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Abstract

Previous work has indicated that people do not use their judgment of a target's speed to determine where to hit it. Instead, they use their judgment of the target's changing position and an expected speed (based on the speed of previous targets). In the present study we investigate whether people also ignore the target's apparent direction of motion, and use the target's changing position and an expected direction of motion instead. Subjects hit targets that moved in slightly different directions across a screen. Sometimes the targets disappeared after 150 ms, long before the subjects could reach the screen. This prevented subjects from using the target's changing position to adjust their movements, making it possible to evaluate whether subjects were relying on the perceived or an expected (average) direction to guide their movements. The background moved perpendicular to the average direction of motion in some trials. This influences the target's perceived direction of motion while leaving its perceived position unaffected. When the background was stationary, subjects hit disappearing targets along their trajectory, just as they hit ones that remained visible. Moving the background affected the direction in which subjects started to move their hand, in accordance with the illusory change in direction of target motion. If the target disappeared, this resulted in a hit that was systematically off the target's trajectory. If the target remained visible, subjects corrected their initial error. Presumably they did so on the basis of information about the target's changing position, because if the target disappeared they did not correct the error. We conclude that people do use the target's perceived direction of motion to determine where to hit it. Thus the perceived direction of motion is treated differently than the perceived speed. This suggests that the motion of an object is not broken down into speed components in different directions, but that speed and direction are perceived and used separately.