When a single brief visual flash is accompanied by two auditory bleeps, it is frequently perceived incorrectly as two flashes. Such illusory multisensory perception is associated with increased activation of retinotopic human primary visual cortex (V1) suggesting that such activity reflects subjective perception [Watkins, S., Shams, L., Tanaka, S., Haynes, J.D., Rees, G., 2006. Sound alters activity in human V1 in association with illusory visual perception. Neuroimage. 31, 1247-1256]. However, an alternate possibility is that increased V1 activity reflects either fluctuating attention or auditory-visual perceptual matching on illusion trials. Here, we rule out these possibilities by studying the complementary illusion, where a double flash is accompanied by a single bleep and perceived incorrectly as a single flash. We replicate findings of increased activity in retinotopic V1 when a single flash is perceived incorrectly as two flashes, and now show that activity is decreased in retinotopic V1 when a double flash is perceived incorrectly as a single flash. Our findings provide strong support for the notion that human V1 activity reflects subjective perception in these multisensory illusions.