Abstract
In the cognitive domain, enormous variation in methodological approach prompts questions about the generalizability of behavioral findings obtained from studies of transcranial direct current stimulation (tDCS). To determine the impact of common variations in approach, we systematically manipulated two key stimulation parameters—current polarity and intensity—and assessed their impact on a task of inhibitory control (the Eriksen Flanker). Ninety participants were randomly assigned to one of nine experimental groups: three stimulation conditions (anode, sham, cathode) crossed with three intensity levels (1.0, 1.5, 2.0 mA). As participants performed the Flanker task, stimulation was applied over left dorsolateral prefrontal cortex (DLPFC; electrode montage: F3-RSO). The behavioral impact of these manipulations was examined using mixed effects linear regression. Results indicate a significant effect of stimulation condition (current polarity) on the magnitude of the interference effect during the Flanker; however, this effect was specific to the comparison between anodal and sham stimulation. Inhibitory control was therefore improved by anodal stimulation over the DLPFC. In the present experimental context, no reliable effect of stimulation intensity was observed, and we found no evidence that inhibitory control was impeded by cathodal stimulation. Continued exploration of the stimulation parameter space, particularly with more robustly powered sample sizes, is essential to facilitating cross-study comparison and ultimately working toward a reliable model of tDCS effects.
Highlights
With the recent surge in use of transcranial direct current stimulation has come a growing uncertainty about the reliability of this neuromodulatory technique
Hyper-polarization of these areas via negative current administered during cathodal stimulation (CtDCS) tends to diminish the amplitude of motor-evoked potentials (MEPs), indicating cortical inhibition (Nitsche and Paulus, 2000; see Fregni et al, 2006; Furubayashi et al, 2008; Jefferson et al, 2009; Stagg et al, 2009)
No reliable effect of stimulation intensity was observed, and we found no evidence that Flanker reaction times (RTs) performance was impeded by cathodal stimulation
Summary
With the recent surge in use of transcranial direct current stimulation (tDCS) has come a growing uncertainty about the reliability of this neuromodulatory technique. TDCS, a form of non-invasive electrical brain stimulation, hinges on a simple premise: hypo-polarization of a cortical area should increase neuronal excitability, while hyper-polarization should induce the opposite effect. This rationale has been largely supported at the neurophysiological level: when primary motor areas are hypo-polarized by positive current administered during anodal stimulation (A-tDCS), motor-evoked potentials (MEPs) recorded from peripheral muscles tend. Hyper-polarization of these areas via negative current administered during cathodal stimulation (CtDCS) tends to diminish the amplitude of MEPs, indicating cortical inhibition (Nitsche and Paulus, 2000; see Fregni et al, 2006; Furubayashi et al, 2008; Jefferson et al, 2009; Stagg et al, 2009). Cathodal stimulation of the underlying substrate should lead to decrements in that function
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