Is digit-color synaesthesia strictly unidirectional? Preliminary evidence for an implicitly colored number space in three synaesthetes

Background. The unidirectionality of associations constitutes an undisputed law of synaesthesia. In digit-color synaesthesia a digit elicits a concurrent color percept, but if the particular color is presented, it is usually devoid of numerical properties. Based on scattered case reports of synaesthetes whose phenomenal experience runs counter to the law of unidirectionality, we investigated implicit numerical encoding of colors in three digit-color synaesthetes. Material and methods. Those colors regularly elicited by numerals were presented in the center of a computer screen in a spatial stimulus-response paradigm with bimanual responses. Results. Left hand reaction times (RTs) were faster to colors representing small numbers, and right hand RTs were faster to colors representing large numbers. This SNARC effect (Dehaene, 1992) for purely chromatic information was absent for non-synaesthetic matched control subjects, who had previously learned the synaesthetes' digit-color associations. Conclusions. This finding presents evidence for an at least implicit coactivation of number magnitude by color stimulation in synaesthetes, and thus questions the universality of the unidirectionality principle.