In a famous paper published in 1983, Libet et al. showed that the recordable cerebral activity (readiness-potential) that precedes a freely voluntary motor act occurs at least several hundred milliseconds before the reported time of conscious intention to act. The actual movement occurs 200-250 msec after the reported time of intention to act. The data are pretty spooky when you think about it. They say that your brain ("it") has started working on a action well before "you" think you are initiating it! This article has sparked a continuing debate on whether we actually have free will. Libet has suggested that the ~200-250 msec period between awareness of intention and the actual action was sufficient to permit a "veto" of the action if it was judged inappropriate. In this interpretation, we might be said to be "free won't" rather than "free will".
Lau et al. have now done a more nuanced version of LIbet's experiments. In a previous paper they showed that, when participants were required to estimate the onset of their intentions using Libet's procedure, the activity in the presupplementary motor area (pre-SMA) was enhanced ~228 msec before motor execution. In their most recent work they show that when participants were required to estimate the onset of their motor executions (instead of their intentions), the activity in the cingulate motor area was enhanced. This latter condition, judged to be more natural and have less task-demanding instructions. The perceived onset of intention could be as late as ~120 msec before the motor execution . "Together, these findings raise the question of whether the conscious control of spontaneous action can be done within a much shorter time window than we had expected, or whether, as suggested by Wegner (The illusion of conscious will Cambridge, MA: MIT Press, 2002), our impression of conscious control is simply illusory."
I think Wegner has it right. His book, and his interpretation of our sense of agency as an after the fact ' emotion of authorship' is a must-read for anyone interested in issues of conscious volition.