Subjects with grapheme-color synesthesia report that a given number is always associated with a certain color (e.g., '5' may be green). We have shown that synesthetic colors can lead to segregation or pop-out (Ramachandran & Hubbard, 2001) and decreasing contrast reduces the vividness of the synesthetic colors monotonically. Below about 10% contrast the color disappeared, even though the letter was still visible (Hubbard & Ramachandran, SFN 2002). We now show that the synesthetic color induction depends on physical (retinal) rather than perceived contrast by using the Mach Card Illusion and cast shadows. We also find that synesthetic colors can lead to apparent motion. We alternated two frames: in frame 1 a cluster of 5s was embedded in a matrix of randomly placed 2s; in frame 2, the cluster of 5s was displaced by 4 degrees and the 2s randomly repositioned. Controls saw random incoherent motion; synesthetes saw apparent motion of the 5s based on the synesthetically induced colors. These results suggest tha...