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(chǎn)生運(yùn)動(dòng)的意圖，然而電擊前運(yùn)動(dòng)皮層（蘭），可以使他們軀體活動(dòng)而并不察覺(jué)。

    日程安排得很緊的人可以作證，計(jì)劃要做的事有時(shí)實(shí)際上做的卻是另外兩件事。一項(xiàng)新的研究表明，只是因?yàn)槟愕哪X子不能做出如此利索的辨別。研究人員發(fā)現(xiàn)當(dāng)你揮手向人致意的時(shí)候，譬如，揮動(dòng)手的意圖只是產(chǎn)生了手已經(jīng)揮動(dòng)的感覺(jué)，而不是手已經(jīng)產(chǎn)生物理動(dòng)作的本身。此項(xiàng)發(fā)現(xiàn)進(jìn)一步揭示了腦子是怎樣跟蹤軀體動(dòng)作的。

    盡管神經(jīng)科學(xué)已經(jīng)揭示了很多關(guān)于腦子是怎樣處理體驗(yàn)的，然而意圖的起源至今仍然是個(gè)神秘的事情。以往的研究把意圖與后頂葉皮層和前運(yùn)動(dòng)皮層聯(lián)系在一起，這兩個(gè)皮層是大腦中與軀體的動(dòng)作和活動(dòng)的認(rèn)知密切相關(guān)的兩個(gè)區(qū)域。但是，這兩個(gè)區(qū)域各自的作用, 以及它們兩者之間怎樣協(xié)調(diào)工作至今還不清楚。

    法國(guó)布朗的認(rèn)知神經(jīng)科學(xué)研究中心的神經(jīng)系統(tǒng)科學(xué)家安吉拉·西里古（Angela Sirigu）在醫(yī)治大腦后頂葉受傷的病人時(shí)，頭腦的這個(gè)部分在意志活動(dòng)方面的作用激起了她的強(qiáng)烈興趣。西里古說(shuō)，病人在他們開(kāi)始想要活動(dòng)的時(shí)候之所以無(wú)法得到確認(rèn)，是因?yàn)樗麄儾荒鼙O(jiān)測(cè)到他們自己的意圖。

    西里古參加了法國(guó)里昂大學(xué)和皮埃爾·威斯?fàn)柲�里昂醫(yī)院的神經(jīng)外科醫(yī)生卡邁因·莫托萊塞(Carmine Mottolese)的研究人員在其通用手術(shù)室里所進(jìn)行的試驗(yàn)。作為外科手術(shù)的部分準(zhǔn)備工作，神經(jīng)外科醫(yī)生有時(shí)要對(duì)施行了局部麻醉，處于清醒狀態(tài)的病人作電的刺激，以描繪病人的腦電圖并把外科手術(shù)并發(fā)癥減小到最低程度。在幾個(gè)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(yáng)穴等大腦區(qū)域，而西里古的研究小組則要求病人描述出他們的感覺(jué)。

    在刺激了頂葉皮層以后，病人報(bào)告說(shuō)，他們“要”活動(dòng)他們的手臂、腿、嘴唇或胸部，然而實(shí)際上這些部位并沒(méi)有動(dòng)作。當(dāng)莫托萊塞在同一個(gè)大腦部位加大了刺激的強(qiáng)度后，病人認(rèn)為他們已經(jīng)移動(dòng)了打算活動(dòng)的那部分軀體，盡管這些部位仍然沒(méi)有什么動(dòng)作。另一方面，如果用電刺激了前運(yùn)動(dòng)皮層，結(jié)果是軀體的一些部位真的出現(xiàn)了動(dòng)作，可是病人卻并沒(méi)有能意識(shí)到。

    明天發(fā)行的《自然科學(xué)》雜志里所報(bào)道的研究結(jié)果表明，“我們需要的是，意識(shí)到我們正在做什么的意圖”， 西里古說(shuō)。她還說(shuō)，“大腦的意圖和實(shí)現(xiàn)這種意圖所得到結(jié)果的預(yù)測(cè)，它們構(gòu)建了已經(jīng)產(chǎn)生動(dòng)作的體驗(yàn)。”

    英國(guó)倫敦大學(xué)學(xué)院認(rèn)知神經(jīng)學(xué)家帕特里克·合格德(Patrick Haggard)說(shuō)，“我認(rèn)為此項(xiàng)研究是極為鼓舞人心的。它正在鼓勵(lì)人們?nèi)ニ伎寄抢锟赡艽嬖谥�意志的神�?jīng)系統(tǒng)科學(xué)，而意志的概念被認(rèn)為是人類本質(zhì)的核心。