PLANNING AN ACTION

The motor control of a sequence of two motor acts forming an action wa s studied in the present experiment. The two analysed motor acts were reaching-grasping an object (first target) and placing it on a second target of the same shape and size (experiment 1). The aim was to deter mine whether extrinsic properties of the second target (i.e. target di stance) could selectively influence the kinematics of reaching and gra sping. Distance, position and size of both targets were randomly varie d across the experimental session. The kinematics of the initial phase of the first motor act, that is, velocity of reaching and hand shapin g of grasping, were influenced by distance of the second target. No ki nematic difference was found between movements executed with and witho ut visual control of both hand and targets. These results could be due to computation of the general program of an action that takes into ac count extrinsic properties of the final target. Conversely, they could depend on a visual interference effect produced by the near second ta rget on the control of the first motor act. In order to dissociate the effects due to second target distance from those due to visual interf erence, two control experiments were carried out. In the first control experiment (experiment 2) subjects executed movements directed toward s spatial locations at different distances from the first target, as i n experiment 1. However, the near second target was not presented and subjects were required to place the object on an arbitrary near positi on. Distance of the second (either real or arbitrary) target affected the reaching component of the first motor act, as in experiment 1, but not the grasp component. In the second control experiment (experiment 3), the pure visual interference effect was tested. Subjects were req uired to reach and grasp the object and to lift it in either presence or absence of a second near stimulus. No effect on the initial phase o f the first motor act was observed. The results of the this study sugg est a dissociation in the control of reaching and grasping, concerning not only visual analysis of extrinsic properties of the immediate tar get but also visual analysis of the final target of the action. In oth er words, the notion of modularity for the motor control can be extend ed to the construction of an entire action.