In this study, we aimed to characterize the effect of anodal and cathodal direct current stimulation (tDCS) on contrast sensitivity inside the central 10 degrees of the visual field in healthy subjects. Distinct eccentricities were investigated separately, since at the cortical level, more central regions of the visual field are represented closer to the occipital pole, i.e. closer to the polarizing electrodes, than are the more peripheral regions. Using a double-blind and sham-controlled within-subject design, we measured the effect of stimulation and potential learning effect separately across testing days. Anodal stimulation of the visual cortex compared to sham stimulation yielded a significant increase in contrast sensitivity within 8° of the visual field. A significant increase in contrast sensitivity between the conditions "pre" and "post" anodal stimulation was only obtained for the central positions at eccentricities smaller than 2°. Cathodal stimulation of the visual cortex did not affect contrast sensitivity at either eccentricity. Perceptual learning across testing days was only observed for threshold perimetry before stimulation. Measuring contrast sensitivity changes after tDCS with a standard clinical tool such as threshold perimetry may provide an interesting perspective in assessing therapeutic effects of tDCS in ophthalmological or neurological defects (e.g. with foveal sparing vs. foveal splitting).

Purpose: TDCS can increase excitability in the visual cortex. It is a matter of current debate if tDCS can improve visual performance. Promising parameters to measure detection sensitivity may be those of the signal detection theory (= SDT), as it allows differentiating between response bias and detection sensitivity changes. The measure of detection sensitivity can be used to predict actual performance under a wide variety of different response criteria. Methods: Here we test if the SDT can quantify tDCS-induced effects in a visual contrast discrimination task in healthy subjects. Results: Anodal stimulation of the visual cortex improved performance, as calculated by detection sensitivity for stimuli presented in the center of the visual field. More peripheral locations in the visual field were unaffected by anodal stimulation. Cathodal stimulation and sham stimulation of the visual cortex had no consistent effect on detection sensitivity. The response bias was not affected by any type of stimulation. Conclusions: Neuroplastic changes in the visual cortex induced by anodal tDCS can be measured by SDT, suggesting SDT could prospectively be a useful approach for monitoring restorative tDCS-effects on visual function in patients with central visual deficits.

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