Combination of binocular and monocular visual information in ball catching

When attempting to catch a ball passing to the side, information about the ball's time to passage (TTP) and crossing distance (Xc) is available to the observer from both monocular and binocular retinal image variables. Although it has been demonstrated in passive psychophysical tasks that observers are sensitive to these different variables, it is not clear how these different information sources are combined during active interception. We investigated this question using a simulated catching task. 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. To explore the relative weighting of binocular and monocular we used two separate manipulations (i) cue dissociation: the TTP and Xc specified by monocular information were mismatched with that specified by binocular information, and (ii) selective adaptation: observers adapted to a rate of change of size and/or rate of change of disparity prior to catching trials. Results from both manipulations indicated that for short values of TTP catching movements were based on an average of the TTP and Xc specified by binocular and monocular information. For longer values of TTP, monocular information was given more weight. We also observed some interesting individual differences: observers that used a strategy of reaching forward to intercept the ball well in front of their face weighted monocular information more heavily than observers that caught the ball to the side of their body.