No effect of anodal high-definition transcranial direct current stimulation during motor sequence learning in older people.

Transcranial direct current stimulation (tDCS) targeting the primary motor cortex (M1) has previously been shown to enhance motor learning in older adults. However, findings across studies are inconsistent, which may be partly due to variability in stimulation parameters and montages across experimental protocols. We aimed to replicate the effect of 1 mA conventional anodal tDCS over the M1 hotspot during one session of motor sequence learning on retention (24 h) compared to sham stimulation in older adults. In addition, we aimed to compare the effect of high definition (HD tDCS) versus conventional tDCS on motor sequence learning to explore if stimulation montage and according focality affects the effect sizes found. In a pre-registered, double-blind, randomized, sham-controlled, parallel study involving 52 older adults, we investigated the effects of conventional and HD tDCS on motor sequence learning using a serial reaction time task (SRTT). We employed a hybrid design, whereby the primary contrast (active stimulation vs. sham) was assessed using the parallel study framework, while a crossover design was embedded to compare the stimulation montage. Although all groups showed motor sequence learning over time, both during practice sessions (main effect of time: p < 0.0001), as well as across session (main effect of time: p < 0.0001), we observed no significant effects on learning between stimulation groups (main effect of stimulation: p = 0. 68) or montages (main effect of montage: p = 0. 66). For all groups, motor sequence learning improvements were maintained but not further enhanced after 24 h of consolidation (HD tDCS: p = 0.64; conventional tDCS: p = 0.76; HD sham: p = 0.69; conventional sham: p = 0.57). Our findings indicate that neither conventional nor HD tDCS enhanced motor sequence learning in older adults. To better understand potential long-term or cumulative effects, we recommend that future studies investigate the effects of repeated tDCS sessions administered throughout the motor learning process.