Zapping the parietal cortex (in yellow and red) gave brain tumor surgery patients the intention of moving, whereas stimulating the premotor cortex (blue) made them move without realizing it.

    As anyone with a busy schedule can attest, intending to do something and actually doing it are two different things. But your brain doesn't make such neat distinctions, according to a new study. Researchers have found that when you wave at someone, for example, the intention to move your hand creates the feeling of it having moved, not the physical motion itself. The discovery sheds new light on how the brain tracks what the body does.

    Although neuroscience has revealed much about how the brain processes experiences, the origin of intention has remained a mystery. Past studies linked it to the posterior parietal cortex and the premotor cortex, two regions of the brain also associated with motion and awareness of movement, but each region's role and how they work together remained unclear.

    Neuroscientist Angela Sirigu of the Centre de Neuroscience Cognitive in Bron, France, became intrigued by the posterior parietal's role in willed actions when working with patients who had injured that part of their brains. The patients couldn't define when they began to want to move, says Sirigu, because they couldn't monitor their own intention.

    Sirigu joined researchers at the University of Lyon in France and neurosurgeon Carmine Mottolese of Lyon's Hôpital Pierre Wertheimer to take advantage of a common operating room practice. As part of their preparation for surgery, neurosurgeons sometimes electrically stimulate the brains of their patients, who are awake under local anesthetic, to map the brain and minimize surgical complications. During brain tumor surgery on seven patients, Mottolese stimulated their frontal, parietal, and temporal brain regions, and Sirigu's team asked the patients to describe what they felt.

    After stimulation of the parietal cortex, patients reported "wanting" to move their arms, legs, lips, or chest but didn't actually move them. When Mottolese stimulated the same region more intensely, patients believed that they had moved the body parts they'd intended to move even though they hadn't. Stimulating the premotor cortex, on the other hand, resulted in real movements, but the patients were never conscious of their motions.

    The results, reported in tomorrow's issue of Science, suggest that "we need intention to be aware of what we are doing," says Sirigu. The brain's intention and its prediction of what will result from carrying out that intention create our experience of having moved, she says.

    "I think this study is extremely exciting," says Patrick Haggard, a cognitive neuroscientist at University College London in the United Kingdom. "It's quite encouraging to think that there could be a neuroscience of volition," he says. "And this idea of volition is about as central to our nature as it gets."

    電擊頂葉皮層（在圖上黃和紅的部分）使患有腦瘤的病人產生運動的意圖，然而電擊前運動皮層（蘭），可以使他們軀體活動而并不察覺。

    日程安排得很緊的人可以作證，計劃要做的事有時實際上做的卻是另外兩件事。一項新的研究表明，只是因為你的腦子不能做出如此利索的辨別。研究人員發現當你揮手向人致意的時候，譬如，揮動手的意圖只是產生了手已經揮動的感覺，而不是手已經產生物理動作的本身。此項發現進一步揭示了腦子是怎樣跟蹤軀體動作的。

    盡管神經科學已經揭示了很多關于腦子是怎樣處理體驗的，然而意圖的起源至今仍然是個神秘的事情。以往的研究把意圖與后頂葉皮層和前運動皮層聯系在一起，這兩個皮層是大腦中與軀體的動作和活動的認知密切相關的兩個區域。但是，這兩個區域各自的作用, 以及它們兩者之間怎樣協調工作至今還不清楚。

    法國布朗的認知神經科學研究中心的神經系統科學家安吉拉·西里古（Angela Sirigu）在醫治大腦后頂葉受傷的病人時，頭腦的這個部分在意志活動方面的作用激起了她的強烈興趣。西里古說，病人在他們開始想要活動的時候之所以無法得到確認，是因為他們不能監測到他們自己的意圖。

    西里古參加了法國里昂大學和皮埃爾·威斯爾莫里昂醫院的神經外科醫生卡邁因·莫托萊塞(Carmine Mottolese)的研究人員在其通用手術室里所進行的試驗。作為外科手術的部分準備工作，神經外科醫生有時要對施行了局部麻醉，處于清醒狀態的病人作電的刺激，以描繪病人的腦電圖并把外科手術并發癥減小到最低程度。在幾個患有腦瘤病人身上，莫托萊塞用電刺激他們的前額、頂葉和太陽穴等大腦區域，而西里古的研究小組則要求病人描述出他們的感覺。

    在刺激了頂葉皮層以后，病人報告說，他們“要”活動他們的手臂、腿、嘴唇或胸部，然而實際上這些部位并沒有動作。當莫托萊塞在同一個大腦部位加大了刺激的強度后，病人認為他們已經移動了打算活動的那部分軀體，盡管這些部位仍然沒有什么動作。另一方面，如果用電刺激了前運動皮層，結果是軀體的一些部位真的出現了動作，可是病人卻并沒有能意識到。

    明天發行的《自然科學》雜志里所報道的研究結果表明，“我們需要的是，意識到我們正在做什么的意圖”， 西里古說。她還說，“大腦的意圖和實現這種意圖所得到結果的預測，它們構建了已經產生動作的體驗。”

    英國倫敦大學學院認知神經學家帕特里克·合格德(Patrick Haggard)說，“我認為此項研究是極為鼓舞人心的。它正在鼓勵人們去思考那里可能存在著意志的神經系統科學，而意志的概念被認為是人類本質的核心。