Coordinating Initial and Final Action Goals in Planning Grasp-to-Rotate Movements: An ERP Study

Action goals have often been investigated in previous studies within a single action. However, most of the manual actions (such as prehension) are not restricted to a single action towards the object but can involve multiple follow-up actions to achieve a further purpose. The coordination of the initial (grip posture) and final (task purpose) action goals within such complex actions is still not fully understood. In the present experiment, the neural mechanisms underlying the goal coordination were investigated with the help of event-related potentials (ERP). With the "first cue - second cue - imperative signal" design, the action goals were presented separately in different sequences (either "final-initial" or "initial-final"), and participants were instructed to plan and execute a grasp-to-rotate movement with either free-choice or specified grasping. Results revealed that shorter reaction times were needed for the final-initial than for the initial-final trials only when the movement requires a free-choice grasping. At the moment when the goal information was incomplete (at the first cue), final goals evoked a larger anterior P2 than initial goals, whereas initial goals elicited a larger anterior N2 and a more robust frontal negativity (400--550 ms) than final goals. When the goal information was complete (at the second cue), we only found a larger P2 for final goals than for initial goals in free-choice grasping. Moreover, a larger N2 was also found for the specified than for the free-choice grasping in initial-final trials. These neurophysiological results indicate that final goals are more critical than initial grip postures in planning prehensile movements. The initial and final action goals seem to be preferably coordinated in a hierarchical manner, that is, the final task purpose is processed with precedence, whereas the initial grip posture is selected depending on the final task purpose.