Anodal tDCS alters perceived visual contrast in humans

Cortical gain modulation is thought to alter perceptual experience, most prominently demonstrated by the effects of spatial attention on contrast appearance. Whether externally induced changes in cortical excitability produce comparable perceptual consequences remains unclear. Here, we tested whether anodal transcranial direct current stimulation (tDCS) over occipital cortex alters perceived contrast. Using a contrast-appearance paradigm, participants judged which of two gratings appeared higher in contrast. Anodal stimulation produced systematic shifts in the point of subjective equality (PSE), such that stimuli presented in the contralateral visual field were perceived as higher in contrast. This effect reversed across stimulus configurations and was absent under sham stimulation, ruling out response bias. Psychometric slopes did not change significantly, consistent with stable discrimination precision around the PSE. These findings demonstrate that tDCS alters contrast appearance, suggesting that changes in cortical excitability can directly influence perceptual experience through gain-like modulation of sensory responses.