This is the first part in a four part series on the science of free will.
First, some history. Though philosophers have debated free will for over 2000 years, scientists only began to take it on experimentally in the 1980s, when Benjamin Libet (1916-2007), a physiologist at the University of California San Francisco, performed a now-classic experiment. Libet instructed participants to flex their wrists whenever they felt the urge to do so, within a window of a few seconds. Subjects watched a rapidly moving clock and were instructed to note to themselves, and later report to the researchers, the time on the clock when they had come to a decision to move. At the same time, their brain activity was monitored by EEG. Libet was looking for a distinct change in brain activity that he called the "readiness potential," which he believed was an indicator of the brain preparing for movement.
Libet found that the readiness potential appeared, on average, 350 milliseconds before subjects reported that they had made a decision. This meant that the order of the events was: 1) A subject's brain prepared to move the wrist, 2) The subject said to himself, "I have decided to move my wrist," 3) The subject's wrist moved. To Libet, this suggested that the subject's decision was not truly the cause of the movement, since the brain was preparing the movement a fraction of a second before the subject made a conscious decision. Libet came to the conclusion that our conscious control over our actions is limited--or may not exist at all. Wow, sounds like free will just took a pretty hard blow. But did it really?
Jeff Miller of the University of Otago, New Zealand, was set on finding out whether the readiness potential signal was in fact a definite indicator of movement preparation. So, he recreated Libet's experiment, with a twist: This time, subjects did not move on every trial. His team found no evidence of stronger signal before a decision to move than before a decision not to move. They observed the readiness potential both before movements and when no movement happened, meaning it was not a consistent indicator of movement preparation. Since the readiness potential does not cause movement, something else could be the true cause of the movement, "maybe even the person's free will," said Miller.
What exactly does the readiness potential indicate, then? As Miller explained via email, he believes "it reflects some kind of general engagement with this task. I realize that's a very vague answer, but we need to pin down precisely what experimental conditions are necessary to produce this pattern of EEG activity before we can really say what it reflects."
Libet did leave some space in his conclusion for free will to exist, but in a more limited role. He thought there might be conditions in which the conscious mind takes over and "vetoes" spontaneous behavior. He noted that, "subjects have reported some recallable conscious urges were 'aborted.'" In these instances the subject's subconscious presented the urge, or option of how to act, and his conscious mind chose whether or not to act on it.
Critics like neuroscientist John Dylan Haynes, however, argue that the "veto" isn't necessarily a product of free will, either. "Every conscious process, even a veto, will have its brain correlate, its unconscious precursor," says Dylan Haynes. To Dylan Haynes, the very idea of a veto--or, as Mele refers to it, "free-won't"--is an artifact of the discredited philosophy known as "dualism," the notion that mind or consciousness and body or brain/subconscious activity are two separate entities.
Libet's critics also take issue with the design of his experiment, specifically with how subjective the self-reported timing method was. Marcel Brass of the University of Ghent, Belgium has worked on an experiment that proves that a subject's perception of the time of their intention can be manipulated by playing a tone at different intervals after they perform an action. Mark Hallett at the National Institute of Health worked on an experiment that aimed to provide a more objective measure of the time of intention using the subject's real-time decision of whether or not there was a thought to move when a tone occurred.
What did we get out of Libet's studies then? "The work was an excellent stimulus for useful discussions about the challenge of relating neuroscience to philosophical questions about consciousness and free will," says Miller. "Of course these are tough questions and they will not be settled any time soon."
To learn about the more recent work inspired by Libet's experiment, come back to read the next installments in this series.