Planet Earth

Free Will Starts…Now

The LoomBy Carl ZimmerDec 2, 2003 1:50 AM


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Two decades ago, a neuroscientist named Benjamin Libet published a classic experiment on conscious will. He had his subjects rest a finger on a button as they stared at a specially designed clock. It had only one hand, which swept through a revolution once every 2.5 seconds. Libet would ask his subjects to push the button at their own choosing. In some runs, he asked them to note the position of the clock hand when they actually pushed the button. In other runs they had to note its position when they first began to think about pushing it. Libet measured the brain activity of his subjects with EEG, and also attached electrodes to their hands to monitor their muscle activity. His subjects turned out to be good at timing the moment when they pushed the button, with an accuracy within just a few milliseconds. But they were not so good with their own intentions. Near the top of the brain there's a region known as the motor area where neurons fire to make the body move in particular ways. Libet found that EEG recordings from the motor area in his subjects' brains began to shift into a new pattern 1.5 seconds before the subjects pressed the button. Libet interpreted this as the mental preparation that goes into initiating an action. But his subjects consistently claimed that they began thinking about moving their hand about half a second after the EEG recordings began to change. In other words, they had already started preparing to make a voluntary movement for half a second before they felt like they were making a voluntary movement. A lot of scientists have questioned different parts of Libet's experiments over the years, but the results have held up pretty well. It seems that we only become conscious of our will after we begin to do something. This week a team of European neuroscientists published a fitting tribute to Libet for the twentieth anniversary of his experiment. They ran Libet's experiment again, but some of the people they chose as their subjects had damage to certain parts of the brain. As they report in Nature Neuroscience, some kinds of brain damage make no difference to people's performance. But something fascinating happened to people who suffered damage to the parietal cortex, located at the back of the head. Like the healthy controls, they could nail the moment they actually pressed the button, to within a few milliseconds. But they also noted that they intended to press the button just around the time they actually did press the button. In other words, they were completely unconscious of their action until the action was already taking place. What's most fascinating about this experiment is that the subjects with a damaged parietal cortex do not act all that strangely. They are perfectly able to carry out their will--to press buttons when they want to, to say what they want to say, to walk down the street where they want to go. Their very ordinariness offers a hint about the nature of both consciousness and will. The evidence from many different studies suggests that intentions, plans, and similar thoughts are born within the prefrontal cortex at the front of the brain. These prefrontal neurons send out branches into a number of other regions of the brain, where models of intentions can be created. These models create predictions--if I do this, I should expect this sensory feedback. If I don't get that feedback, I've screwed up. Some of these models may be completely unconscious--they may offer quick checks between what we expect to feel when we do something and what we really feel. There's also a model in the parietal cortex, but the authors of the new study suggest it does something different. It works at a higher level, seeing whether our actions match up with their desired goals. Normally the intentions formed in the prefrontal cortex trigger a model in the parietal cortex as the brain prepares the action in the motor area. Our conscious experience usually depends on the model, and not the intention itself.

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