The Behavioral Effects of tDCS on Visual Search Performance Are Not Influenced by the Location of the Reference Electrode

Ellison, Amanda; Ball, Keira L.; Lane, Alison R. · 2017 · Crossref

DOI: 10.3389/fnins.2017.00520

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Summary

This study investigates whether the placement of the reference electrode in transcranial direct current stimulation (tDCS) influences behavioral outcomes in visual search tasks. While tDCS is commonly used to modulate neuronal excitability, standard protocols often ignore the potential cognitive impact of the reference electrode, typically placed on the contralateral frontal pole. The authors aimed to determine if placing the anode (reference) ipsilaterally versus contralaterally to the cathode affects performance when stimulating two regions known to be causally involved in conjunction visual search: the right posterior parietal cortex (rPPC) and the right frontal eye fields (rFEF). Thirty-five participants were randomly assigned to one of five conditions in a between-groups design. All participants completed two testing sessions one week apart, receiving sham stimulation in the first session to establish a baseline. In the second session, four groups received 15 minutes of cathodal tDCS (1.0 mA) over either rPPC or rFEF, with the anode placed on either the ipsilateral or contralateral supraorbital cortex. A fifth control group received sham stimulation in both sessions to isolate practice effects. Participants performed a conjunction visual search task, identifying targets among distractors across five blocks of trials per session. Reaction times for target-present trials were the primary dependent variable. The results indicated that reference electrode placement did not significantly modulate tDCS effects. Cathodal stimulation of the rPPC increased search times immediately after stimulation, regardless of whether the reference electrode was ipsilateral or contralateral. This slowing effect was robust and statistically significant compared to the sham group. In contrast, cathodal stimulation of the rFEF did not produce significant changes in search times compared to sham. Crucially, across all active stimulation conditions, the typical improvement in search speed associated with practice (observed in the sham/sham group) was negated. Participants in active tDCS groups failed to show the expected reduction in reaction times across trial blocks, whereas the sham-only group demonstrated significant speeding. The study concludes that for conjunction visual search tasks, the location of the reference electrode on the frontal pole is not a critical determinant of behavioral outcomes when the active cathode is placed over rPPC or rFEF. However, the findings highlight that tDCS can disrupt learning and practice effects, even when immediate performance changes are not apparent. This suggests that researchers must carefully contextualize tDCS effects, particularly in between-participant designs, as stimulation may mask or alter the trajectory of skill acquisition independent of immediate performance metrics.

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• visual search