Grapheme-color synesthesia induction with V4 transcranial direct current stimulation

Grapheme-color synesthesia (GC-S) is a neurological condition in which the perception of a grapheme elicits the experience of color or even the visual representation of that grapheme as colored. Previous research using transcranial magnetic stimulation on GC-synesthetes demonstrated enhanced excitability of the visual cortex. Consequently, we hypothesized that using anodal "offline" transcranial direct current stimulation on the visual cortex in area V4 followed by visual training could boost cortical excitability in the target areas and thus produce effects similar to GC-S in non-synesthetes. We discovered that after anodal stimulation, participants had a considerably smaller mean change in reaction time on white symbols than after sham and cathodal stimulation. Our findings indirectly support the cross-activation hypothesis.