Synesthetically induced colors evoke apparent-motion perception

Synesthesia is a condition in which certain otherwise normal individuals see colors when they hear tones, or, when they look at black-and-white numbers, each number is tinged with a specific color (eg 5 is red and 2 is green). We constructed a display in which a random matrix of 5s had a vertical column of 2s 'embedded' in it. This was shown in frame 1 of a movie, followed by a similar display in frame 2 in which the element locations were uncorrelated but the bar as a whole was shifted horizontally. When normal subjects viewed the display, they just saw random jitter or twinkle; there was no impression of a bar moving horizontally. But, when our subject (JC) who had grapheme-color synesthesia viewed the display, he reported seeing a bar moving left or right depending on the trial. We conclude that, in at least some subjects, a synesthetically induced color that does not exist on the retina can nonetheless influence motion perception.