THE PERCEPTUAL GUIDANCE OF GRASPING A MOVING OBJECT

The purpose of this experiment was to examine the type of strategy subjects used in catching an object approaching on a non-intersection trajectory at different, but predictable, constant velocities. A small treadmill mounted on a table top supported a target dowel to be 'caught' by the subject's hand, which rested at the end of the treadmill. The treadmill produced six constant velocity approaches administered in blocks of trials in ascending order. The object and the subject's finger movements were kinematically examined using a three-dimensional motion analysis system. The results indicated that a type of perceptual-motor coupling between the value of the optical variable tau (tau) at initiation of the catching movement and the duration of the catching movement was being used, rather than a consistent tau strategy. In addition, a relatively constant ratio of the value of tau at initiation and tau at the start of the catching movement was observed across the six object velocities. These results show limitations in the use of a critical tau strategy, at least in this type of task. A functional relationship between the information in tau and how a movement is performed may allow for greater flexibility in the coordination of interceptive acts.