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domingo, maio 10, 2009

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Um post muito interessante de Ed Yong do Science Blog Not Exactly Rocket Science.

Ed Yong is an award-winning science writer based in London. Not Exactly Rocket Science is his attempt to make the latest scientific discoveries interesting to everyone by beating jargon, confusion and elitism with the stick of good writing. He finds writing about himself in the third person strange and unsettling.

When it comes to the human brain, even the simplest of acts can be counter-intuitive and deceptively complicated. For example, try stretching your arm.

Nerves in the limb send messages back to your brain, but the subjective experience you have of stretching isn't due to these signals. The feeling that you willed your arm into motion, and the realisation that you moved it at all, are both the result of an area at the back of your brain called the posterior parietal cortex. This region helped to produce the intention to move, and predicted what the movement would feel like, all before you twitched a single muscle.

Michel Desmurget and a team of French neuroscientists arrived at this conclusion by stimulating the brains of seven people with electrodes, while they underwent brain surgery under local anaesthetic. When Desmurget stimulated the parietal cortex, the patients felt a strong desire to move their arms, hands, feet or lips, although they never actually did. Stronger currents cast a powerful illusion, convincing the patients that they had actually moved, even though recordings of electrical activity in their muscles said otherwise.

But when Desmurget stimulated a different region - the premotor cortex - he found the opposite effect. The patients moved their hands, arms or mouths without realising it. One of them flexed his left wrist, fingers and elbow and rotated his forearm, but was completely unaware of it. When his surgeons asked if he felt anything, he said no. Higher currents evoked stronger movements, but still the patients remained blissfully unaware that their limbs and lips were budging.

These contrasting responses tell us two important things. Firstly, they tell us that our feelings of free will originate (at least partially) in the parietal cortex. It's the activity of these neurons that creates a sense that we initiate actions of our own accord. Secondly, they show that the sense of moving doesn't depend very much on actually doing so - it depends on calculations that are made in the parietal cortex, long before the action itself begins.

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Dualist philosophers like Descartes believed that the mind and consciousness exist outside the physical world, producing our actions by interacting with the physical meat of our brains. The idea has become commonplace, but it's challenged by neuroscientific studies like this one, which show that the conscious intention to move emerges from electrical activity in neurons, tangible objects that are all too real.

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