Melbourne School of Psychological Sciences - Theses
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ItemAssessment of the effects of transcranial direct current stimulation (tDCS) on neurophysiologic, cognitive, and behavioral outcome measures( 2016)Transcranial Direct Current Stimulation (tDCS) is a form of non-invasive brain stimulation that serves to modulate brain activity via the passing of a weak, electrical current through the head. Despite public and clinical excitement, the effects of tDCS reported in the literature have been inconsistent. Accordingly, the reliability of this device remains unclear. The purpose of this PhD was to explore the consistency and reliability of inter- and intra-subject tDCS effects. First, I conducted research exploring the effects of four tDCS parameters (polarity, current density, electrode location, and stimulation-to-task relationship) on simple visual motor reaction time (smRT). 150 individuals were assigned to one of 5 experimental groups (2 mA anodal, 2 mA cathodal, 1 mA anodal, 1 mA cathodal, or sham) across 3 different conditions (M1 target with the reference over an orbitofrontal, bilateral, or extracephalic location). Participants received 20min stimulation while undertaking an smRT task 5min preceeding, during, and 5min following stimulation. Results revealed tDCS demonstrated no significant effect on any smRT parameter. In my next study, 30 participants underwent 7min of anodal, cathodal, or sham stimulation using a bilateral M1 electrode montage. Participants undertook an smRT task for 5min preceeding, during, and 20min following stimulation: however, the cueing stimulus utilized was auditory rather than visual. As before, this experiment produced null results. Next, 2 participants undertook 8 separate sessions of 1min cathodal tDCS utilizing a bilateral M1 montage. Participants undertook an smRT task for 1min preceding, during, and following stimulation. Again, this experiment produced null-results. To determine a cognitive measure reliably amenable to stimulation, I next conducted a quantitative review of every inter-lab replicated cognitive task in the tDCS literature. Of the 59 analyses exploring 52 outcome measures across three-tiers of inclusion stringency (gleaned from 138 studies), none demonstrated a significantly reliable response to tDCS. To determine a physiologic outcome measure amenable to stimulation, I conducted a second quantitative review of every inter-lab replicated physiologic outcome measure in the tDCS literature. Of the 31 measures (gleaned from 145 studies), only 1 reached statistical significance: motor evoked potential (MEP) amplitude modulation. At the time of the analysis, no data had been generated exploring intra-subject reliability of this outcome. In my final study, I explored the effect of multiple identical tDCS sessions on MEP amplitude modulation within individuals. 14 participants each underwent 9 sessions of 10min tDCS (3 anodal, 3 cathodal, 3 sham). MEP amplitudes were measured prior to and for 30min following stimulation. Results demonstrated tDCS effects on MEP amplituted are highly variable within individuals across sessions. Group-wide analysis demonstrated no significant effect of tDCS on MEP amplitude compared to sham. Regression analysis demonstrated modulation of MEP amplitude may be explainable by regression-to-the-mean and non-experimental variables. Based on the experimental investigations conducted and the comprehensive reviews of the existing literature, there is currently insufficient evidence that tDCS produces a reliable effect. In the conclusion, I explore potential explanations for the current state of the tDCS literature and suggest future directions for this tool.