The effect of optokinetic nystagmus on the perceived position of briefly flashed targets

Stimuli flashed briefly around the time of an impending saccade are mislocalized in the direction of the saccade and also compressed towards the saccadic target. Similarly, targets flashed during pursuit eye movements are mislocalized in the direction of pursuit. Here, we investigate the effects of optokinetic nystagmus (OKN) on visual localization. Subjects passively viewed a wide-field drifting grating that elicited strong OKN, comprising the characteristic slow-phase tracking movement interspersed with corrected "saccade-like" fast-phase movements. Subjects reported the apparent position of salient bars flashed briefly at various positions on the screen. In general, bars were misperceived in the direction of the slow-phase tracking movement. Bars flashed around the onset of the fast-phase movements were subject to much less mislocalization, pointing to a competing shift in the direction of the fast-phase, as occurs with saccades. However, as distinct from saccades, there was no evidence for spatial compression around the time of the corrective fast-phase OKN. The results suggest that OKN cause perceptual mislocalizations similar to those of smooth pursuit and saccades, but there are some differences in the nature of the mislocalizations, pointing to different perceptual mechanisms associated with the different types of eye movements.