Introduction: Transcranial direct current stimulation (tDCS) is a noninvasive electrical brain stimulation technique that has been used extensively over prefrontal cortex in an effort to enhance verbal working memory (WM). However, inconsistent and contradictory outcomes from similar stimulation protocols have created a strong need to examine methodologies in greater detail. This review undertook an in-depth look at both positive and negative prefrontal tDCS-WM findings in adult populations to shed light on methodological parameters that may be driving the inconsistent outcomes in the literature.
Method: To facilitate comprehension of the protocols employed in each study and aid between-study comparisons, we illustrated study design alongside key findings. To aid clinical translation, we reviewed separately by population (healthy young adults versus WM impaired) performance changes during stimulation (online) versus following stimulation (offline), which offers more therapeutic promise.
Results: Our dissection of the literature highlighted design factors that are likely adding unnecessary noise and obscuring outcomes. Of note, a major blind spot in the literature relates to cognitive factors, including influential characteristics pertaining to the details of the WM test used to assess tDCS effects and participant characteristics that influence WM abilities and the organization of WM in the brain, and can impact the efficacy of tDCS-WM protocols through complex interactions.
Conclusions: By attending to both cognitive- and tDCS-related factors in the design phase of the study, future researchers can reduce unintended variation that may obscure positive outcomes or lead to spurious results, thereby advancing the field forward toward developing more effective tDCS-WM protocols.
Keywords: Clinical populations; cognitive therapy; electrical brain stimulation; healthy adults; prefrontal cortex.