Human and robotic catching of dropped balls and balloons: Fielders still try to make the image of the projectile rise

When humans navigate to catch balls that are projected toward them in the sagittal plane, they run along a path that keeps the tangent of the optical image of the ball rising at a constant rate. This heuristic, known as optical acceleration cancellation (OAC), produces a near least-energy running solution (provided the optical ball velocity maintained is approximately that initially perceived by the fielder), but it also will guarantee success at other constant optical velocities (provided the fielder can move rapidly enough). In this study, we examined optical behavior of fielders and robotic simulations in the less common case in which a target projectile is dropped, so the initial optical ball velocity is downward. We examined whether fielders attempt to maintain the initial downward optical velocity, or try to run fast enough to reverse the optical direction of the ball (and maintain an arbitrary rising optical speed). Robotic simulations indicated that either strategy can work for slow falling objects like balloons, but for normal balls, realistic lag times do not allow a fielder to move fast enough during the final portion of navigation. Tests with human fielders confirmed that navigating to the interception point of falling objects is quite difficult. Fielders generally selected a running path that did not allow the image of the projectile to descend. For falling balloons this resulted in fielders running rapidly until just a step or two before the point of interception. For falling balls, fielders typically ran as fast as they could. The findings support that fielders try to maintain a rising optical trajectory even in the extreme case of falling balls with initial downward optical trajectories. This further generalizes the range of cases in which OAC is used in interception and confirms that it is not the initial velocity, but the upward direction that appears to be the universal regularity that is maintained while catching.