Disruption of synaesthesia following TMS of the right posterior parietal cortex

This study examines the role of four regions of the parietal lobe in grapheme-colour synaesthesia. TMS applied over a right parieto-occipital region disrupts performance on a synaesthetic priming task. TMS over the left parietal or a more anterior right parietal site did not have a reliable effect on synaesthesia even though one of the sites had been implicated in synaesthesia by previous fMRI studies. The same disruption is found for synaesthetes who experience colours in their “mind's eye” as well as those who project colours onto the inducing grapheme. This region may be important for binding graphemes and colours to different spatial reference frames.