Different strategies for using motion in depth information in catching

Relative to the large amount of research on temporal prediction, there have been very few studies that have examined spatial prediction (i.e., how to know where to move your hand to catch an approaching ball). Furthermore, previous studies in this area have had the limitation that the catcher was restricted to using only lateral movement of the hand. We investigated catching behavior in the more natural situation where hand movements were unconstrained. Movements of the index finger and thumb of the right hand were tracked as participants tried to 'catch' a simulated approaching ball. A sensation of motion in depth was created using combinations of changing image size and changing binocular disparity. Five different directions of motion in depth and five different values of ball time to contact (TTC) were randomly interleaved. Our participants used two distinct interception strategies: 'Waiters' (n=4) used only lateral movements of the hand and did not vary the interception point as a function of ball TTC while 'Interceptors' (n=4) used both forward and lateral hand movements. For the 'Interceptors' there was a significant negative correlation between the forward position of the hand and the ball TTC. These findings suggest that interception models based solely on lateral hand position (e.g., Peper et al, 1994) may not generalize to all catchers. In a separate experiment we mismatched the binocular and monocular cues to motion in depth. For both strategy groups, catching behavior suggested that the perceived direction and TTC of the ball was determined by a combination of disparity and size.