Abstract
Transcranial direct current stimulation (tDCS) is a well-established technique for non-invasive brain stimulation (NIBS). However, the technique suffers from a high variability in outcome, some of which is likely explained by the state of the brain at tDCS-delivery but for which explanatory, mechanistic models are lacking. Here, we tested the effects of bi-parietal tDCS on perceptual line bisection as a function of tDCS current strength (1 mA vs 2 mA) and individual baseline discrimination sensitivity (a measure associated with intrinsic uncertainty/signal-to-noise balance). Our main findings were threefold. We replicated a previous finding (Giglia et al., 2011) of a rightward shift in subjective midpoint after Left anode/Right cathode tDCS over parietal cortex (sham-controlled). We found this effect to be weak over our entire sample (n = 38), but to be substantial in a subset of participants when they were split according to tDCS-intensity and baseline performance. This was due to a complex, nonlinear interaction between these two factors. Our data lend further support to the notion of state-dependency in NIBS which suggests outcome to depend on the endogenous balance between task-informative ‘signal’ and task-uninformative ‘noise’ at baseline. The results highlight the strong influence of individual differences and variations in experimental parameters on tDCS outcome, and the importance of fostering knowledge on the factors influencing tDCS outcome across cognitive domains.
Highlights
Transcranial direct current stimulation is a noninvasive tool for research into healthy brain function and is being increasingly investigated for its therapeutic and neuro-enhancing potential in various cognitive domains (Brunoni et al, 2012; Oliveri, 2011)
This is illustrated in the left-biased subjective midpoint judgments, which are all significantly displaced to the left of veridical centre, as the 95% confidence intervals of the group-averaged point of subjective equality (PSE) do not overlap zero for any of the groups [1 mA- ‘high discrimination sensitivity’ group: mean: À2.05 pixels/confidence interval (CI): À2.33 to À1.78; 1 mA-‘low discrimination sensitivity’ group: mean: À1.24 pixels/CI: À1.79 to À.60; 2 mA-‘high discrimination sensitivity’ group: mean: À2.13 pixels/CI: À2.44 to À1.81; 2 mA‘low discrimination sensitivity’ group: mean:À3.43 pixels/CI: À3.96 to À2.90]
We found only a weak overall effect in a group of 38 participants in line with recent metaanalyses of weak effects of Transcranial direct current stimulation (tDCS) on cognitive outcome measures (Horvath et al, 2015a; Jacobson, Koslowsky et al, 2012), yet the effect was found to be strong in a subset of our participants when they were split according to individual baseline discrimination sensitivity; a measure associated with intrinsic uncertainty during perceptual decision making (Gold & Ding, 2013)
Summary
Transcranial direct current stimulation (tDCS) is a noninvasive tool for research into healthy brain function and is being increasingly investigated for its therapeutic and neuro-enhancing potential in various cognitive domains (Brunoni et al, 2012; Oliveri, 2011). The classic anodal-facilitation/ cathodal-inhibition distinction does not always hold for cognitive functions (Jacobson, Koslowsky, & Lavidor, 2012; Vallar & Bolognini, 2011) and recent meta-analyses cast doubt on the reliability of tDCS effects on neurophysiological and cognitive outcome measures in healthy participants (Horvath, Forte, & Carter, 2015a, 2015b). An explanation for this may lie in the trait- and/or state-dependent nature of tDCS effects. For a better understanding of tDCS effects, it is of importance to map those factors, and the relationships between them, that may determine tDCS outcome across different cognitive domains
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