Abstract
In grapheme-color synesthesia, observers perceive colors that are associated with letters and numbers. We tested the dynamic properties 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. First we tested our observers for color identifications on all letters of the alphabet, numbers up to 12, and roman numerals. Two tests more than 48 hours apart were in 100% agreement, showing that our subjects were true synesthetes. Peripheral crowding eliminated synesthetic color, so our synesthetes were both associators, not projectors. Rotating letters at 36deg/sec changed their synesthetic colors abruptly as letter identification changed or failed. Rotated characters, in a constant-width sans-serif font, were N, M, T, A, H, S, K and X, and the number 9. These characters were chosen to have varying symmetries and varying letter transformations under rotation; rotating M, for example, produced a W 180deg and an E at 270deg with the corresponding changes in synesthetic color. Morphing letters also changed color together with a change in letter identification, for example P to R with growth of the diagonal line element. The transformations were E-F, P-R, and I-J. Abrupt disappearance of a colored character on a white background yielded a negative color 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, in the enclosed regions of the number 8. These results show close linking of synesthetic color with character identity rather than image properties, in contrast to physical color.