Transcranial Direct Current Stimulation Effects on Single and Paired Flash Visual Evoked Potentials

Transcranial direct current stimulation (tDCS) applied over the occipital cortex has a controversial effect on the visual cortex excitability. Paired flash visual evoked potentials (paired F-VEPs) offer a unique method to express neural inhibition within the visual system. However, no studies have explored the effects of tDCS on F-VEPs in humans. The aim of this study was to evaluate the changes of single- and paired-F-VEPs during and after tDCS in healthy humans. Twenty-six healthy volunteers participated. F-VEPs were recorded from occipital electrodes with closed eyes. Stimuli were single flashes, intermingled to flash pairs at the interstimulus interval of 125, 62.5, 50, 33.3, 16.6, and 11.1 ms (internal frequency of 8, 16, 20, 30, 60, and 90 Hz). The single F-VEP was split into a “main complex” and a “late response.” As to paired stimuli, the “test” F-VEP emerged from electronic subtraction of the single-F-VEP to the paired-F-VEP. In experiment 1, the return electrode was located on the scalp and we s...