The cyclopean eye and its implications: vergence state and visual direction.

The cyclopean illusion (Hering, 1861) is an anomalous lateral shift in the apparent direction to a monocularly seen target, which arises when a change in vergence is made by the opposite (nonobserving) eye. Surprisingly, this directional illusion does not arise, when the observed target is an afterimage or an intermittently illuminated (4-8 Hz) object. Instead, during convergence, a monocularly imprinted afterimage seems to move toward the observer, and a stroboscopically illuminated target seems to remain fully stationary. The apparent displacement of an afterimage in depth is particularly puzzling. Since binocular interactions in the persistence of monocularly induced afterimages can be demonstrated, it is conceivable that long-persistent afterimages arise, in part, from binocularly driven neurons in the visual cortex, and that a monocularly induced afterimage can thereby become the perceptual equivalent of a binocularly fused target.