When C, a digit-color synaesthete, views black digits, she reports that eac
h digit elicits a highly specific color (a photism), which is experienced a
s though the color was externally projected onto the digit. We evaluated th
is claim by assessing whether C's photisms influenced her ability to percei
ve visually presented digits. C identified and localized target digits pres
ented against backgrounds that were either congruent or incongruent with th
e color of her photism for the digits. The results showed that C was poorer
at identifying and localizing digits on congruent than incongruent trials.
Such differences in performance between congruent and incongruent trials w
ere not found with nonsynaesthete control participants. These results sugge
st that C's colored photisms influence her perception of black digits. We p
ropose a model in which color information influences the perception of digi
ts through reentrant pathways in the visual system.