Six men and six women tracked stimuli that demanded responses of unequal probability. The control-display relation was directionally incompatible. Half the subjects used their non-preferred hand. Many large directional errors occurred in early practice and these were amended after a mean delay of 0·24 sec. With continued practice, small errors persisted mainly in responses of low probability but the mean amendment time fell to 0·11 sec. These errors provided new, highly sensitive measures that revealed differences in performance associated with sex, hand preference and probability (P < 0·01).
The results are compatible with hypotheses that the speed, direction and extent of movement are determined by negative proprioceptive feedback and integral-error control (Gibbs, 1954).