Transcranial direct current stimulation over the frontal eye field has no effect on visual search performance
Top-down attention for the goal-directed (de-)prioritization of information is fundamental for successful everyday-life behavior and poses tremendous problems when negatively impacted by disease. Attention-targeting enhancement and rehabilitation attempts using non-invasive brain stimulation techniques like transcranial direct current stimulation (tDCS) are therefore of major importance. tDCS-driven excitation of the left frontal eye field (FEF; a key region within fronto-parietal attention networks) has recently been suggested to improve attention-guided visual search with stronger effects for lower baseline performers. Here, we report two preregistered tDCS experiments that tested 1) whether the previously observed visual search improvement could be boosted through stimulation over the allegedly more dominant right FEF and 2) whether tDCS-related visual search improvements might depend on search field size. To this end, in experiments one and two, N=29 and N=31 healthy participants performed a visual search task, in which they searched for an upside-down T amongst upright Ts and Ls within small or large search fields, before and during the application of anodal (excitatory) or sham (control) tDCS over the right or left FEF, respectively. In contrast to previous studies, in both experiments (i.e., independent of stimulation site and search field size) we found neither tDCS-specific (anodal > sham) visual search improvements, nor stimulation-specific baseline dependencies (larger improvements for lower baseline performers were observed in both tDCS conditions, suggesting rather stimulation-unspecific effects like regression to the mean). Together, our results provide evidence against reliable top-down attention-guided visual search improvements through FEF tDCS.