Dissociating Cognitive and Motoric Precursors of Human Self-Initiated Action.

Across-trial variability of EEG decreases more markedly before self-initiated than before externally triggered actions, providing a novel neural precursor for volitional action. However, it remains unclear whether this neural convergence is an early, deliberative stage or a late, execution-related stage in the chain of cognitive processes that transform intentions to actions. We report two experiments addressing these questions. Participants viewed randomly moving dots on a screen. At a random time, all dots started moving coherently to the left or right side of the screen. Participants were rewarded for correctly responding to the direction of coherent dot movement. However, the waiting time before coherent dot motion onset could be extremely long. Participants had the option to skip waiting by pressing a "skip" key. These self-initiated "skips" were compared with blocks where participants were instructed to skip. EEG variability decreased more markedly before self-initiated compared with externally triggered "skip" actions, replicating previous findings. Importantly, this EEG convergence was stronger at frontomidline electrodes than at either the electrode contralateral or ipsilateral to the hand assigned to the "skip" action in each block (Experiment 1). Furthermore, convergence was stronger when availability of skip responses was "rationed," encouraging deliberate planning before skipping (Experiment 2). This suggests that the initiation of voluntary actions involves a bilaterally distributed, effector-independent process related to deliberation. A consistent process of volition is detectable during early, deliberative planning and not only during late, execution-related time windows.