The impact of task-constraints on the planning and control of interceptive hitting movements.

On the basis of research in self-paced aiming movements, Glover [S. Glover, Separate visual representations in the planning and control of action, Behav. Brain Sci., 27 (2004) 3-24] proposed a dichotomy between visual systems that accommodate planning and on-line control of action. Specifically, the planning-versus-control model posits that the on-line control system solely adjusts the spatial parameters of a movement. We examined whether this proposition is also adequate for interceptive hitting movements that require being at a specific location at the right time. Participants had to hit an approaching ball by first moving the bat away from the interception point (preparatory phase) and subsequently make the hitting movement (strike phase). The ball had to be projected to a landing location that could be near or far. To examine the ability of actors to use online visual information to adapt to unexpected changes in impact requirements, we perturbed the distance of the landing location from near to far during the unfolding of the movement. No adjustments were observed when the perturbation occurred at the onset of the strike phase. When the perturbation occurred at the onset of the preparatory phase the impact velocity increased and, therefore, the ball traveled a larger distance. This was realized by decreasing the duration of the strike phase and increasing the movement amplitude. So, both spatial and temporal characteristics of the hitting movement were adjusted to comply with an online perturbation of target distance. This observation is incongruent with Glover's recent proposition that the online visual system only accommodates spatial parameters.