Noninvasive brain stimulation of the left frontal eye field alters fixational control but leaves visually guided...

To investigate the effects of anodal transcranial direct current stimulation (tDCS) over the left frontal eye field (left frontal eye field) on oculomotor functions in visually normal adults, focusing on saccades, fixation stability, vergence stability, stereopsis, and fixational eye movements.

Nineteen right-handed adults with normal vision participated in a randomized single-blind crossover study involving both sham and anodal tDCS. Stimulation (0 μA for sham, 2000 μA for anodal) was applied for 25 min over the left frontal eye field with 20-s fade-in/out. Eye movements were recorded using the Eyelink 1000plus system at 1000 Hz. Tasks included 15-s fixation on a 0.5° target, visually guided saccades at ±5°, ±10°, ±15°, and a stereopsis test using random dot stereograms. Measurements were taken at baseline, 10, and 30 min post-stimulation. Data were analyzed using general linear model repeated measures analysis and post hoc tests.

There were no significant changes in saccadic latency, saccadic gain, or stereopsis across time points for either condition. Fixation stability and vergence stability also showed no substantial difference in both the sham and anodal conditions. However, fixational saccade amplitude significantly increased following anodal stimulation (Pillai's Trace = 0.005, F(1, 18) = 43.84, p < 0.001), corresponding with a significant increase in fixational saccade peak velocity (Pillai's Trace = 0.003, F(1, 18) = 9.70, p = 0.006). Both amplitude and peak velocity significantly decreased after sham stimulation.

Anodal tDCS over the left frontal eye field did not significantly affect gross oculomotor metrics but did modulate fixational saccade dynamics. These findings suggest that miniature eye movements may be more sensitive to neuromodulation than overt saccades, highlighting their potential as biomarkers for cortical excitability. Further research is needed to explore these effects in clinical populations and optimize stimulation protocols for targeted oculomotor modulation.