Offline beats online: transcranial direct current stimulation timing influences on working memory.

The n-back task is an established measure of an individual's working memory. In this task, participants have to continuously update their working memory to react to a stimulus correctly. For the verbal n-back task in particular, the left dorsolateral prefrontal cortex (lDLPFC) plays a key role in working memory updating and a higher activation of the lDLPFC has been linked to an increase in performance. Transcranial direct current stimulation (tDCS) has been used extensively to investigate its effect on working memory but it remains unclear whether online or offline tDCS produces the largest modulation of task performance. To tackle this question, we used anodal tDCS to stimulate the lDLPFC in a repeated measures design and compared offline, online and sham stimulation. A 9 cm anode was always placed over the lDLPFC while the 35 cm cathode was positioned over the left deltoid. In a sample of N = 63 healthy adults, we found a significantly larger performance improvement, in correct reaction times as well as accuracy rates, after offline tDCS compared to online or sham stimulation. This pattern of results provides evidence that the stimulation timing plays a crucial role in tDCS application and it confirms the role of the lDLPFC in verbal working memory performance.