Age-dependent Effects of Titrating Anodal Transcranial Direct Current Stimulation (tDCS) Intensity on Motor Sequence Learning

Optimising the currently heterogenous efficacy of transcranial direct current stimulation (tDCS) interventions for motor learning requires identifying stimulation parameters facilitating performance while accounting for age-related differences in baseline performance and mechanisms underlying neuroplasticity. We systematically explored anodal tDCS intensity effects on implicit motor sequence learning (IMSL) in young and older adults. The study utilised a randomised, double-blind, counterbalanced crossover design. Ninety-six healthy participants (48 young adults, 48 older adults) completed a serial reaction time task (SRTT) with online sham or anodal tDCS over M1 at intensities of 1, 2, and 3 mA. The next day, memory consolidation was assessed in a recall test. Both age groups demonstrated IMSL and consolidation across conditions. While 1 mA tDCS improved IMSL in young adults by reducing reaction times, higher intensities had no significant benefit compared to sham. In older adults, anodal tDCS did not affect general task performance compared to sham, but 1mA tDCS acutely impaired selective sequence learning. The results demonstrate age-dependent and non-linear dose-dependent effects of anodal tDCS on IMSL. This underscores the necessity for age-adapted protocols for experimental and clinical tDCS applications. Future research should explore neurophysiological reasons for reduced tDCS efficacy in older adults found in the present study.