Dynamic phenomenology of grapheme-color synesthesia

In grapheme-color synesthesia, observers perceive colors that are associated with letters and numbers. We tested the dynamic limits of this phenomenon by exposing two synesthetes to characters that rotate smoothly, that morph into other characters, that disappear abruptly, or that have colors either consistent or inconsistent with the corresponding synesthetic color. Rotating letters changed their synesthetic colors abruptly as letter identification changed or failed. Morphing letters also changed color together with a change in letter identification. Abrupt disappearance of a black character on a white background yielded a negative afterimage, but maintenance of the same synesthetic color. Our synesthetes could maintain both physical and synesthetic color in the same character, without conflict. Neon color spreading in one observer occurred for physical but not synesthetic color. These and other results show close linking of synesthetic color with character identity rather than image properties, in contrast to physical color.