Contributions from Impulse and Limb-Target Regulation to Schmidt's Linear Speed-Accuracy Tradeoff

The purpose of the present study was to make inferences about the sensorimotor mechanisms that result in the linear speed-accuracy tradeoff. Theoretical and empirical research have suggested that movements in Schmidt's task involve minimal online control. We tested this hypothesis with three types of trajectory analysis that could differentiate online control from impulse and limb-target regulation. Thirty participants made discrete pointing movements with goal movement times and amplitudes that ranged from 333 to 194 ms and 100 to 300 mm, respectively. Linear mixed-effects model analysis revealed a strong linear relationship between mean velocity and variable spatial error. Proportion of movement time after maximum velocity and the number of secondary submovements suggested that most conditions had minimal limb-target regulation. However, spatial variability throughout movement execution suggested that all conditions involved online control. This online control without limb-target regulation was likely the result of impulse regulation. We suggest that impulse regulation was used because it can modulate movement velocity without a large cost to temporal precision. This contributes to the primary and secondary goals of Schmidt's tasks, which are temporal precision and spatial accuracy. Future theoretical models of Schmidt's law should distinguish between online control from impulse and limb-target regulation.