Lateral ball interception: hand movements during linear ball trajectories.

Part of understanding how acts are coordinated is identifying the information that guides movements. In the case of catching a ball within arm's reach, that identification has been complicated by empirical disparities concerning hand-movement reversals during catching. Jacobs and Michaels found unilateral reversals in a paradigm in which balls swung down in an arc; this implicated a particular optical variable, the ratio of lateral velocity to expansion velocity. Montagne et al. reported bilateral reversals when balls approached along a linear trajectory, which implicated a different variable, lateral ball position. The research reported here attempted to replicate Montagne et al.'s findings. In Experiment 1, participants caught balls rolling toward them across a table, under full lighting using monocular or binocular viewing; in Experiment 2, participants caught luminous balls with a luminous glove in an otherwise dark room. Using Montagne et al.'s criterion, we observed no movement reversals in any condition, though some aspects of hand movements suggested the relevance of lateral ball position. The results of Experiment 3, which asked perceivers to indicate only where rods pointed, suggested that lateral position effects were a bias that is unrelated to interception. The ratio of lateral velocity to expansion appears to be a better variable for explaining hand trajectories in lateral interception.