Development of synaesthetic consistency: Repeated autonomous engagement with graphemes and colours leads to consistent associations.

Synaesthetic consistency is the hallmark of synaesthesia and plays an important role in the definition and validation of synaesthesia. It has been hypothesised that the acquisition of initially unspecified synaesthetic associations is based on consolidation processes. Thus, we investigated in non-synaesthetes whether repeatedly engaging with grapheme-colour associations mimics the developmental trajectory of synaesthetic consistency in genuine grapheme-colour synaesthesia. This was the case for the two tested experimental groups, irrespective of whether they were instructed to memorize their chosen associations, but not for the passive control group. Moreover, consolidated associations of the experimental groups resembled those frequently found in genuine synaesthesia. Furthermore, the acquisition of consistent grapheme-colour associations resulted in a transfer of benefits to performance in recognition memory for abstract stimuli, as also found in genuine synaesthesia. Our findings suggest that consistent synaesthetic associations are based on consolidation processes due to repeated engagement with graphemes and colours.