Unconscious brain activity shapes our decisions

Not Exactly Rocket ScienceBy Ed YongApr 13, 2008 10:01 PM


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Our brains are shaping our decisions long before we become consciously aware of them. That's the conclusion of a remarkable new study which shows that patterns of activity in certain parts of our brain can predict the outcome of a decision seconds before we're even aware that we're making one.

It seems natural to think that we carry out actions after consciously deciding to do so. I decide to start typing and as a result, my hands move around a keyboard. But according to modern neuroscience, that feeling of free will may be an illusion. For over twenty years, experiments have suggested that, unbeknownst to us, a large amount of mental processing goes on in unconsciously before we become aware that we intend to act.

The first such study was done by Benjamin Libet in 1983. Libet asked volunteers to press a button at a time of their choice, and to remember the position of the second hand of a wristwatch when they first felt the urge to move. While this happened, Libet measured the activity of their supplementary motor area (SMA), a part of the brain involved in planning movements. Astonishingly, he found that the SMA became active about half a second before the volunteer felt a conscious desire to press the button.

The seminal experiment suggested that the brain makes decisions on a subconscious level and that people only believe that they consciously drove their actions in hindsight. The experiment seemed to put a dent in beliefs about free will and understandably, it has proved to be controversial. Some have criticised the techniques that Libet used, claiming that inaccurate measurements could explain that the small gap between brain activity and conscious awareness.

For those who were convinced by the experiment's results, a slew of important questions remained. Is the SMA the source of the decision, or is it responding to other parts of the brain even higher up the chain of command? And is this unconscious activity a sign that the relevant areas of the brain are revving into readiness, or does it actually predict the action that is eventually taken? Now, Chong Siong Soon and colleagues from the Max Planck Institute have addressed these queries with an elegant update of Libet's work.

Pick a button, any button

Soon asked 36 volunteers to watch a letter in the centre of a computer screen, that changed every half-second. As the letters streamed by, the volunteers had to press one of two buttons, whenever they felt like it. When that happened, they were given a choice of four letters and had to choose the one that was up when they made their button-pressing decision.

As they did this, Soon measured their brain activity using a scanning technique called functional magnetic resonance imaging (fMRI). He then used a statistical technique called pattern recognition to see if he could match the patterns of activity in different parts of the brain at different times with the choice to press the right or left button.

On average, the volunteers took about 22 seconds to press the button and felt that they consciously decided to do so about a second or less before they made the movement. But the fMRI data told a much different story. Two parts of the brain - the frontopolar cortex and the precuneus -showed activity that predicted the choices that the volunteers made and in the frontopolar cortex, this activity happened a whopping 7 seconds before the subjects were consciously aware of their decisions.

These astonishing results suggest that by the time we become consciously aware of a decision to move, our choices have already been influenced for several seconds by the actions of the frontopolar cortex. The study goes well beyond Libet's original work. It shows that this preliminary activity is far from a general and non-specific curiosity, but can actually predict a decision. Nor can it be explained away by inaccuracies in measurement - the timescales involved were far too long for that.

These associations have never been seen until now because neither the frontopolar cortex not the precuneus were more active in total in the time leading up to the button press. Instead, it was the pattern of firing neurons within these areas that predicted the final decision, and it took the use of pattern recognition techniques for this effect come to light.

Chain of events

The involvement of the frontopolar cortex isn't surprising. It fulfils the role of an executive manager and is involved in retrieving memories and controlling other high-level parts in the brain. Soon thinks that it is the source of the decision itself, with the precuneus simply storing the decision until it reaches a conscious level. When he changed his experiment so that volunteers were shown a cue to tell them when to make their choice, the frontopolar cortex still showed predictive activity before the signal, but the precuneus only did so in the time between signal and action.

Soon found, as Libet did, that the SMA was also active before the volunteers became conscious about their intentions. But he also showed that its pattern of activity can predict when the final decision is made. Again, this information is available at an unconscious level about 5 seconds before the volunteers actually move. The frontopolar and parietal cortex aren't involved in timing until the few milliseconds before the movement, so it seems that the two parts of the brain have different and complementary duties. One shapes the outcome of a choice and the other affects its timing.

Soon tentatively suggests that the sparks of a decision begin in the frontopolar cortex. From there, the decision is 'prepared' by the buildup of activity in the precuneus and later, the SMA. It is held there for a short while before we become consciously aware of it and act. This unconscious part of the decision-making process may be for our own good. At least one experiment showed that people with damage to the relevant parts of the brain don't show any signs of unconscious preparation and make poorer decisions in a gambling experiment.

Studies like these have important philosophical implications. If our brains unconsciously make our decisions for us, is there any room for free will? Libet himself thinks so, but only in a restricted way. He asserts that for all the brain's unconscious preparation, people can still consciously decide to stop performing an action in the final milliseconds before thought becomes deed. In this view, it's more a case of "free won't" than free will.

Reference: Soon, C.S., Brass, M., Heinze, H., Haynes, J. (2008). Unconscious determinants of free decisions in the human brain. Nature Neuroscience DOI: 10.1038/nn.2112

Images: Thinker photo by Satyakamk

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