Grapheme-color synesthesia, one of the most common variants of the phenomenon, is triggered by linguistic entities like letters and numerals (inducers) and generates perceptions of colors (concurrents) (Cytowic & Wood,
1982). These associations are acquired in early childhood and remain highly consistent over time (Baron-Cohen, Wyke, & Binnie,
1987). Evidence suggests that the neural representations of phenomenal colors experienced in synesthesia (synesthetic colors) resemble those during conscious color vision under standard perceptual conditions (ink colors): fMRI studies indicated activation in areas V4/V8 during synesthetic experiences (Nunn et al.,
2002) and psychophysics indicated that these experiences have color opponent properties (Nikolić, Lichti, & Singer,
2007), suggesting the involvement of neurons responsible for color perception. However, the neuronal representations of the inducers are less clear. Neuronal activations during synesthesia involve a variety of striate and extra striate areas, including left dorso-lateral pre-frontal cortex, left intraparietal, and inferior temporal areas (Aleman, Rutten, Sitskoorn, Dautzenberg, & Ramsey,
2001; Sperling, Prvulovic, Linden, Singer, & Stirn,
2006). Some of these areas are likely to be directly involved in the generation of the perceptual associations (binding) between the grapheme and the respective color, such as those in parietal and temporal regions (Robertson,
2003). Others may be dealing with the incongruencies between the ink color of the inducer and its synesthetic color, such as the pre-frontal areas (Cohen Kadosh, Cohen Kadosh, & Henik,
2007). The common understanding of the nature of the inducer is consistent with the name of the phenomenon—
syn +
esthesia meaning ‘union of senses’—, suggesting that the associations occur at the ‘lower’ perceptual level of representation (Cytowic & Wood,
1982; Harrison & Baron-Cohen,
1997; Ramachandran & Hubbard,
2001a). However, more recent studies suggested the possibility that this, commonly accepted, view does not describe accurately the nature of synesthesia. Several results suggest that the meaning of the inducers plays an important role in the process (Dixon, Smilek, Duffy, Zanna, & Merikle,
2006; Rich & Mattingley,
2003; Ward, Tsakanikos, & Bray,
2006; see also Ramachandran & Hubbard,
2001b). Thus, concurrents may be activated only after semantic decoding is completed at ‘higher’ processing levels. Synesthesia may not occur due to unification of senses.