A Higher-Order Approach to the Understanding of Grapheme-Colour Synesthesia

Synesthesia describes neurological conditions in which stimulation of one sensory or cognitive pathway leads to the automatic engagement of another pathway (Ward, 2013). Research on such conditions have been suggested to have profound implications in consciousness studies, as one who undergoes synesthesia obtains additional phenomenal experiences that are not directly triggered by corresponding sensory stimulation (Ramachandran & Hubbard, 2001; Ward, 2013; van Leeuwen et al., 2015). In the present work, we examine investigations of neural mechanisms underlying grapheme-colour synesthesia, the phenomenon in which an individual experiences automatic colour association with specific letters or digits (Dixon et al., 2004), to discuss the role of semantic integration in forming phenomenal experience. Several studies have supported that top-down modulation via higher-order brain regions is essential to grapheme-colour synesthesia. Specifically, neuroimaging data demonstrates that the colour area V4 is activated via a top-down circuit through the superior parietal lobe (SPL) after stimulation of non-colour-sensitive grapheme areas among associators (Grossenbacher & Lovelace, 2001; Hopé et al., 2011; Hubbard et al., 2011; van Leeuwen et al., 2011), synesthetes who report experiencing colour as appeared to be “in the mind’s eye” (see Dixon et al., 2004). Associative parietal regions including the SPL have been shown to involve in semantic integration (Spurgnoli et al., 2021), a process necessary for the mediation of synesthesia (van Leeuwen et al., 2015).

While research on grapheme-colour synesthesia emphasises the importance of semantics to phenomenal experience, there has been little investigation into its mechanism. We argue that existing evidence of how grapheme-colour synesthesia functions aligns with the framework of the higher-order theory (HOT) of consciousness. The HOT stresses that phenomenal experience is the further representation of a non-conscious mental state as a result of cognitive integration, which is supplied by perceptual, mnemonic, and conceptual information (Brown et al., 2019). Such multimodal processes are supported by the higher-order networks comprising not only the prefrontal regions but also posterior cortical areas such as the SPL (Lau & Rosenthal, 2011). We thus advance a neurocognitive model that accounts for grapheme-colour synesthesia in terms of higher-order representation that includes non-sensory-triggered perceptual content via cognitive integration. Referring to the model, we draw on neurolinguistics research of comprehension and memory as well as neuroanatomical findings to propose that varied multimodal semantic integration pathways could be responsible for differed phenomenal experiences of the same stimuli, as in between those who are grapheme-colour synesthete and those who are not.

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