The effect of varying the second target location on movement integration; one-target advantage and target perturbation

The one-target advantage results from control processes associated with the implementation of the second segment during execution of the first. Vision mediates this integration by continually monitoring segment 1; aiding in timing the implementation of segment 2 and reducing movement variability at target 1. Here, we compared the costs to movement integration associated with having to adjust pre-planned movements following an unexpected target perturbation. Participants performed two-target movements with full vision and when vision was occluded at target 1. The location of target 2 remained fixed or was perturbed at movement onset. On perturbed trials, target 2 shifted closer to or further from to the start position. Linear regressions of target 2 error versus movement time 2 on perturbed trials revealed greater y-intercepts for the vision occluded compared to the full vision condition. Hence, the cost in movement time associated with adjusting aiming trajectories following a target shift was greater when participants were denied vision. Furthermore, participants in the vision occluded condition had significantly longer RT's compared to the full vision condition. Thus, the integration of movements involving a perturbation in location of target 2 was easier when visual feedback was available throughout the entire movement. This supports the use of vision to mediate the transition between segments and also highlights continuous visual use during the second segment. However, when vision was occluded during segment 2, participants still planned and executed movements with an interdependent fashion but relied more heavily on the accurate movement planning of movement 2.