Judging an object's velocity when its distance changes due to ego-motion.

This paper examines how one accounts for ones own movements when judging the velocity of a moving object, with emphasis on ego-motion perpendicular to the direction in which the object is moving. The "object" was a square that was tracked with smooth pursuit eye movements as it moved horizontally across a computer screen. Half-way through the presentation, the image on the screen changed in a manner simulating ego-motion in depth. At the same time, the speed with which the square moved across the screen also changed. Subjects were asked to report whether the target moved faster, at the same speed, or more slowly after the simulated ego-motion. The change in target velocity that was required for it to appear to continue to move at the same speed was determined for simulations containing different aspects of the information that is normally at our disposal. The results show that the change in the size of the image of the target, the expansion or contraction of the image of the surrounding, and differences in target motion between the two eyes (giving rise to vergence eye movements), all contribute to rendering the perceived object velocity independent of ego-motion.