Tecchio et al. (2010) employed AtDCS to upregulate M1 activity after practice to enhance consolidation of the practiced implicit 5-FU sequence. This post-practice application of AtDCS may have specifically enhanced consolidation processes and improved offline learning. Nevertheless, our findings support the previously reported role of M1 in offline memory stabilization (Kantak et al., 2010; Kang & Paik, 2011). To our knowledge, our study is the first to investigate the effects of AtDCS applied over PMd during practice on performance and learning of an implicit SRTT sequence. Contrary to our hypothesis,
AtDCS applied over PMd did benefit motor performance during practice and at EoA compared with sham stimulation. Although not statistically significant, the effect size was high, indicating that the effect was likely to be real and may have reached significance with a larger sample size. There may be multiple mechanisms that may implement this effect. Although
we used a smaller anode than those previously used, evidence exists that AtDCS applied over PMd is known to increase the excitability within the M1 via corticocortical connections (Boros et al., 2008). Although it is not clear how explicit and implicit systems interact during practice at a neural substrate level, the behavioral evidence for the effect of explicit knowledge on implicit motor performance is also mixed. Although PMd is thought to be predominantly a part of the explicit memory system, there is evidence that it may be engaged during early performance of any sequence learning task that links the visuospatial Bortezomib cues to compatible responses, an important
characteristic of our task (Grafton et al., 1998, 2002; van der Graaf et al., 2006). Our findings are different from those observed by Boyd & Linsdell (2009) who observed that enhancing PMd excitability during the immediate post-practice period led to better offline learning of a continuous tracking task. In our study, we applied AtDCS during practice of the implicit sequence task, therefore not directly affecting the motor memory consolidation phase. It is likely that AtDCS over PMd during practice led to a motor memory representation that did not through demonstrate offline stabilization. Although somewhat beneficial to online practice performance of the implicit motor sequence learning task, AtDCS over PMd attenuated offline stabilization of the implicit motor sequence compared with sham and M1 AtDCS. This emphasizes the well-known performance–learning distinction which suggests that factors that enhance practice performance may not always enhance retention of motor skills (Kantak & Winstein, 2011). Even after practice ends, functional properties and representation of the skill continue to evolve in the brain and help stabilize motor performance over the retention interval (online learning).