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

Philip Tseng,Mu-Chen Wang,Yu-Hui Lo,Chi-Hung Juan

doi:10.3389/fnins.2018.00421

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.

Highlights

Learning regularities that exist in the environment can help the visual system achieve optimal efficiency while reducing processing load
With cathodal tDCS, antisaccades away from the high-probability cue location became faster. These observed effects were specific to rFEF because anodal tDCS over the supplementary eye field (SEF) did not yield similar patterns. These results imply a substantial difference between the effects of anodal and cathodal tDCS over rFEF on the processing of probabilistic information, but not in a way that is consistent with the conventional assumption of tDCS bidrectionality
It was found that anodal tDCS over rFEF facilitated prosaccdes to low probability locations, whereas cathodal tDCS facilitated antisaccades away from the high probability cue location

Summary

Introduction

Learning regularities that exist in the environment can help the visual system achieve optimal efficiency while reducing processing load. To investigate how the visual system processes probabilistic information, eye movement task involving pro- and anti-saccade is an excellent candidate because of its simplicity, and welldelineated cognitive architecture and neurophysiological basis (Schall and Hanes, 1993; Basso and Wurtz, 1997; Carpenter, 1999; Schall and Thompson, 1999; Schall, 2001, 2004, 2009; Schiller and Kendall, 2004). In this task, prosaccades are eye movements toward a cue, while antisaccades are eye movements moving away from the cue. Prosaccades toward targets in the high probability locations are faster than targets in other locations (e.g., Miller, 1988; Geng and Behrmann, 2005; Liu et al, 2010)

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Discussion

Conclusion