MANUAL INTERCEPTION OF MOVING TARGETS .2. ONLINE CONTROL OF OVERLAPPING SUBMOVEMENTS

We studied the kinematic characteristics of arm movements and their re lation to a stimulus moving with a wide range of velocity and accelera tion. The target traveled at constant acceleration, constant decelerat ion, or constant velocity for 0.5-2.0 s, until it arrived at a locatio n where it was required to be intercepted. For fast moving targets, su bjects produced single movements with symmetrical, bell-shaped velocit y profiles. In contrast, for slowly moving targets, hand velocity prof iles displayed multiple peaks, which suggests a control mechanism that produces a series of discrete submovements according to characteristi cs of target motion. To analyze how temporal and spatial aspects of th ese submovements are influenced by target motion, we decomposed the ve rtical hand velocity profiles into bell-shaped velocity pulses accordi ng to the minimum-jerk model. The number of submovements was roughly p roportional to the movement time, resulting in a relatively constant s ubmovement frequency (similar to 2.5 Hz). On the other hand, the submo vement onset asynchrony showed significantly more variability than the intersubmovement interval, indicating that the submovement onset was delayed more following a submovement with a longer duration. Examinati on of submovement amplitude and its relation to target motion revealed that the subjects achieved interception mainly by producing a series of submovements that would keep the displacement of the hand proportio nal to the first-order estimate of target position at the end of each submovement along the axis of hand movement. Finally, we did not find any evidence that information regarding target acceleration is properl y utilized in the production of submovements.