Published online 11 April 2008 | Nature | doi:10.1038/news.2008.751

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Brain activity predicts decisions before they are consciously made.

Think it over: your brain might pre-empt your consciousness when deciding what to do. Punchstock

Your brain makes up its mind up to ten seconds before you realize it, according to researchers. By looking at brain activity while making a decision, the researchers could predict what choice people would make before they themselves were even aware of having made a decision.

The work calls into question the ‘consciousness’ of our decisions and may even challenge ideas about how ‘free’ we are to make a choice at a particular point in time.

“We think our decisions are conscious, but these data show that consciousness is just the tip of the iceberg,” says John-Dylan Haynes, a neuroscientist at the Max Planck Institute for Human Cognitive and Brain Sciences in Leipzig, Germany, who led the study.

“The results are quite dramatic,” says Frank Tong, a neuroscientist at Vanderbilt University in Nashville, Tennessee. Ten seconds is "a lifetime” in terms of brain activity, he adds.

On the button

Haynes and his colleagues imaged the brains of 14 volunteers while they performed a decision-making task. The volunteers were asked to press one of two buttons when they felt the urge to. Each button was operated by a different hand. At the same time, a stream of letters were presented on a screen at half-second intervals, and the volunteers had to remember which letter was showing when they decided to press their button.

When the researchers analysed the data, the earliest signal the team could pick up started seven seconds before the volunteers reported having made their decision. Because of there is a delay of a few seconds in the imaging, this means that the brain activity could have begun as much as ten seconds before the conscious decision. The signal came from a region called the frontopolar cortex, at the front of the brain, immediately behind the forehead.

This area may well be the brain region where decisions are initiated, says Haynes, who reports the results online in Nature Neuroscience1.

The next step is to speed up the data analysis to allow the team to predict people's choices as their brains are making them.

Mind over matter

The results build on some well-known work on free will done in the 1980s by the late neurophysiologist Benjamin Libet, then at the University of California, San Francisco. Libet used a similar experimental set-up to Haynes, but with just one button and measuring electrical activity in his subjects' brains. He found that the regions responsible for movement reacted a few hundred milliseconds before a conscious decision was made.

But Libet's study has been criticized in the intervening decades for its method of measuring time, and because the brain response might merely have been a general preparation for movement, rather than activity relating to a specific decision.

Haynes and his team improved the method by asking people to choose between two alternatives — left and right. Because moving the left and right hands generates distinct brain signals, the researchers could show that activity genuinely reflected one of the two decisions.

But the experiment could limit how ‘free’ people’s choices really are, says Chris Frith, who studies consciousness and higher brain function at University College London. Although subjects are free to choose when and which button to press, the experimental set-up restricts them to only these actions and nothing more, he says. “The subjects hand over their freedom to the experimenter when they agree to enter the scanner," he says.

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What might this mean, then, for the nebulous concept of free will? If choices really are being made several seconds ahead of awareness, “there’s not much space for free will to operate”, Haynes says.

But results aren't enough to convince Frith that free will is an illusion. “We already know our decisions can be unconsciously primed,” he says. The brain activity could be part of this priming, as opposed to the decision process, he adds.

Part of the problem is defining what we mean by ‘free will’. But results such as these might help us settle on a definition. It is likely that “neuroscience will alter what we mean by free will”, says Tong.