For the seemingly effortless
acquisition of new motor skills, a distributed network of cortical and subcortical structures is required. While a specific significance of the primary motor cortex (M1) for implicit motor sequence learning has been highlighted, the premotor cortex (PMC) has been related to consolidation of the newly learned sequence. To gain more insight into the contribution of the PMC to implicit sequence learning and early consolidation transcranial direct current stimulation (tDCS) was applied to the left PMC. In total 36 healthy right-handed volunteers (age: 24.0 ± 0.4 years, 18 male) participated in the study. Half of them received anodal, cathodal or sham tDCS prior to motor learning (experiment 1) while the other half received tDCS during learning (experiment 2). Stimulation duration was ten minutes, respectively. To induce implicit motor sequence learning a serial reaction time task (SRTT) with a repetitive fixed sequence
interleaved by a random pattern was employed. Reaction times (RT) were determined separately for random and sequential trials at baseline (t1), end of
acquisition (t2), and after presentation of an interfering random pattern (t3) in order to estimate consolidation of the trained sequence.
In both experiments, at t2, facilitation of RT during sequential as compared to random trials was found in the sham condition indicating sequence learning. For active tDCS at t2 RT did not significantly differ between random and sequential trials suggesting a non-specific facilitation of RT. At t3 cathodal tDCS applied prior to SRTT yielded significantly faster RT in sequential as compared to random trials suggesting reduced susceptibility to interference. The present data support the hypothesis that the left PMC is not involved in the acquisition of an implicitly learned motor sequence. Interestingly enough, cathodal tDCS applied prior to learning resulted in reduced susceptibility to an interfering pattern revealing evidence for the assumption that suppression of left PMC excitability is beneficial for early consolidation of an implicit motor sequence.
