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."

    电击顶叶皮层（在图上黄和红的部分）使患有脑瘤的病人产生运动的意图，然而电击前运动皮层（兰），可以使他们躯体活动而并不察觉。

    日程安排得很紧的人可以作证，计划要做的事有时实际上做的却是另外两件事。一项新的研究表明，只是因为你的脑子不能做出如此利索的辨别。研究人员发现当你挥手向人致意的时候，譬如，挥动手的意图只是产生了手已经挥动的感觉，而不是手已经产生物理动作的本身。此项发现进一步揭示了脑子是怎样跟踪躯体动作的。

    尽管神经科学已经揭示了很多关于脑子是怎样处理体验的，然而意图的起源至今仍然是个神秘的事情。以往的研究把意图与后顶叶皮层和前运动皮层联系在一起，这两个皮层是大脑中与躯体的动作和活动的认知密切相关的两个区域。但是，这两个区域各自的作用, 以及它们两者之间怎样协调工作至今还不清楚。

    法国布朗的认知神经科学研究中心的神经系统科学家安吉拉·西里古（Angela Sirigu）在医治大脑后顶叶受伤的病人时，头脑的这个部分在意志活动方面的作用激起了她的强烈兴趣。西里古说，病人在他们开始想要活动的时候之所以无法得到确认，是因为他们不能监测到他们自己的意图。

    西里古参加了法国里昂大学和皮埃尔·威斯尔莫里昂医院的神经外科医生卡迈因·莫托莱塞(Carmine Mottolese)的研究人员在其通用手术室里所进行的试验。作为外科手术的部分准备工作，神经外科医生有时要对施行了局部麻醉，处于清醒状态的病人作电的刺激，以描绘病人的脑电图并把外科手术并发症减小到最低程度。在几个患有脑瘤病人身上，莫托莱塞用电刺激他们的前额、顶叶和太阳穴等大脑区域，而西里古的研究小组则要求病人描述出他们的感觉。

    在刺激了顶叶皮层以后，病人报告说，他们“要”活动他们的手臂、腿、嘴唇或胸部，然而实际上这些部位并没有动作。当莫托莱塞在同一个大脑部位加大了刺激的强度后，病人认为他们已经移动了打算活动的那部分躯体，尽管这些部位仍然没有什么动作。另一方面，如果用电刺激了前运动皮层，结果是躯体的一些部位真的出现了动作，可是病人却并没有能意识到。

    明天发行的《自然科学》杂志里所报道的研究结果表明，“我们需要的是，意识到我们正在做什么的意图”， 西里古说。她还说，“大脑的意图和实现这种意图所得到结果的预测，它们构建了已经产生动作的体验。”

    英国伦敦大学学院认知神经学家帕特里克·合格德(Patrick Haggard)说，“我认为此项研究是极为鼓舞人心的。它正在鼓励人们去思考那里可能存在着意志的神经系统科学，而意志的概念被认为是人类本质的核心。