Role of image acceleration in judging landing location of free-falling projectiles.

The vertical acceleration of the projective image of a free-falling object specifies whether the object will land behind or in front of the observation site. Human sensitivity to this visual cue was investigated in 4 studies. Experiments 1 and 2 examined sensitivity to both constant and accelerating vertical acceleration. Detection of acceleration required a total change in velocity that was about 20% of the average velocity. In Experiments 3 and 4, subjects judged where computer-simulated free-falling objects would land relative to the observation site by viewing the initial segment of the flight objects whose trajectories remained in the sagittal plane of the observer. Judgments were influenced significantly by the magnitude and direction of the image velocity change even when no error feedback was available, implicating image acceleration as a source of information for judging the landing site of free-falling objects.