In the early 1980s, psychologist Benjamin Libet conducted a relatively simple experiment that critically shaped the way we think about free-will. Participants sat facing a clock, keeping a finger on a button, and were instructed to lift the finger whenever they pleased, remembering the clock time corresponding to the time when they decided to move the finger. All the while, EEG was being recorded. Libet found that 300-500 msec before participants moved (and about 150 msec before reporting that they decided to move), that a strong negative signal was found in the EEG waveforms. If the brain "knows" you are going to move before you do, do you really have conscious control over your own behavior?
The Libet experiment has been replicated, if not uncontroversial among philosophers. However, a new paper in Psychological Science questions whether the readiness potential is an artifact of observing a moving clock.
In the new study, Jeff Miller and colleagues presented participants with two decision-making conditions: a clock condition, similar to that of Libet, and a condition without the clock. If the time between the decision and the movement is constant, then one can look at the EEG from the time to move, with or without the clock. The authors found that participants in the clock condition showed the readiness potential, but the participants in the no-clock condition did not, suggesting that the act of monitoring the clock modulated the EEG signal, not the preparation for making a decision.
I think that this study is innovative as we need new methods to study the time course of decision making. I do think it asks more questions than it answers, though. For example, there are differences between the clock and no-clock groups that go beyond the presence of a clock: being asked to keep a time in mind provides a load to working memory that the no-clock participants did not have.
Is it time to give up on Libet? I'm not so sure. Is it time to reconsider with new methods? Absolutely!
Miller J, Shepherdson P, & Trevena J (2010). Effects of Clock Monitoring on Electroencephalographic Activity: Is Unconscious Movement Initiation an Artifact of the Clock? Psychological science : a journal of the American Psychological Society / APS PMID: 21123855