A persistent memory advantage is specific to grapheme-colour synaesthesia

For people with synaesthesia ordinary stimuli such as digits or letters induce concurrent experiences such as colours. Synaesthesia is associated with a memory advantage and the aim of this study was to investigate whether this advantage persists across time. We tested recognition memory of four different types of synaesthesia with different inducer-concurrent pairings across two sessions with a one-year retention interval. In the study phase, participants learned three kinds of stimuli (i.e., related to their inducer, related to their concurrent, or synaesthesia-unrelated): music, words and colours. Recognition memory was tested after one hour and after one year. After one hour, grapheme-colour and grapheme-colour-and-sound-colour synaesthetes showed synaesthesia-specific advantages. After one year, only grapheme-colour synaesthetes still showed an advantage. The results imply that a benefit through enhanced colour-processing is particularly strong and that synaesthesia can lead to a long-lasting memory benefit.