Transcranial direct current stimulation enhances retention of a second (but not first) order conditional visuo-motor sequence

This study examined the role of the left inferior frontal gyrus in the implicit learning and retention of a 'simple' first order conditional (FOC) sequence and a relatively 'complex' second order conditional (SOC) sequence, using anodal transcranial direct current stimulation (a-tDCS). Groups of healthy adults received either a-tDCS (n = 18) over the left inferior frontal gyrus or sham/placebo (n = 18) stimulation. On separate days, participants completed a serial reaction time (SRT) task whilst receiving stimulation. On one of the days, participants were presented with a FOC sequence and in another, a SOC sequence. Both the learning and short-term retention of the sequences were measured. Results showed a-tDCS enhanced the short-term retention of the SOC sequence but not the FOC sequence. There was no effect of a-tDCS on the learning of either FOC or SOC sequences. The results provide evidence of prefrontal involvement in the retention of a motor sequence. However, its role appears to be influenced by the complexity of the sequence's structure. Additionally, the results show a-tDCS can enhance retention of an implicitly learnt motor sequence.