Abstract
Does the enhanced excitability of the visual cortex induced by anodal tDCS affect the contrast processing? The current study aims to answer this question by examining the effect of anodal tDCS on perceptual aspect of contrast processing, the appearance. We hypothesized, if anodal tDCS enhances neural excitability and consequently enhances contrast processing (i.e., contrast gain and sensitivity), a stimulus presented to the visual hemifield contralateral to the hemisphere where anodal tDCS was applied should appear high contrast than the one presented to the ipsilateral hemifield. In each experimental trial, two 2-deg Gabor patches (3 cpd) were presented simultaneously 4-deg away from the fixation. The contrast of one of the Gabor patches was fixed at 15.49% (standard), and the contrast of the other (test) Gabor patch was varied from 8 to 30% (9 log increments). The location (left or right) and contrast of the test patch was completely randomized, and each contrast level was presented 20 times in random order. Participants’ task was to report the orientation of grating with higher contrast than the other using pre-assigned keys. In the first block, we measured participants’ baseline contrast appearance perception. Then, in the second block, participants performed the same task while tDCS (2mA anodal) was applied to their right occipital cortex (O2 in the international 10-20 system coordinate). A sham control experiment was conducted in a different day. We found that the PSE test contrast was lower when the test was presented at the contralateral hemifield to the hemisphere where tDCS was applied than pre-stimulation baseline. This means that tDCS indeed reduced the test contrast required to be appear the same as standard contrast. This effect was not observed in the sham control condition. Our results demonstrate that enhanced neural excitability due to anodal tDCS affects contrast processing and alters appearance.