Effects of planning duration on planning noise

Faster movements are typically more variable, known as Fitts’ law. Are movements that are initiated faster also more variable? Recent behavioral work suggests that movement preparation and initiation are independent processes, implying that movement variability is independent of reaction time (RT). Neurophysiological work, however, has associated larger neural variability during motor preparation with longer RT and larger movement variability, implying that movement variability decreases with increasing RT. To test between these hypotheses, we recorded over 30000 human reaching movements to visually-cued targets, half of them accompanied by a beep to evoke a wide RT range. Results show that the initial reach variability decreases with increasing RT, for RTs up to ~300 ms, while other kinematics and endpoint accuracy remained unaffected. We conclude that movement preparation time determines movement variability. We suggest that RT differences reflect a trade-off between movement initiation and precision, extending Fitts’ law from movement execution to movement planning.