When Letters Shape Vision:Neural Evidence for Synaesthesia

Synaesthesia is a variation within the neurotypical population in which atypical cross-modal associations occur. For example, grapheme–colour synaesthetes experience a colour sensation when viewing letter shapes. The mechanisms underlying synaesthesia remain debated. I propose that synaesthesia may draw on a more general mechanism—expertise—and reflect a cognitive strategy developed and refined to efficiently process a particular information category (e.g., letters).<br/>We have previously demonstrated that attentional parameters modulated by synaesthetic congruence (Ásgeirsson et al., 2015) resemble those modulated by perceptual expertise (Dall et al., 2021), and we have argued that long-term memory templates play a central role in perceptual processing (Brogaard &amp; Sørensen, 2024). Although several findings support this account, attempts to train synaesthesia in non-synaesthetic individuals have met with limited success.<br/>However, studies using EEG indicate that semantic information—presumably instantiated in long-term memory templates—can influence both perceptual (Thierry et al., 2009) and conceptual processing (Casaponsa et al., 2024). Using a variation of the visual mismatch negativity paradigm in a case–control design, Zelazny, Dietz, and Sørensen (submitted) demonstrate a synaesthesia-related neural response to an oddball stimulus. These findings support the expertise account and identify a potential neural marker of synaesthesia that does not rely on behavioural consistency measures.<br/>