Recent research almost unambiguously refutes the hypothesis that the timing of interceptive actions is solely based on the relative rate of expansion [i.e. tau(psi)]. The aim of the present experiment was to evaluated the merits of eight alternative informational variables that recently have been proposed in the literature (i.e. psi, theta, delta, tau(psi), tau(theta), tau(delta), tau(psi, theta), zeta). Participants (n=7) were required to regulate the spatio-temporal characteristics of their reach and grasp to catch a ball approaching on a constant spatial trajectory. To identify the information used to regulate the timing of the catch we examined the qualitative effects of ball speed (0.5, 1.0, 1.5, 2.0, and 2.5 m/s) and viewing (monocular versus binocular) on the kinematics of the catch. Subsequently, we directly assessed the quantitative relationship between informational variables and the timing of reach onset and hand closure. The findings raised serious doubts against the use of variables that specified the time-to-contact between the ball and the point of observation (i.e. relative rate of expansion and disparity). Further, optical variables solely confined to the trajectory of the ball (i.e. the absolute rate of expansion) did yield positive results for the timing of reach onset but not for the timing of hand closure. Only variables that were related to the closure of the gap between hand and ball were found to contribute to the timing of hand closure. These results suggest that information related to the constriction of the optical gap between end-effector and ball becomes more important with approach, whereas the contribution of the absolute rate of expansion becomes less leading.
