Temporal properties of preparation phase for arm-pointing movements in various directions.

In this study, we investigated how the temporal properties of the preparation phase for upper limb movements are affected by the reaching direction. Twelve right-handed participants performed three motor tasks, two types of reaching movements, and one finger-lifting movement. Reaching movements were performed from the home position to 15 target locations (5 directions and 3 distances), as quickly and precisely as possible, under two conditions: with and without pre-cueing of the target. The finger lifting movement was performed by lifting the index finger (from the home position) upward in the air as quickly as possible. The reaction time (RT), movement time (MT), and temporal kinematics of the index finger were obtained for each condition. In addition, differential RTs (DRT) were induced, implicitly representing the time required for the motor planning process to execute reaching movements. The results showed that the anisotropy of the DRTs was largest in the left-forward direction, suggesting that the temporal costs of the motor planning process depended on the movement direction. For the kinematic analysis, the MT in the left-forward direction was the largest among all directions. In addition, the time from peak velocity to termination of movement (TFPV) was significantly longer in the left-forward direction when the target pre-cueing was not provided than when it was provided. These results suggest that if a reaching movement under no-cue condition is initiated with insufficient time for the motor planning process, especially in the direction requiring large temporal costs, it relies more heavily on the online control process to accomplish the intended performance. It is likely that humans achieve their intended movements by allocating the amount of time before and after movement initiation according to the difficulty of control that varies in relation to the movement direction.