Suppression of OKN and VOR by afterimages and imaginary objects.

Optokinetic nystagmus (OKN) is suppressed if attention is directed to a centrally placed afterimage superimposed on a moving display. Imagining a stationary object has little or no effect. An afterimage does not provide the retinal slip and misfoveation error signals provided by a stationary object and we have shown that an effective error signal does not arise from occlusion or masking of the display by the afterimage. Although a lack of relative motion between afterimage and moving display could indicate when OKN gain is one, there is no unique relative motion signal associated with a gain of zero. Subjects could partially inhibit the vestibulo-ocular reflex (VOR) in the dark when they imagined a head-fixed object. They could suppress the response more effectively by attending to an afterimage, but the suppression was still only partial. When OKN and VOR were evoked simultaneously, pursuit movements of the eyes could not be suppressed until the vestibular inputs had subsided. We conclude that signals associated with OKN, are fully available to the mechanism that assesses the headcentric motion of objects but that signals associated with VOR are only partially available to that mechanism.