Motor programmes for goal-directed movements are continuously adjusted according to changes in target location.

We have studied fast arm movements in response to double-step stimuli in two-dimensional space. In a previous paper we found that such movements did not start in the direction of the first or the second target, but in a direction between the two targets. The initial movement direction was found to depend in a continuous fashion on the inter-stimulus interval and on the reaction time. Therefore we concluded that the internal representation of a discrete target displacement is a gradually shifting internal target, moving from the first to the second target location. In this paper we investigate whether the arm movements also show a modification of the trajectory during the movement. An inter-stimulus interval of 100 ms was chosen, because then the initial movement direction is the same as in the response to a single-step displacement. We found that on average double-step trajectories deviate significantly from their original trajectory within 60 ms, and in some cases even within 30 ms of the start of the movement. We conclude that a motor programme is centrally modified according to a changed target location. We hypothesize that the generation of the motor programme starts after the target presentation, and that the activation levels for the appropriate muscles are continuously adjusted to move the hand in the direction of the current internal representation of the target.