Synaesthetes who experience colours when perceiving or representing numbers exhibit structural and functional differences in cortical areas that are involved in number and colour processing compared to non-synaesthetes. Using transcranial magnetic stimulation or transcranial direct current stimulation, the authors showed that in humans, grapheme-colour synaesthesia is characterized by enhanced cortical excitability in the primary visual cortex and can be augmented or attenuated with cathodal or anodal stimulation, respectively.