A Systematic Review and Meta-Analysis of the Effects of Transcranial Direct Current Stimulation (tDCS) Over the Dorsolateral Prefrontal Cortex in Healthy and Neuropsychiatric Samples: Influence of Stimulation Parameters

- We meta-analyzed 233 within-subject trials investigating the effect of single-session DLPFC tDCS on cognitive outcomes.
- No main effects of c-tDCS on RT nor response accuracy were found.
- No effects of stimulation parameters on c-tDCS effects on cognition were found.
- a-tDCS significantly decreased RT in healthy participants, and increased response accuracy in neuropsychiatric patients.
- In healthy participants, increased current density/charge are associated to increased a-tDCS effects on response accuracy.
- In neuropsychiatric patients, online task performance is associated to increased a-tDCS effects on response accuracy.

BackgroundResearch into the effects of transcranial direct current stimulation of the dorsolateral prefrontal cortex on cognitive functioning is increasing rapidly. However, methodological heterogeneity in prefrontal tDCS research is also increasing, particularly in technical stimulation parameters that might influence tDCS effects.ObjectiveTo systematically examine the influence of technical stimulation parameters on DLPFC-tDCS effects.MethodsWe performed a systematic review and meta-analysis of tDCS studies targeting the DLPFC published from the first data available to February 2016. Only single-session, sham-controlled, within-subject studies reporting the effects of tDCS on cognition in healthy controls and neuropsychiatric patients were included.ResultsEvaluation of 61 studies showed that after single-session a-tDCS, but not c-tDCS, participants responded faster and more accurately on cognitive tasks. Sub-analyses specified that following a-tDCS, healthy subjects responded faster, while neuropsychiatric patients responded more accurately. Importantly, different stimulation parameters affected a-tDCS effects, but not c-tDCS effects, on accuracy in healthy samples vs. patients: increased current density and density charge resulted in improved accuracy in healthy samples, most prominently in females; for neuropsychiatric patients, task performance during a-tDCS resulted in stronger increases in accuracy rates compared to task performance following a-tDCS.ConclusionsHealthy participants respond faster, but not more accurate on cognitive tasks after a-tDCS. However, increasing the current density and/or charge might be able to enhance response accuracy, particularly in females. In contrast, online task performance leads to greater increases in response accuracy than offline task performance in neuropsychiatric patients. Possible implications and practical recommendations are discussed.