Spatial trajectories and reaction times of aimed movements: effects of practice, uncertainty, and change in target location.

Abstract

1. The spatial and temporal characteristics of arm movements in two (X-Y) dimensions were studied in three rhesus monkeys a) during the acquisition of an aiming motor skill, b) under conditions of spatial and temporal uncertainty, and c) when the location of the target changed during the reaction or movement time, from 50 to 400 ms after the presentation of the target. 2. The acquisition of the aiming skill was marked by an exponential reduction in the spatial variability of the movement trajectories. The reaction time remained virtually unchanged, whereas the peak velocity increased by a small amount. The skill was transferred appreciably to the other hand within a short period of training. 3. Handpath variability increased under conditions of spatial uncertainty. Temporal uncertainty in target presentation had no effect. Reaction time did not change significantly in either condition. 4. Change of target location during the reaction or movement time elicited a graded movement toward the first target, followed by reversal of direction and movement to the second target. The duration of the movement toward the first target was a linear function of the time that elapsed from the presentation of the first target to the change of targets (interstimulus interval, ISI): the later the change occurred, the longer the movement toward the first target. In contrast to the gradation of hand movement with change in target location, the eyes always fully made a saccade to the first or second target. 5. The trajectory of the initial hand movement deviated occasionally toward the second of the two targets when the interstimulus interval was short and the targets were adjacent. 6. No marked delays were observed, beyond the reaction time, when responding to the first or the second target. A large increase in peak velocity was attained after reversing the movement, i.e., on the way to the second target. 7. The results of this study indicate that the process that generates the aimed movement becomes less variable with practice and is influenced by the uncertainty of the subject about the location of the target but not by the time of its appearance. 8. The orderly modification of the movement produced by change in target location suggests that the aimed motor command is emitted in a continuous, ongoing fashion as a real-time process that can be interrupted at any time by the substitution of the original target by a new one. The effects of this change on the ensuing movement appear promptly, without delays beyond the usual reaction time. No appreciable “psychological refractory period” is observed under these conditions, and the second stimulus has continual and effective access to the process generating the aimed arm movement,