MISMATCH NEGATIVITY AS AN OBJECTIVE MARKER OF GRAPHEME–COLOUR SYNAESTHESIA

Synaesthesia is a variation in how some people experience their brains information processing. While the phenomenon is highly heterogeneous some forms are more common than others, e.g. grapheme-colour synaesthesia where observers also have a colour sensation when reading letters. Various theories have been put forth to explain how some people develop synaesthesia, from faulty pruning during development and disinhibition gone wrong. I suggest that a more parsimonious explanation is that synaesthesia reflect variation based on a common mechanism, namely expertise. The brain does not come equipped with an IKEA manual, so the brain needs to blindly bind information that is relevant together. While some studies have demonstrated that colour associations in grapheme-colour synaesthesia can be learned from the environment (e.g., Witthoft & Winawer, 2006), this has typically not garnered much attention. We have suggested that the colour associations become embedded in the mental grapheme templates during development in synaesthetes (Brogaard & Sørensen, 2023; Mannix & Sørensen, 2021). To investigate this hypothesis, we constructed a case-controlled electroencephalographic experiment investigating the Mis-Match Negativity (MMN) of processing oddball stimulus presented with either a congruent or an incongruent colour. The congruent colour for one synaesthete was incongruent to the other and vice versa, thus controlling for low-level visual properties. In posterior electrodes we demonstrate a colour dependent MMN around 150 ms with a stronger deflection for the incongruent oddball. In addition, to the theoretical implications this is important as this is also a marker for synaesthesia that does not rely on subjective report.