Do preparation or control processes result in the modulation to Fitts' law for movements to targets with placeholders?

It is remarkable that the movement time of a goal-directed movement, the result of complex coordination in the nervous system, can be predicted by a simple mathematical equation. That equation is Fitts' law, and it is one of only a few laws that capture human motor performance. It has recently been shown that reaches to targets with placeholders modulate Fitts' law (e.g. Adam et al. in Psychol Sci 17(9):794-798, 2006). The purpose of this study was to further test whether the modulation to Fitts' law is a result of processes related to movement preparation or movement execution. Preparation and control processes were isolated with trajectory analysis; specifically, the durations of the primary submovement and the secondary submovement were selected to reflect the preparation and control processes, respectively. The time available for movement preparation was also manipulated by precuing the target in some blocks. We found that the modulation to Fitts' law in total movement time with target placeholders occurred during the secondary submovement, suggesting that control processes were the locus of the modulation. However, extending the duration of preparation with a precue eliminated the modulation in total movement time, which suggests that preparation processes were the locus of the modulation. Based on these results, it is premature to isolate unequivocally the modulation to either preparation or control processes. The modulation to Fitts' law during the secondary submovement presents the possibility that facilitated online control may contribute to the modulation.