Better together? The cognitive advantages of synaesthesia for time, numbers, and space.

Synaesthesia for time, numbers, and space (TNS synaesthesia) is thought to have costs and benefits for recalling and manipulating time and number. There are two competing theories about how TNS synaesthesia affects cognition. The "magnitude" account predicts that TNS synaesthesia may affect cardinal magnitude judgements, whereas the "sequence" account suggests that it may affect ordinal sequence judgements and could rely on visuospatial working memory. We aimed to comprehensively assess the cognitive consequences of TNS synaesthesia and distinguish between these two accounts. TNS synaesthetes, grapheme-colour synaesthetes, and nonsynaesthetes completed a behavioural task battery. Three tasks involved cardinal and ordinal comparisons of temporal, numerical, and spatial stimuli; we also examined visuospatial working memory. TNS synaesthetes were significantly more accurate than nonsynaesthetes in making ordinal judgements about space. This difference was explained by significantly higher visuospatial working memory accuracy. Our findings demonstrate an advantage of TNS synaesthesia that is more in line with the sequence account.