New information on the effects of transcranial direct current stimulation on n-back task performance

The n-back task is prototypical tool widely used to evaluate working memory (WM) abilities in healthy and clinical populations. Previous studies finding beneficial effects of transcranial direct current stimulation (tDCS) over the dorsal lateral prefrontal cortex (DLPFC) on n-back task performance were limited by the number of n-back "memory loads" utilized and the assessment of performance only immediately following stimulation. Our aims were to investigate both the immediate and lasting effects of six sessions of bilateral tDCS over the DLPFC on n-back performance. We used a 1-, 2-, 3-, and 4-back shaped WM task at three time points: pre-stimulation (T1), immediately following a final (6th) stimulation (T2), and 1 month following the final stimulation (T3). Twenty-five right-handed participants were randomly assigned to active or sham stimulation. Performance was evaluated by percentage hits, false alarms, and reaction times (RTs) for correct responses. Results showed lack of improvement in all outcome measures for both groups at T2. Except for general faster RT in the active group, no lasting effect on percentage hits and false alarms was found for all memory loads among both groups at T3. However, lenient analysis indicated improvement in RT for the 1-back memory load among the active group from T1 to T3. These results question the previously found effectiveness of tDCS over the DLPFC in improving short-term n-back performance and cast doubt on its long-term effectiveness.