神经科学之舞

如果直译标题“The Neuroscience of Dance”应该是“舞蹈的神经科学”但那样看起来显得像专业文献一样拒人千里了，故而我翻成“神经科学之舞”。
舞，应该是充满激情的活动。这样翻译也想暗示现代神经科学的发展正是如火如荼，同时也指出文章的内容与舞蹈有关。

文章来源：《科学美国人》2008年7月刊（Scientific American July 2008 Volume 299 Number 1）
神经科学之舞
新近的脑成像研究揭开了人类舞蹈技能背后神经机制的神秘面纱

作者：Steven Brown 和 Lawrence M. Parsons
翻译：Runner
关于作者：
Steven Brown是加拿大（安大略省）麦克马斯特大学心理学、神经科学及行为学部，神经艺术实验室主任。他的研究方向是人类沟通交流（包括语言表达、音乐、手语、舞蹈以及情绪表达）的神经学机制。
Lawrence M. Parsons是英国舍菲尔德大学心理系教授。他的研究范围包括小脑的功能、重唱的神经机制、会话中的话轮转换（话轮是人们日常会话的基本单位,话轮转换是会话分析的核心问题.——译者）以及演绎推理。


关键概念：
舞蹈——舞蹈是人类的一种基本表达方式。很可能它是随着音乐发展而发展的，而音乐是产生节奏感的一种方式。舞蹈需要专门的心智技巧：当一部分脑区掌握方向感，指挥我们的躯干运动的时候，另一些脑区则负责调整我们的动作步调使之跟音乐相匹配。

无意识协动——这是一种神经活动的过程。这种神经活动会引起我们的脚无意识地随着音乐节拍轻叩（这也反映了舞蹈是人类的一种天然禀赋）。当皮层下脑区绕开听觉区域逆向传导电活动的时候就会发生无意识协动。

正文：

绝大多数人觉得我们拥有节奏感是理所当然的。是啊，这是再自然不过的事情了：当听到音乐时我们就会轻轻叩脚，或者摇摆身子，我们甚至没有意识到自己的肢体在随着音乐运动。但实际上，这是在进化过程中人类获得的特有的禀赋。其他动物没有也不可能有这样的活动。人类这种无意识协动的天赋正是舞蹈的核心所在。它使得舞蹈成为一种由节奏感、一连串动作和姿态组成的表达方式。到目前为止，舞蹈可以被认为是一致性程度最高的团体活动，没有哪种社会性活动需要人们在时间和空间上达到如此精确的配合和协作。

虽然舞蹈被认为是人类的一种基本的表达方式，但以前它很少引起神经生物学家的关注。最近，研究人员首次在业余舞者和职业舞者身上分别做了脑成像的研究。他们的研究致力于回答这样一些问题：舞蹈家们如何运动？他们是怎样调整步伐的？人们是怎么学会一系列组合动作的？研究结果揭示了哪怕完成最基本的舞步也需要头脑进行复杂的协调配合。

我有节奏感

长期以来，神经科学家研究的都是单一的动作，比如脚踝翻转或者叩指动作。这些研究让我们知道了大脑是怎样协调简单的动作。就拿单脚跳这个简单的动作来说——更别说让你边跳边拍脑袋这样复杂的组合动作了——就需要运用大脑的感觉运动系统来感知测算空间，平衡体态，确认目标，估计时间等等。简单来说，视觉信息由一处叫作后顶叶皮层的区域（位于脑部后方）转化为行动的命令，再把这些信号传递到前运动皮质和辅助运动区那些编排动作的区域，然后这些指令被投射到皮层主运动区。由主运动区产生的神经冲动沿脊髓传递到肌肉，并且引起肌肉的收缩从而产生动作。同时，肌肉中的感受器会把身体确切的空间相位信息通过脊髓传递到大脑皮层。另外，位于脑后方的小脑皮层下环路和位于脑中央的基底神经节环路都会依据感受到的反馈随时修正运动指令，从而使我们实际做出的动作更为精准。然而问题是，对于像芭蕾舞步这样优美而复杂的动作是否也是由同样的神经机制来控制完成的呢？
为了解答这个问题，我们与德州健康科学中心的Michael J. Martinez合作，第一次利用脑成像技术来研究舞蹈运动。正电子发射断层扫描成像技术（PET）可以用来记录大脑血流的变化，这种血流变化反映了大脑活跃的程度。如果记录到大脑的某个区域血液供应有所增加则反映了这个区域的神经元变得更活跃了。我们的男女各五名受试者都是业余的探戈舞蹈爱好者。我们的受试者头部固定，平躺在PET扫描仪上做脑部PET扫描，但他们的腿可以活动，脚也可以在一个倾斜的面板上面滑动。第一次扫描的时候我们让受试者一边用耳机听探戈舞曲，一边用脚在斜面上完成一种格子舞步，这种舞步是来源于阿根廷探戈里一种叫作salida的基本步法。然后我们在同样的条件下对受试者进行第二次脑部扫描，只是这次我们记录的是受试者腿部肌肉随着音乐刚开始收缩和舒张，而腿部还没有产生实际空间运动时的信号。这样，把两次扫描记录到的信号区域相减就可以追踪到与腿部空间运动相关并产生特定运动模式的关键脑区。
正如预料的那样，通过这两种信号的比较排除了许多脑部基本的运动区，剩下的是顶叶的部分区域。这些区域在人类和其它哺乳动物对空间感知和定位方面都发挥重要的作用。就跳舞而言，空间感知是主要的运动感觉：你就算闭上眼睛也随时能感觉到自己躯干和四肢的位置。这多亏了存在于我们肌肉组织中的感受器。这些感受器能感知每一个关节的旋转和每一条肌肉上产生的张力，并且把这些信号传递给大脑，大脑再根据这些信号相应地指挥我们的身体做出一系列的反应。在实验中我们观察到楔前叶特别活跃。楔前叶是顶叶区的一部分，它离运动感觉投射区中代表腿部运动感知的那部分区域非常接近。我们相信楔前叶包含一个运动感觉投射的图谱，可以让人们在运动的时候意识到自己的躯体在空间里的定位。无论你是跳华尔兹还是简单地走直线，楔前叶都会以一种“躯体为中心”或者说“自我为中心”的角度在脑中绘制出你的运动路线。
接下来我们把这些扫描结果同那些在没有音乐的条件下扫描记录到的结果进行比较，扣除掉在两种条件下都会被激活的脑部区域后，我们希望能找到真正使我们的动作和音乐协调起来的脑区。这样的比较结果实际上排除掉了大脑所有的运动区，而主要有差异的区域出现在小脑接受脊髓传入的那一部分——小脑蚓部。虽然在有或没有音乐的条件下控制腿部的运动都会动用这个脑区，但是在配合音乐完成舞步时记录到的这个区域的血流量要远远高于没有音乐时的血流量。
虽然我们的研究结果支持这样一种假说，即小脑蚓部所起的作用类似一个中继站，监控并且传导来自大脑不同区域的信息以协调编排我们的动作（参见《科学美国人》2003年八月刊里由James M. Bower 和 Lawrence M. Parsons所写的《重新认识小脑》一文），但是小脑作为一个整体它更像一个节拍器。它接收来自听觉、视觉、躯体感觉系统（躯体感觉系统能使我们针对来自声音、光线、碰触等各种刺激做出相应的动作反应）的感觉传入，并且它像大脑一样也有一个全身的运动感觉投射图。
通过对第二组实验数据（即上述分别在有和没有音乐条件下完成探戈舞步扫描记录到的脑区）进行分析后，我们还获得了意外的结果，而这些结果能更有力地解释为什么人类在听到音乐时会无意识地随着节拍叩脚（无意识协动）。我们发现听觉传导通路的后半部分，即一个叫作内侧膝状核（MGN）的皮层下结构，只在有音乐的情况下才被激活。起先我们以为这种激活是由于听觉刺激，也就是音乐引起的，但是我们用一组对照实验排除了这种可能：对照组的受试者能听到音乐，但他们的腿部没有运动，在这种情况下他们的内侧膝状核并没有被激活。
于是我们推断内侧膝状核的活跃程度跟听觉与运动的同步化直接相关，而不是仅仅跟听觉有关。根据这个发现，我们提出了一个“低端通路”假说。这个假说是指，当听觉的神经信号绕开大脑皮层的高级听觉区域而直接投射到小脑的听觉与计时环路时就会发生无意识协动。

人人都能跳舞吗？

当我们观看和学习舞蹈动作的时候大脑的其它区域也参与其中。伦敦大学的Beatriz Calvo－Merino 和 Patrick Haggard 及其同事研究了当人们观看他们自己所精通的舞蹈时，会不会有某些特定的脑区优先被激活，也就是说跳芭蕾的人在看到芭蕾舞表演时他们的某些脑区被激活，而跳卡泼卫勒舞（一种模仿非洲裔巴西人武术的舞蹈）的人在看同样的芭蕾舞表演时他们的那些脑区就不会被激活。
Beatriz Calvo－Merino 和 Patrick Haggard的研究组分别给芭蕾舞蹈家、卡泼卫勒舞蹈家以及不会跳舞的人观看三秒钟的无声芭蕾舞视频片段或无声卡泼卫勒舞视频片段，同时用核磁共振成像仪对这些受试者进行脑部扫描。研究人员发现舞蹈家的前运动皮层会受到视频刺激的影响，并且只有当他们看到的是他们掌握的舞蹈动作时，他们的前运动皮层才会被激活。其他一些研究结果对这个现象给出了可能的解释。研究人员以前就观察到这样的现象：当人们看到一些简单的动作时，他们的前运动皮层负责完成这些动作的区域不需要真正完成动作就会被激活。这就提示人们能够对看到的动作在内心进行预演，这种预演练习也许可以帮助人们学习并理解新的动作。研究人员想要知道的是，人类行为到底在多大程度上依赖于脑袋里的这种“模仿环路”。
在后续的研究工作中，Calvo-Merino和她的同事们把受试的芭蕾舞蹈家按性别分类，然后给他们看专用于男演员或女演员的芭蕾舞步视频，同时记录比较他们脑部扫描到的信息。和前面的实验结果类似，当男性受试者看到男用舞步而女性受试者看到女用舞步的时候，他们的前运动皮层才被最大程度地激活。
事先在脑海中对动作进行预演的过程对于学习运动技巧来说是非常重要的。达特茅斯大学的Emily S. Cross,和Scott T. Grafton以及他们的同事在2006年研究的问题是，脑中的模仿环路是否在学习的过程中被激活。在为期数周的时间里，研究人员每周都对正在学习复杂现代舞的舞蹈家们进行核磁共振成像扫描。在扫描的过程中受试者要观看五秒钟的视频片段，视频内容要么是受试者已经掌握的动作，要么是其他无关的动作。在看过视频后受试者被要求评价他们自己能在多大程度上完成刚才看到的动作。受试者前运动皮层的活跃程度在学习训练的过程中不断增加，与之相关的是，他们也越来越肯定自己有能力完成在视频里看到的舞蹈动作。这些结果进一步证实了Calvo-Merino和她同事们的研究结果。
双方的研究都突出了这样一个事实：学习一系列复杂的动作不但直接激活了运动系统来控制肌肉的收缩，而且也激活了一套运动规划系统。这套运动规划系统包含关于躯体完成特定运动的能力的信息。一个人在某种运动模式上越老练他就越容易想象在这种运动模式下产生的感觉，也就越容易完成那些动作。
我们的研究结果表明，在头脑中模仿一个动作——不管是跳舞也好，打网球发球也好还是挥杆打高尔夫球也好——不但是一个视觉的活动，同时也是一个运动感觉的活动。而要真正掌握一个动作还需要肌肉的感觉系统参与，需要有对运动的想象，也需要存在于大脑中的运动规划区的参与。

摇摆，恰恰以及社会角色

也许最令神经生物学家着迷的一个问题就是人类为什么跳舞。音乐和舞蹈是紧密联系的，在许多情况下音乐甚至是在舞蹈中产生。墨西哥城的阿兹特克人在舞蹈节上戴的绑腿挂满了恰恰果。他们每挪动一步都会发出声响。在许多其他的文化里，人们在跳舞时都会在身上或衣服上佩戴诸如响片、珠子等等能够发出声音的物件，并且一边跳舞一边拍手，打响指，跺脚。因此我们提出了一个“躯体打击乐”假说，这个假说认为舞蹈最初是从一些发声现象演化而来，并且舞蹈和音乐特别是打击乐，共同作为一种产生节奏感的方式而演化发展。第一件打击乐器也许就是来源于舞蹈服饰上的某些物件，类似于阿兹特克人佩戴的恰恰果。
然而跟音乐不同的是，舞蹈具有很强的模仿和表现的作用，这就提示也许舞蹈曾经是一种早期的语言形式。舞蹈的确是肢体语言的精髓。在我们的研究过程中还发现一个有趣的现象，当受试者在完成任何一个规定动作的时候，我们都发现他们的右侧大脑半球对应于左侧大脑半球布罗卡区的位置被激活，而布罗卡区是左侧大脑额叶里跟语言表达能力直接相关的区域。过去十多年的研究发现，布罗卡区也控制手语表达。
这个发现奠定了语言进化中所谓的手势理论。这个理论的拥护者主张，语言在成为一种发声的表达方式之前是由手势表达系统演化而来的。我们的研究首先发现了腿部运动会激活右侧大脑半球对应于布罗卡区的部分，这对于舞蹈起初是一种表达交流的形式的观点是一个有力的支持。
右侧大脑半球上与布罗卡区对称的那部分脑区在人类获得舞蹈能力的过程中到底起什么作用呢？答案似乎跟语言能力没有直接的关系。加州大学的Marco Iacoboni及其同事在2003年的一项研究里运用“重复经颅磁刺激”暂时性地破坏布罗卡区或其在右侧大脑半球的对称区域的功能，在这两种情况下受试者的右手在模仿别人指头活动时也都出现了障碍。（“重复经颅磁刺激”是一种无创性的激活脑神经细胞的方法。其原理是通过快速改变磁场引起的电磁效应可以在脑组织中产生微弱的电流，从而激活特定区域的神经细胞。这种方法被用于脑部环路功能和联系的研究，也被用于多种脑部疾病的治疗——译者）Iacoboni的研究组得出的结论是，这些脑区在模仿过程中是不可或缺的，因此这些区域在人类学习和传播文化的行为中也起关键作用。我们提出了另一种假说。虽然我们的研究实质上并不是针对模仿动作，但跳探戈和模仿别人指头动作都要求大脑准确地组织一系列相互关联的运动。那么类似于布罗卡区帮助我们组织词句一样，它的对称区域也许就负责把运动单元组织起来成为一系列流畅的动作。
舞蹈、语言和音乐三者的起源问题一直都纠缠不清。我们希望未来的神经影像学研究能够增加我们对舞蹈的起源和演化背后的大脑机能的了解。我们把舞蹈看作是语言表达能力和音乐节奏感的结合，这种结合不但使人们可以用身体来表达思想，而且在大家共同起舞的时候，人们之间的凝聚力也加强了。

附英文原文：
brain biology
The Neuroscience of Dance
Recent brain-imaging studies reveal some of the complex neural choreography behind our ability to dance.
By Steven Brown and Lawrence M. Parsons
[THE AUTH ORs]
Steven Brown (left) is director of the NeuroArts Lab in the department of psychology, neuroscience and behavior at McMaster University in Ontario. His research focuses on the neural basis of human communication, including speech, music, gesture, dance and emotion.
Lawrence M. Parsons (right) is a professor in the department of psychology at the University of Sheffield in England, where his research includes studying the function of the cerebellum and the neuroscience of dueting, turn-taking in converstaion and deductive inference.
KEY CONCEPTS n
Dance is a fundamental form of human expression that likely evolved together with music as a way of generating rhythm.
n It requires specialized mental skills. One brain area houses a representation of the body’s orientation, helping to direct our movements through space; another serves as a synchronizer of sorts, enabling us to pace our actions to music. N
Unconscious entrainment—the process that causes us to absent- mindedly tap our feet to a beat— reflects our instinct for dance. It occurs when certain subcortical brain regions converse, bypassing higher auditory areas.
—The Editors
Tantalizing Tango Finding
In a study published in December 2007, Gammon M. Earhart and Madeleine E. Hackney of the Washington University School of Medicine in St. Louis found that tango dancing improved mobility in patients with Parkinson’s disease. The condition stems from a loss of neurons in the basal ganglia, a problem that interrupts messages meant for the motor cortex. As a result, patients experience tremors, rigidity and difficulty initiating movements they have planned. The researchers found that after 20 tango classes, study subjects “froze” less often. Compared with subjects who attended an exercise class instead, the tango dancers also had better balance and higher scores on the Get Up and Go test, which identifies those at risk for falling.
Ballet for Better Balance?
Roger W. Simmons of San Diego State University has found that, when thrown off balance, classically trained ballet dancers right themselves far more quickly than untrained subjects, thanks to a significantly faster response to the disturbance by nerves and muscles. As the brain learns to dance, it also apparently learns to update feedback from the body to the brain more quickly.

So natural is our capacity for rhythm that most of us take it for granted: when we hear music, we tap our feet to the beat or rock and sway, often unaware that we are even moving. But this instinct is, for all intents and purposes, an evolutionary novelty among humans. Nothing comparable occurs in other mammals nor probably elsewhere in the animal kingdom. Our talent for unconscious entrainment lies at the core of dance, a confluence of movement, rhythm and gestural representation. By far the most synchronized group practice, dance demands a type of interpersonal coordination in space and time that is almost nonexistent in other social contexts.
Even though dance is a fundamental form of human expression, neuroscientists have given it relatively little consideration. Recently, however, researchers have conducted the first brainimaging studies of both amateur and professional dancers. These investigations address such questions as, How do dancers navigate though space? How do they pace their steps? How do people learn complex series of patterned movements? The results offer an intriguing glimpse into the complicated mental coordination required to execute even the most basic dance steps.
I Got Rhythm
Neuroscientists have long studied isolated movements such as ankle rotations or finger tapping. From this work we know the basics of how the brain orchestrates simple actions. To hop on one foot—never mind patting your head at the same time—requires calculations relating to spatial awareness, balance, intention and timing, among other things, in the brain’s sensorimotor system. In a simplified version of the story, a region called the posterior parietal cortex (toward the back of the brain) translates visual information into motor commands, sending signals forward to motion-planning areas in the premotor cortex and supplementary motor area. These instructions then project to the primary motor cortex, which generates neural impulses that travel to the spinal cord and on to the muscles to make them contract [see box on next page].
At the same time, sensory organs in the muscles provide feedback to the brain, giving the body’s exact orientation in space via nerves that pass through the spinal cord to the cerebral cortex. Subcortical circuits in the cerebellum at the back of the brain and in the basal ganglia at the brain’s core also help to update motor commands based on sensory feedback and to refine our actual motions. What has remained unclear is whether these same neural mechanisms scale up to enable maneuvers as graceful as, say, a pirouette.
To explore that question, we conducted the first neuroimaging study of dance movement, in conjunction with our colleague Michael J. Martinez of the University of Texas Health Science Center at San Antonio, using amateur tango dancers as subjects. We scanned the brains of five men and five women using positron-emission tomography, which records changes in cerebral blood flow following changes in brain activity; researchers interpret increased blood flow in a specific region as a sign of greater activity among neurons there. Our subjects lay flat inside the scanner, with their heads immobilized, but they were able to move their legs and glide their feet along an inclined surface [see box on page 81]. First, we asked them to execute a box step, derived from the basic salida step of the Argentine tango, pacing their movements to the beat of instrumental tango songs, which they heard through headphones. We then scanned our dancers while they flexed their leg muscles in time to the music without actually moving their legs. By subtracting the brain activity elicited by this plain flexing from that recorded while they “danced,” we were able to home in on brain areas vital to directing the legs through space and generating specific movement patterns.
As anticipated, this comparison eliminated many of the basic motor areas of the brain. What remained, though, was a part of the parietal lobe, which contributes to spatial perception and orientation in both humans and other mammals. In dance, spatial cognition is primarily kinesthetic: you sense the positioning of your torso and limbs at all times, even with your eyes shut, thanks to the muscles’ sensory organs. These organs index the rotation of each joint and the tension in each muscle and relay that information to the brain, which generates an articulated body representation in response. Specifically, we saw activation in the precuneus, a parietal lobe region very close to where the kinesthetic representation of the legs resides. We believe that the precuneus contains a kinesthetic map that permits an awareness of body positioning in space while people navigate through their surroundings. Whether you are waltzing or simply walking a straight line, the precuneus helps to plot your path and does so from a body-centered or “egocentric” perspective.
Next we compared our dance scans to those taken while our subjects performed tango steps in the absence of music. By eliminating brain regions that the two tasks activated in common, we hoped to reveal areas critical for the synchronization of movement to music. Again this subtraction removed virtually all the brain’s motor areas. The principal difference occurred in a part of the cerebellum that receives input from the spinal cord. Although both conditions engaged this area—the anterior vermis—dance steps synchronized to music generated significantly more blood flow there than self-paced dancing did.
Albeit preliminary, our result lends credence to the hypothesis that this part of the cerebellum serves as a kind of conductor monitoring information across various brain regions to assist in orchestrating actions [see “Rethinking the Lesser Brain,” by James M. Bower and Lawrence M. Parsons; Scientific American, August 2003]. The cerebellum as a whole meets criteria for a good neural metronome: it receives a broad array of sensory inputs from the auditory, visual and somatosensory cortical systems (a capability that is necessary to entrain movements to diverse stimuli, from sounds to sights to touches), and it contains sensorimotor representations for the entire body.
Unexpectedly, our second analysis also shed light on the natural tendency that humans have to tap their feet unconsciously to a musical beat. In comparing the synchronized scans with the self-paced ones, we found that a lower part of the auditory pathway, a subcortical structure called the medial geniculate nucleus (MGN), lit up only during the former set. At first we assumed that this result merely reflected the presence of an auditory stimulus—namely, music—in the synchronized condition, but another set of control scans ruled out this interpretation: when our subjects listened to music but did not move their legs, we detected no blood flow change in the MGN.
Thus, we concluded that MGN activity related specifically to synchronization and not simply listening. This finding led us to postulate a “low road” hypothesis that unconscious entrainment occurs when a neural auditory message projects directly to the auditory and timing circuits in the cerebellum, bypassing high-level auditory areas in the cerebral cortex.
So You Think You Can Dance?
Other parts of the brain engage when we watch and learn dance movements. Beatriz Calvo- Merino and Patrick Haggard of University College London and their colleagues investigated whether specific brain areas become active preferentially when people view dances they have mastered. That is, are there brain areas that switch on when ballet dancers watch ballet but not, say, capoeira (an Afro-Brazilian martial art stylized as a dance and performed to music)?
To find out, the team took functional magnetic resonance imaging scans of ballet dancers, capoeira dancers and nondancers as they viewed three-second, silent video clips of either ballet or capoeira movements. The researchers found that expertise had a major influence on the premotor cortex: activity there increased only when subjects viewed dances that they themselves could execute. Other work offers a likely explanation. Investigators have found that when people watch simple actions, areas in the premotor cortex involved in performing those actions switch on, suggesting that we mentally rehearse what we see—a practice that might help us learn and understand new movements. Researchers are examining how widely humans rely on such imitation circuits.
In follow-up work, Calvo-Merino and her colleagues compared the brains of male and female ballet dancers as they watched video clips of either male or female dancers performing gender-specific steps. Again, the highest activity levels in the premotor cortex corresponded to men viewing the male-only moves and to women viewing the female-only moves.
The ability to rehearse a movement in your mind is indeed vital to learning motor skills. In 2006 Emily S. Cross, Scott T. Grafton and their colleagues at Dartmouth College considered whether imitation circuits in the brain increase their activity as learning takes place. Over the course of several weeks, the team took weekly functional MRI scans of dancers as they learned a complex modern dance sequence. During the scans, subjects viewed five-second clips that exhibited either the movements they were mastering or other, unrelated steps. After each clip, the subjects rated how well they thought they could execute the movements they saw. The results affirmed those of Calvo-Merino and her colleagues. Activity in the premotor cortex increased during training and was indeed correlated to the subjects’ assessments of their ability to perform a viewed dance segment.
Both investigations highlight the fact that learning a complex motor sequence activates, in addition to a direct motor system for the control of muscle contractions, a motor-planning system that contains information about the body’s ability to accomplish a specific movement. The more expert people become at some motor pattern, the better they can imagine how that pattern feels and the more effortless it probably becomes to carry out.
As our research shows, however, the ability to simulate a dance sequence—or tennis serve or golf swing—in the mind is not simply visual, as these studies might suggest; it is kinesthetic as well. Indeed, true mastery requires a muscle sense, a motor image, as it were, in the brain’s motion-planning areas of the movement in question.
Shake, Rattle and (Social) Role
Perhaps the most fascinating question for neuroscientists to explore is why people dance in the first place. Certainly music and dance are closely related; in many instances, dance generates sound. Aztec danzantes in Mexico City wear leggings containing seeds from the ayoyotl tree, called chachayotes, which make a sound with every step. In many other cultures, people put noise-making objects—from taps to castanets to beads—on their bodies or clothes while they dance. In addition, dancers frequently clap, snap and stomp. As a result, we have postulated a “body percussion” hypothesis that dance evolved initially as a sounding phenomenon and that dance and music, especially percussion, evolved together as complementary ways of generating rhythm. The first percussion instruments may well have been components of dancing regalia, not unlike Aztec chachayotes.
Unlike music, however, dance has a strong capacity for representation and imitation, which suggests that dance may have further served as an early form of language. Indeed, dance is the quintessential gesture language. It is interesting to note that during all the movement tasks in our study, we saw activation in a region of the right hemisphere corresponding to what is known as Broca’s area in the left hemisphere. Broca’s area is a part of the frontal lobe classically associated with speech production. In the past decade research has revealed that Broca’s area also contains a representation of the hands.
This finding bolsters the so-called gestural theory of language evolution, whose proponents argue that language evolved initially as a gesture system before becoming vocal. Our study is among the first to show that leg movement activates the right-hemisphere homologue to Broca’s area, which offers more support for the idea that dance began as a form of representational communication.
What role might the homologue to Broca’s area have in enabling a person to dance? The answer does not appear to involve speech directly. In a 2003 study Marco Iacoboni of the University of California, Los Angeles, and his colleagues applied magnetic brain stimulation to disrupt function in either Broca’s area or its homologue. In both cases, their subjects were then less able to imitate finger movements using their right hand. Iacoboni’s group concluded that these areas are essential for imitation, a key ingredient in learning from others and in spreading culture. We have another hypothesis as well. Although our study did not involve imitative movements per se, dancing the tango and copying finger actions both demand that the brain correctly order series of interdependent movements. Just as Broca’s area helps us to correctly string together words and phrases, its homologue may serve to place units of movement into seamless sequences.
We hope that future neuroimaging studies will provide fresh insight into the brain mechanisms behind dance and its evolution, which is highly intertwined with the emergences of both language and music. We view dance as a marriage of the representational capacity of language and the rhythmicity of music. This interaction allows people not only to tell stories using their bodies but to do so while synchronizing their movements with others’ in a way that fosters social cohesion.