Anodal and Cathodal tDCS Over the Right Frontal Eye Fields Impacts Spatial Probability Processing Differently in Pro- and Anti-saccades

Abstract

Learning regularities that exist in the environment can help the visual system achieve optimal efficiency while reducing computational burden. Using a pro- and anti-saccade task, studies have shown that probabilistic information regarding spatial locations can be a strong modulator of frontal eye fields (FEF) activities and consequently alter saccadic behavior. One recent study has also shown that FEF activities can be modulated by transcranial direct current stimulation, where anodal tDCS facilitated prosaccades but cathodal tDCS prolonged antisaccades. These studies together suggest that location probability and tDCS can both alter FEF activities and oculomotor performance, yet how these two modulators interact with each other remains unclear. In this study, we applied anodal or cathodal tDCS over right FEF, and participants performed an interleaved pro- and anti-saccade task. Location probability was manipulated in prosaccade trials but not antisaccade trials. We observed that anodal tDCS over rFEF facilitated prosaccdes toward low-probability locations but not to high-probability locations; whereas cathodal tDCS facilitated antisaccades away from the high-probability location (i.e., same location as the low-probability locations in prosaccades). These observed effects were specific to rFEF as tDCS over the SEF in a separate control experiment did not yield similar patterns. These effects were also more pronounced in low-performers who had slower saccade reaction time. Together, we conclude that (1) the overlapping spatial endpoint between prosaccades (i.e., toward low-probability location) and antisaccades (i.e., away from high-probability location) possibly suggest an endpoint-selective mechanism within right FEF, (2) anodal tDCS and location probability cannot be combined to produce a bigger facilitative effect, and (3) anodal rFEF tDCS works best on low-performers who had slower saccade reaction time. These observations are consistent with the homeostasis account of tDCS effect and FEF functioning.

Saccades, attentional orienting and disengagement: the effects of anodal tDCS over right posterior parietal cortex (PPC) and frontal eye field (FEF)

“…Regarding the gap trials, instead, the association between baseline performance and tDCS effect was stronger after FEF and PPC stimulation, but only for leftward saccades. Overall, these results are consistent with previous evidence showing baseline-dependent effects of parietal tDCS on spatial attention [e.g., Learmonth et al 2015], as well as of FEF tDCS on eye movements [Tseng et al 2018]. Hence, it seems that the level of the baseline performance mediates sideand disengagement-specific effects of tDCS [e.g., Benwell 2015], but further research is needed to confirm this.…”

“…Although this approach proved to be valid on primary cortices [Nitsche and Paulus 2000], polarization of associative cortices yielded mixed findings with respect to excitatory/inhibitory influences on behavior. With regard to eye movements, some studies applied tDCS over the FEFs in pro-and anti-saccades tasks [Kanai et al 2012;Tseng et al 2018;Reteig et al 2018]. However, the results are not conclusive: whereas Kanai et al [2012] found stimulation-and task-specific effects on saccades directed to the contralateral visual hemifield, Tseng et al [2018] observed that effects of anodal tDCS over the right FEF depended on the probability of target location and the individual level of performance.…”

“…With regard to eye movements, some studies applied tDCS over the FEFs in pro-and anti-saccades tasks [Kanai et al 2012;Tseng et al 2018;Reteig et al 2018]. However, the results are not conclusive: whereas Kanai et al [2012] found stimulation-and task-specific effects on saccades directed to the contralateral visual hemifield, Tseng et al [2018] observed that effects of anodal tDCS over the right FEF depended on the probability of target location and the individual level of performance. On the other hand, Reteig et al [2018] found no effects of either anodal or cathodal stimulation.…”