Published online 7 May 2009 | Nature | doi:10.1038/news.2009.454

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Researchers pinpoint a brain area that controls the desire to move.

Scientists have found a way to investigate our intentions. Getty

We control our actions by generating desires and plans. But the process of generating these intentions, known as volition, has been hard to study.

Michel Desmurget, a neuropsychologist at the University of Lyon in France, and his colleagues have found a way around the problem by designing an experiment that probes the neural circuits involved in volition in the brains of fully conscious people. They stimulated different parts of the brain to artificially generate a conscious experience of intending, or wanting, to make an action.

The researchers showed that this intention is processed in a different part of the brain, the parietal cortex, to the one involved in triggering the actual physical movement, the premotor cortex. The results are published in Science1 today.

Stanislas Dehaene, a consciousness researcher at the Collège de France in Paris, says the paper "fills an important gap" in our knowledge of "how we become aware of our actions".

External signals

Neuroscientists had suspected the involvement of the parietal cortex in forming intentions from earlier experiments, for example recording brain activity indirectly from outside the skull while subjects were told to generate an intention to move. But there was a confounding factor. Movement in response to an external signal, rather than in response to a thought, involves quite different neural circuitry. "Yet when we instruct our subjects to generate internally an intention to move — that in itself could in fact be an external signal," says Patrick Haggard, a cognitive neuroscientist at University College London.

The researchers bypassed the problem by taking direct control of the experimental conditions. They carried out tests on the brains of seven patients who were undergoing surgery to remove tumours, and who had volunteered to take part in the experiment.

They electrically stimulated at different sites in the brain while simultaneously measuring electrical activity in 12 muscles in the face, hand, wrist, elbow, knee and foot using an electromyograph. The patients were asked to report when they felt an 'urge' to move, and when they thought they had moved.

Illusion of movement

When the researchers directly stimulated one part of the parietal cortex, patients reported feeling that they would like to move, although they did not do so. When the same area was re-stimulated at a higher intensity, they had the feeling they had already moved — even though they still had not. One, for example, when stimulated at lower intensity, said, "I felt a desire to lick my lips" and when stimulated at a higher intensity, said, "I moved my mouth, I talked, what did I say?".

But patients whose premotor cortex was directly stimulated did move, and sometimes the movements were very large. However, they were unaware that they had done so and were unaware that they had generated any intention to move.

This approach "provides an exciting way to study an experience that comes so close to what it means to be a human being", says Haggard.

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Desmurget thinks that the illusion of having made a movement at higher stimulation intensities may result from wider circuitry in the parietal cortex being drawn in as current increases — areas that simulate in real time how the movement should unfold if the original intention were actually carried out. Normally such computation would be validated by feedback from the premotor cortex which provides information from the body to determine if the hoped for movement has actually happened. "But in the experiment, the parietal cortex is stimulated in isolation — the brain is not getting any error signal, and is relying on the out-dated parietal simulation."

Dehaene says that it is "striking" that the experiment is founded on introspective methods in which what people say about their experiences is treated as data. He thinks that "neuroscience needs to return to a quantified form of introspection in order to make progress on the front of consciousness".