In 1964, a German experiment asked people to randomly tap their fingers — whenever they wanted — while having their brain's electrical activity monitored. The scientists discovered something nifty: The Bereitschaftspotential, a little burst of electrical activity the subjects' brains gave off in the milliseconds just before the finger-tap. Neuroscientists were fascinated: We now had a glimpse of the brain's crucial planning activities.
In the 80s, things got super weird. The American physiologist Benjamin Libet repeated the experiment and observed the Bereitschaftspotential occurring about 350 milliseconds before the subject decided to move their fingers. In other words, your brain and body were deciding to move your finger before you yourself were aware of your intent to do so. Free will was an illusion.
Armchair philosophers went to town on this, as you can imagine: Consciousness is an illusion! We're well and truly just self-deluding bags of meat! etc etc.
Then in 2012, Aaron Schurger — a scientist at Paris' National Institute of Health and Medical Research — proposed a different explanation for the Bereitschaftspotential. As he knew from his research, the brain is constantly a hive of activity and electrophysiological noise, and like any natural phenomenon with tons of little jittering components, it produces wave-like crests of activity.
So maybe the Bereitschaftspotential was just that. Maybe it was just the product of a noisy brain. A couple of top neuroscientists wondered if the original 1964 finger-tap experiment had been misinterpreted, and its correlations misunderstood. Since the decision to tap your finger randomly isn't terribly consequential, maybe the subjects were unconsciously timing their fingertaps with the Bereitschaftspotential. Or to put it another way, the ebb and flow of the Bereitschaftspotential produced moments of easier activation for the motor system. In other words, the Bereitschaftspotential's timing was just an artifact of that particular experiment.
But what if you did a controlled experiment designed to strip away that artifact? Recently, Schurger did precisely this, and presto — the eerie precognitive timing of the Bereitschaftspotential vanished.
In a new study under review for publication in the Proceedings of the National Academy of Sciences, Schurger and two Princeton researchers repeated a version of Libet's experiment. To avoid unintentionally cherry-picking brain noise, they included a control condition in which people didn't move at all. An artificial-intelligence classifier allowed them to find at what point brain activity in the two conditions diverged. If Libet was right, that should have happened at 500 milliseconds before the movement. But the algorithm couldn't tell any difference until about only 150 milliseconds before the movement, the time people reported making decisions in Libet's original experiment.
In other words, people's subjective experience of a decision—what Libet's study seemed to suggest was just an illusion—appeared to match the actual moment their brains showed them making a decision.
Now, as Gholipour notes, this debunking of the Bereitschaftspotential theory doesn't move us anywhere closer to solving the mystery of free will. That one's still a fantastic mystery. It just indicates that it'll probably be an even deeper mystery than we've assumed for the last few decades.