Pursuit eye movements to second-order motion targets

We studied smooth-pursuit eye movements elicited by first-and second-order motion stimuli. Stimuli were random dot fields whose contrast was modulated by a Gaussian window with a space constant of 0.5°. For the first-order stimuli, the random dots simply moved across the screen at the same speed as the window; for the second-order stimuli the window moved across stationary or randomly flickering dots. Additional stimuli which combined first-and second-order motion cues were used to determine the degree and type of interaction found between the two types of motion stimuli. Measurements were made at slow (1°/s) and moderate (6°/s) target speeds. At a velocity of 1°/s the initiation, transition, and steady-state phases of smooth pursuit in response to second-order motion targets are severely affected when compared with the smooth pursuit of firstorder motion targets. At a velocity of 6°/s there is a small but significant deficit in steady-state pursuit of second-order motion targets but not much effect on pursuit initiation.