Conditions determining early modification of motor programmes in response to changes in target location.

Arm movements were studied in response to double-step stimuli in two-dimensional space. The inter-stimulus interval (ISI) between the two successive target presentations varied randomly between 25, 50, 75 and 100 ms. When ISI was 25 or 50 ms movement trajectories were sometimes modified by the second target already at the very onset of the movement. This modification was apparent from a change in the initial movement direction. A change in the initial movement direction occurred when the reaction time of the movement minus ISI was greater than 200 ms. The initial movement direction of these modified movements was in between the first and second target. To explain the results we conclude that 1) the internal representation of target position shifts gradually to its final position and 2) the motor programme uses as the goal of the movement this current internal representation that may not yet have reached that final position.