Motion defined exclusively by second-order characteristics does not evoke optokinetic nystagmus.

We showed high-contrast, second-order motion stimuli to subjects whilst recording their horizontal eye movements. These stimuli were very poor at evoking optokinetic nystagmus. Smooth-pursuit eye movements and fixation were reduced by a masking band +/- 2.5 deg above and below an imaginary fixation point. First-order stimuli evoked vigorous optokinetic nystagmus (OKN) under identical conditions and also when matched for apparent contrast. These findings are discussed in terms of the site of detection of second-order motion.