Reaching movements: mode of motor programming influences programming time by itself.

The present study was undertaken to assess the effect of both target spatial dispersion and mode of motor programming (an implicit strategy, continuous or discrete) on the time needed to initiate accurate arm reaching trajectories. For this purpose, we compared three conditions, in which human subjects were required to reach to one of four possible targets, equidistant from a common origin, in four different directions. The directions were varied across conditions in such a way that (1) the total spatial dispersion of targets varied, or (2) the separations between the medial targets varied, or (3) both (1) and (2) varied. We confirmed that a wider target spatial dispersion determines a lengthening of programming time. The major finding of this work is that, even when target spatial dispersion is kept constant, the continuous mode of programming allows a faster specification of the correct trajectory, while the discrete mode yields a slower programming process. Thus, the mode of motor programming influences the programming time course by itself.