Ideaesthesia: Conceptual processes assign similar colours to similar shapes

Abstract One of the main problems in understanding synaesthesia is determining whether it is a sensory or semantic phenomenon-a problem also known as the distinction between ‘low-’ vs. ‘high-level’ synaesthesia. Recently, it has been reported that in grapheme-colour synaesthesia, the colour associated to a letter is chosen by similarity to other letters. Letters of similar shapes tend to elicit similar colours. The authors explained this result by low-level cross-wiring between grapheme and colour brain areas. In the present study we show that when new graphemes are learned, they also associate synaesthetic colours based on shape- and colour similarity. Importantly, these novel associations between graphemes and colors are created within minutes, which is too rapid to be accounted for by cross-wiring. In addition, the dimensions of similarity used to create these associations were more abstract than what would be expected from elementary features processed in the grapheme area. Therefore, we conclude that the result is better explained by high-level conceptual mechanisms that guide the choices and creation of synaesthetic concurrents.