Repeated beta transcranial alternating current stimulation disrupts consolidation and retention of motor sequence learning

Introduction: Implicit motor sequence learning (IMSL) enables unconscious acquisition of everyday motor skills and involves stage-specific neural dynamics. Beta-band (13–30 Hz) activity is believed to support consolidation, yet its causal role across IMSL stages remains unclear. Methods: We conducted the first study examining the effects of repeated 20 Hz transcranial alternating current stimulation (tACS) over the motor cortex on IMSL acquisition, consolidation and retention. Using a double-blind, sham-controlled, between-subjects design, 80 healthy young adults received active or sham tACS during a serial reaction time task (SRTT) across three consecutive days (Sessions 1–3), with retention assessed one week later (Follow-Up). Results: Mixed-effects models revealed sequence-specific learning in both groups, across sessions (all p's < .0001). Active tACS tended to enhance learning in Session 1 (M = 54.62 ms) vs. sham (M = 41.89 ms), p = .063, but significantly impaired learning in Session 2 (active M = 75.97 ms, sham M = 85.95 ms, p = .003), Session 3 (active M = 95.89 ms, sham M = 113.14 ms, p < .0001), and at Follow-Up (active M = 91.79 ms, sham M = 117.21 ms, p < .0001). Conclusion: Repeated beta-tACS tended to improve acquisition, but impaired consolidation and retention. This stage-specific disruption – preserved early learning followed by later deficits – supports a dynamic role for beta oscillations: early desynchronization facilitates flexible encoding, while later synchronization stabilizes learned patterns. Recovery during random blocks suggests transient disruption of motor flexibility followed by compensatory adaptation, rather than premature overgeneralization of sequence knowledge.