Abstract
The stimuli for smooth pursuit are thought to be target position and target velocity. These two cues, present in many continuously moving targets, play different roles in motion detection — velocity-sensitive and position-sensitive mechanisms are dominant at slow and fast speeds, respectively. Using a random dot pattern, we examined how position and velocity cues affect smooth pursuit to slow and fast motion. For this target, use of short dot lifetime minimizes position cues (i.e. the individual dots are too ephemeral to be tracked continuously) and maintains velocity cues by spatial integration. Here, we used a random dot pattern moving at slow (4í/s) or fast (16í/s) speeds. At each speed, dot lifetime was set to 45 msec or 1500 msec so that the target contained mainly velocity cues or both velocity and position cues, respectively. Observers tracked the random dot patterns moving either to the left or to the right (randomized from trial to trial, 100% coherence). Smooth pursuit to the fast motion was normal regardless of dot lifetime. For slow motion, however, smooth pursuit was well initiated and maintained only under the long, but not under the short, dot lifetime condition. The results indicate that smooth pursuit eye movements rely primarily on position cues for slow motion and velocity cues for fast motion.
Supported by AFOSR F49620-98-1-0022 and K08 22349082409.