Vestibular signals can distort the perceived spatial relationship of retinal stimuli

The flash-lag phenomenon is an illusion that affects the perceived relationship of a moving object and a briefly visible one: the moving object appears to be ahead of the flashed one. In practically all studies of this phenomenon, the image of the object moves on the retina as the object moves in space. Therefore, explanations of the illusion were sought in terms of purely visual mechanisms. Here we set up a situation in which the object's motion in space is entirely produced by passive rotation of the subject. No motion occurred on the retina. The visual display (a continuously lit stimulus and a flashed one) was mounted on a vestibular chair. While the subjects fixated this display, they were rotated in the dark at a constant speed and suddenly stopped. Perceptual misalignment (flash-lag) was robust and consistent during both the initial phase of rotation and the postrotary period when neither chair, subject, nor stimulus was actually moving. As a vestibular signal can cause an illusory spatial dissociation in the visual domain, we conclude that the mechanism of the flash-lag must be more general than was thought up-to-now.