THE EFFECTS OF TEMPORAL-PRECISION AND TIME-MINIMIZATION CONSTRAINTS ON THE SPATIAL AND TEMPORAL ACCURACY OF AIMED HAND MOVEMENTS

Discrete aimed hand movements, made by subjects given temporal-accuracy and time-minimization task instructions, were compared. Movements in the temporal-accuracy task were made to a point target with a goal movement time of 400 ms. A circular target then was manufactured that incorporated the measured spatial errors from die temporal-accuracy task, and subjects attempted to contact the target with a minimum movement time and without missing the circular target (time-minimization task instructions). This procedure resulted in equal movement amplitude and approximately equal spatial accuracy for the two task instructions. Movements under the time-minimization instructions were completed rapidly (M = 307 ms) without target misses, and tended to be made up of two submovements. In contrast, movements under temporal-accuracy instructions were made more slowly (M = 397 ms), matching the goal movement time, and were typically characterized by a single submovement. These data support the hypotheses that movement times, at a fixed movement amplitude versus target width ratio, decrease as die number of submovements increases, and that movements produced under temporal-accuracy and time-minimization instructions have different control characteristics. These control differences are related to the linear and logarithmic speed-accuracy relations observed for temporal-accuracy and time-minimization tasks, respectively.