Abstract
When subjects are intentionally preparing a curved trajectory, they are engaged in a time-consuming trajectory planning process that is separate from target selection. To investigate the construction of such a plan, we examined the effect of artificially shortening preparation time on the performance of intentionally curved trajectories using the Timed Response task that enforces initiation of movements prematurely. Fifteen subjects performed obstacle avoidance movements toward one of four targets that were presented 25 or 350 ms before the “go” signal, imposing short and long preparation time conditions with mean values of 170 ms and 493 ms, respectively. While trajectories with short preparation times showed target specificity at their onset, they were significantly more variable and showed larger angular deviations from the lines connecting their initial position and the target, compared to the trajectories with long preparation times. Importantly, the trajectories of the short preparation time movements still reached their end-point targets accurately, with comparable movement durations. We hypothesize that success in the short preparation time condition is a result of an online control mechanism that allows further refinement of the plan during its execution and study this control mechanism with a novel trajectory analysis approach using minimum jerk optimization and geometrical modeling approaches. Results show a later agreement of the short preparation time trajectories with the optimal minimum jerk trajectory, accompanied by a later initiation of a parabolic segment. Both observations are consistent with the existence of an online trajectory planning process.Our results suggest that when preparation time is not sufficiently long, subjects execute a more variable and less optimally prepared initial trajectory and exploit online control mechanisms to refine their actions on the fly.
Highlights
During the process of planning a reaching movement, subjects select a goal for their action (Wolpert and Landy, 2012) and are subsequently engaged in motion planning; either in the form of planning a specific trajectory (Flash and Hogan, 1985; Ganesh et al, 2010) or by designing an optimal control policy, that allows for on-line feedback corrections (Todorov and Jordan, 2002)
For a specific trajectory drawn by the subject, r (t), we examined at each time point t0, what would be the predicted minimum jerk trajectory assumed to be initiated at a time t0 and having at this time position, velocity and acceleration vector values equal to those measured at that time, n.a..mely r = r (t0), v = r (t0), a = r (t0) and j = r (t0)
We argue that the source of the increased variability in the short preparation condition is downstream from the target selection stage and is likely to be the result of an insufficient time for planning the required curved trajectory
Summary
During the process of planning a reaching movement, subjects select a goal for their action (Wolpert and Landy, 2012) and are subsequently engaged in motion planning; either in the form of planning a specific trajectory (Flash and Hogan, 1985; Ganesh et al, 2010) or by designing an optimal control policy, that allows for on-line feedback corrections (Todorov and Jordan, 2002). The concern that reaction time durations do not adequately reflect the cognitive processes underlying motor planning was further supported by a recent study showing that even when subjects are forced to move early, compared to their natural reaction time, they are still capable of producing accurate movements (Haith et al, 2016), suggesting that reaction time might be controlled separately of the movement preparation processes. To overcome this limitation, in this study we investigate the kinematic and geometrical effects that the shortening of the preparation time has on the performed trajectory
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