequency tACS selectively enhances early-phase motor learning through cerebellar modulation.

The cerebellum plays a crucial role in motor learning, facilitating processes such as timing, error correction, and coordination. However, optimizing noninvasive brain stimulation (NIBS) to enhance these processes remains challenging. This study investigated the effects of cerebellar transcranial alternating current stimulation (tACS) at 5 Hz and 50 Hz on motor learning during a serial reaction time task (SRTT). Twenty-six healthy participants completed three sessions, receiving 5 Hz, 50 Hz, or Sham stimulation during SRTT performance. Changes in reaction time and sequence performance were measured during the online stimulation phase, with motor retention assessed 24 h later. We found that 5 Hz tACS significantly improved motor performance during the early stages of sequence learning, as demonstrated by faster reaction times compared to the 50 Hz and Sham conditions. These effects, specific to early acquisition phases, align with the cerebellum's involvement in motor timing and error correction. No significant improvements were observed during offline motor retention, possibly due to the weaker entrainment or lack of prolonged sessions required for long-term plasticity. Furthermore, 50 Hz tACS did not influence SRTT performance, highlighting the frequency-specific nature of tACS-induced modulation. These findings suggest that theta-frequency tACS can selectively enhance cerebellar contributions to motor learning by aligning stimulation with intrinsic oscillations. Although transient, theta-tACS shows promise for modulating motor circuits in both research and clinical contexts. Future studies should investigate theta-tACS in more complex tasks and explore its therapeutic potential for sustained motor rehabilitation outcomes. This study highlights the potential of 5-Hz theta-frequency cerebellar transcranial alternating current stimulation (tACS) to enhance early motor learning. During a serial reaction time task, 5-Hz tACS significantly improved reaction times compared with 50 Hz and Sham conditions, aligning with the cerebellum's role in motor timing and error correction. Though effects were transient, these findings underscore the frequency-specific benefits of tACS and its promise for advancing motor learning research and therapeutic applications.