Synasthesia is a neurological phenomenon where stimulation of one sensory or cognitive modality leads to automatic, involuntary experiences in a second sensory or cognitive pathway/modality. It has been observed that around 4% population have this feature and connection of words/numbers to colours (grapheme-colour synaesthesia) is one of the commonest observed types. Studies have shown that rTMS applied over a right parieto-occipital region disrupts performance on a synaesthetic priming task indicating the role of this region in building up of this phenomenology (Muggleton et al, 2007; expected as the cross talk between visual and somatosensory spatial reference regions). But are you aware of any studies using M1-TMS as the measure of altered excitability while responding to any 'so called' non-specific stimuli (colors/sounds)? Further these or other stimuli could be used to modulate the excitability of brain regions in synesthetes or in other conditions.