Optokinetic nystagmus (OKN) suppression by fixation of a stabilized target: the effect of OKN-stimulus predictability.

In previous work, subjects looked at a target stabilized at the fovea, superimposed on a sinusiodally moving OKN stimulus. The stabilized target (no retinal-slip) suppressed OKN leaving residual eye movements that were often in counterphase with the OKN stimulus motion. In the present study we explored how this type of suppression of OKN is influenced by OKN stimulus predictability: OKN stimulus motion was either sinusoidal or a random walk of half-sinusoids. During fixation of a stabilized target with sinusoidal stimulus motion, OKN was suppressed leaving residual eye movement whose amplitude was typically less than OKN and with a phase lag of about 180 deg (roughly in counterphase with stimulus motion). With random-walk stimulus motion, the residual movement amplitude was even smaller, and at higher frequencies the phase lag decreased to become the same as for OKN. For both stimulus motions, OKN was suppressed when the target was present, but counterphase residual movements appear to depend on stimulus predictability.