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

Abstract

Background:Research 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.Objective: To systematically examine the influence of technical stimulation parameters on DLPFC-tDCS effects. Methods:We 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. Results:Evaluation 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 accurate. 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. Conclusions:Healthy 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.

Stimulation of Dorsolateral Prefrontal Cortex Enhances Adaptive Cognitive Control: A High-Definition Transcranial Direct Current Stimulation Study

“…One hundred twenty healthy right-handed participants (59 men, 61 women; mean Ϯ SD years, 26.02 Ϯ 4.69) were recruited for this study and assigned to four stimulation conditions: left and right DLPFC and M1 HD-tDCS (N ϭ 30 per condition). Each group underwent both active (i.e., excitatory "anodal" HD-tDCS, which is typically used to enhance performance; Coffman et al, 2014;Dedoncker et al, 2016) and sham HD-tDCS in a crossover, double-blind design while completing a visual flanker task to assess potential stimulation effects on behavioral performance. Potential adverse effects and blinding efficacy were also assessed in a systematic fashion.…”

Individual Differences and Long-term Consequences of tDCS-augmented Cognitive Training

“…Several studies have demonstrated selective tDCS benefits among individuals with low, but not high, baseline WM abilities (Gozenman & Berryhill, 2016;Heinen et al, 2016;Tseng et al, 2012), and meta-analyses tend to report stronger effect sizes in clinical or older adult populations compared with the higher-performing young adult population (Dedoncker, Brunoni, Baeken, & Vanderhasselt, 2016;Hill, Fitzgerald, & Hoy, 2016;Hsu, Ku, Zanto, & Gazzaley, 2015;Summers, Kang, & Cauraugh, 2015). Moreover, the evidence extends beyond the WM domain.…”

Optimizing Transcranial Direct Current Stimulation Protocols to Promote Long-Term Learning

“…Furthermore, as discussed earlier, many studies using online stimulation have successfully demonstrated greater learning consolidation after a delay, but no study to our knowledge has done the same with offline stimulation prior to task performance. The potential delayed benefits of online, rather than offline, stimulation then is an important consideration given that meta-analyses suggest both forms of stimulation provide comparable immediate benefits 2 (Dedoncker et al 2016;Hill et al 2016).…”

“…However, this argument has not been empirically evaluated since direct evidence for membrane polarization is based on intracellular recordings in animal models where current is applied directly to cortical slices rather than transcranially (e.g., Purpura and McMurtry 1965). Nevertheless, most meta-analyses converge on small overall effects within healthy young adults, which become larger and more robust when studies with lower-performing populations such as clinical patients and the elderly are included (Dedoncker et al 2016;Hill et al 2016;Mancuso et al 2016). Therefore, it is imperative to develop a stronger theoretical understanding to account for the diverse effects and to optimize protocols for greater reliability and more meaningful results.…”