Transcranial alternating current stimulation reveals atypical 40 Hz phosphene thresholds in synaesthesia.

Grapheme-colour synaesthesia is a condition characterized by atypical binding in which letters and numerals involuntarily elicit colour photisms (Ward, 2013). Previous research using transcranial magnetic stimulation (TMS) demonstrated that synaesthetes display enhanced cortical excitability selectively in primary visual cortex (Terhune, Tai, Cowey, Popescu, & Cohen Kadosh, 2011), as measured by phosphene thresholds (Cowey &Walsh, 2000). Further research found that variability in TMS phosphene thresholds covaries with individual differences in the visuospatial phenomenology of synaesthesia. In particular, projectors, who experience colour photisms as spatially co-localized with the inducing grapheme (Dixon, Smilek, & Merikle, 2004; Ward, Li, Salih, & Sagiv, 2007), exhibit lower phosphene thresholds (reflecting elevated cortical excitability) than associators, who experience colour photisms as endogenous images, and non-synaesthete controls (Terhune et al., submitted for publication). Insofar as top-down visuospatial attention enhances visual cortex excitability