A negative afterimage is the phenomenon where exposure to a vi-
sual stimulus leads to an afterimage of opposite polarity (e.g. perceiv-
ing an illusory black spot after exposure to a white spot). Such
afterimages are normal, and are believed to arise at the level of the
retina [e.g. [14]]. To exclude the possibility that CQ's abnormal after-
images were mediated by abnormalities in his negative afterimage,
we started by assessing his negative afterimages