Abstract
We investigated the role of reference electrode placement (ipsilateral v contralateral frontal pole) on conjunction visual search task performance when the transcranial direct current stimulation (tDCS) cathode is placed over right posterior parietal cortex (rPPC) and over right frontal eye fields (rFEF), both of which have been shown to be causally involved in the processing of this task using TMS. This resulted in four experimental manipulations in which sham tDCS was applied in week one followed by active tDCS the following week. Another group received sham stimulation in both sessions to investigate practice effects over 1 week in this task. Results show that there is no difference between effects seen when the anode is placed ipsi or contralaterally. Cathodal stimulation of rPPC increased search times straight after stimulation similarly for ipsi and contralateral references. This finding does not extend to rFEF stimulation. However, for both sites and both montages, practice effects as seen in the sham/sham condition were negated. This can be taken as evidence that for this task, reference placement on either frontal pole is not important, but also that care needs to be taken when contextualizing tDCS “effects” that may not be immediately apparent particularly in between-participant designs.
Highlights
Transcranial direct current stimulation involves passing an electrical current from an anode to a cathode (Been et al, 2007; Sparing and Mottaghy, 2008) resulting in a change in neuronal excitability
Participants were correct on 95.9% of target present trial and this was not significantly different between testing sessions [Session 1: M = 95.3%; Session 2: M = 96.4%, t(34) = −1.898, p = 0.066] Responses to incorrect trials were removed, as were search times more than two standard deviations above or below the individual’s mean (9.6% of correct present trials were excluded on these grounds)
All data were tested for normality using the Shapiro–Wilk statistic; the data were normal Transcranial direct current stimulation (tDCS) Montage Manipulation
Summary
Transcranial direct current stimulation (tDCS) involves passing an electrical current from an anode to a cathode (Been et al, 2007; Sparing and Mottaghy, 2008) resulting in a change in neuronal excitability. The use of tDCS to understand the neural function in cognitive tasks typically takes a bipolar non-balanced approach (Nasseri et al, 2015), investigating the region of interest over which the cathode or anode is placed. The complementary electrode is termed the reference electrode and is placed most often over the contralateral frontal pole (Nitsche et al, 2008; Stagg and Nitsche, 2011). This approach, at best denigrates, or at worst ignores, the possible involvement that modulation of the underlying frontal regions may have in the processing of tDCS Montage Manipulation the cognitive task at hand. In the experiment reported in this paper, we sought to evaluate the role of the reference electrode in a task which has been extensively investigated with respect to neurostimulation and underpinning functional networks
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