Six is Sapphire, but is Sapphire Six? Bidirectionality and Numerosity in Grapheme-Color Synesthesia

In grapheme-color synesthesia, numbers and letters create a color experience that is consistent, automatic, and unique to each synesthete. Recent studies have examined the way viewing graphemes elicits colors as well as the possibility of bidirectional synesthesia, in which viewing colors may elicit graphemes in the minds of synesthetes (Dixon, Smilek, Cudahy, & Merikle, 2000). This thesis addresses the issue of bidirectionality to see if specific colors elicit the information represented by graphemes in a manner that is cognitively accessible to the synesthete observer. Using psychophysics and event related potential (ERP) waveforms, we found bidirectional synesthesia to exist, as evidenced by synethetes’ ability to accurately complete an arithmetic verification task in which some or all graphemes were replaced with patches of color that matched the synsethetes’ grapheme associations. Synesthete reaction times were just as fast for trials with a color solution as grapheme solution, and were comparable to control participants’ reaction times. The ERP results from showed that the manner in which synesthetes visually process both numbers and colors differs from that of non-synesthetes, with each synesthete showing a wave pattern distinct from controls and from each other. This research adds a crucial piece to the puzzle of how both synesthesia and numerical concepts are processed in the brain, and includes the first study to date that looks at ERP waveforms of synesthetes while performing an arithmetic task requiring bidirectional synesthesia