A new study turns over a new leaf in the hot and curious debate on the existence of free will by removing one of the strongest arguments against it.

The existence of free will came under heavy doubt when experiments in the 1980s revealed the subconscious processes behind our conscious actions. Scientists found that when people are asked to do something (for example, to reach out to press a button spontaneously) their brains start subconsciously preparing for the movement seconds before people themselves become aware of their own decision to move.

This and similar findings led neuroscientists to wonder whether the brain's subconscious process "fates" a person to moving, or if the person still has a choice in the matter once it starts. If the brain activity kicks off an unstoppable process leading to movement, it's hard to argue for free will. How could anyone be free if their brain activity has already decided their actions?

Now the new research shows that people can jump into this subconscious process and consciously control their decision to move until about 200 milliseconds before their muscles contract.

"This has been one of the cornerstone arguments against free will," said John-Dylan Haynes, a neuroscientist at the Charité Medical School in Berlin. "What our experiment shows is that this argument doesn't work. It's a blunt sword."

In the original experiments in the 1980s, neuroscientist Benjamin Libet asked research participants to make spontaneous finger movements while he recorded their brain activity. The results showed a pattern of brain activity—dubbed the "readiness potential"—starting about 500 milliseconds before the movement. Surprisingly, people reported consciously deciding to move only 200 milliseconds before their muscles twitched. In other words, the subconscious appeared to be initiating the decision to move about 300 milliseconds before people thought they'd decided.

In 2008, Haynes and his colleagues found that unconscious brain activity starts to ramp up even earlier, preceding a decision to move by up to 7 seconds. The question, Haynes said, is whether this unconscious brain activity represents an irreversible process. Once the unconscious brain has decided to move, is movement inevitable, like the fall of the last domino in a row after someone flicks over the first? Or can the process be halted, as if someone were to pluck a domino from the middle of the line and interrupt the cascade?

To find out, Haynes and his colleagues Benjamin Blankertz, Matthias-Schultze Kraft and Dan Birman asked participants to play a computer game while their brain waves were measured with an electroencephalogram (EEG). The computer would show the players a green light, and the players had to press a button with their foot before the light turned red. (A foot button was chosen because foot movement produced stronger EEG signals than hand or finger movement.)

In later rounds of the game, the computer got savvier. Using EEG readings of readiness potentials in the players' brains, it predicted when they were about to move and turned the light red in response. The goal was to find out how late in the subconscious brain process participants could change their intention to move in order to beat the game.

The findings, published online December 14 in the journal PNAS, reveal the point of no return: 200 milliseconds before movement.

"Our conscious decisions are not slaves to unconscious brain processes," Haynes said.

In other words, we can stop the fall of the dominoes. But does this ability translate to free will? Haynes says no.

"The Libet experiments have long been used to argue that people are unable to control the influence of unconscious brain processes in their decisions, but our results show that this is not valid," he said. "But the mind is still a brain process."

Haynes, like many neuroscientists, argues that there is no room for true free will in the brain. Just like the heart or the liver, the brain is subject to natural laws, he said. Imagine you are trying to choose between toast and waffles for breakfast one morning. It may feel like a toss-up. But this deterministic view holds that if you could rewind the universe to its beginning and run it right back to the point of the crucial toast versus waffles decision, you'd still make the same choice. (Haynes argues for a "soft" version of determinism, given that physicists have found evidence of true randomness at a quantum level : recreate the exact conditions of a given choice, he says, and you'll face the same probabilities that you'll choose one way or another, which leaves room not for true free will, but for the potential of a different choice.)

The next step for neuroscientists investigating free will in the brain is to level up to more complex decisions. The choice of whether to make a spontaneous movement is much simpler than the choice of where to go to school, or even what to eat for breakfast. Simply studying the point of no return in the laboratory was a complicated experiment that took five years, Haynes said, so moving the studies into the real world will be even tougher.

"We haven't really had an epiphany on how to do that yet," he said.