Abstract
Fast arm movements are often described as consisting of two phases. The first is a ballistic phase in which the hand quickly moves to the vicinity of the target. The second is a guidance phase in which the hand is slowly guided towards a suitable position on the target. Visual information about the position of the hand is often only considered to be relevant during the second stage. That obtaining as good visual information about the target as possible is important for fast arm movements is evident from the fact that people generally look at the target. If the target is moving, they sometimes look at its future position. They seldom look at their hand. Presumably, there is no need to do so because they can feel where their hand is. However, people do use peripheral vision to guide their hand, as can be demonstrated by perturbing the position of a cursor representing the hand. People are known to respond to such perturbations. Here, we examine how such responses depend on the stage of the movement. We find that people respond to small perturbations of a representation of their hand in much the same way as they respond to small perturbations of the target’s position. The responses are weaker for the representation of the hand, presumably because of the conflicting haptic information. Importantly, the hand appears to be guided in the same manner throughout its movement. We find no evidence for there being different phases in goal-directed arm movements. Instead, we find that movements are constantly guided by visual information, but that the vigour of the response to a perturbation increases as the time that is available for adjusting the movement to the perturbation decreases.