Smooth eye movements with step-ramp stimuli: the influence of attention and stimulus extent

Subjects regarded targets which executed step-ramp motion in one of two conditions, either active (looking at the target) or passive (similar to the subject condition in passive or stare optokinetic nystagmus). The targets jumped 2 deg to one side and returned at a constant velocity. Targets were single round targets or extended targets with a high edge content. Many subjects showed substantial smooth eye movement responses in the passive condition, suggesting that a substantial component of initial smooth movement responses can occur independent of attention. Additional features of the smooth eye movement responses included (i) oculomotor "twitches" (brief, short-latency smooth eye movements in the step direction), (ii) overshoots (eye velocity greater than target velocity), and (iii) oscillations. Most of the response features can be accounted for by a model with a gain that is dependent on the subject's attentional level and on stimulus parameters.