Contribution of somatosensory input to mechanisms of movement suppression during action observation

执行日常运动，例如抓握物体，看起来和感觉起来毫不费力，但需要肌肉和关节的复杂机械装置以及包括脊髓和大脑回路的复杂控制系统。有越来越多的证据表明，这一控制系统的大部分是活跃的运动观察。二十多年前，Rizzolatti教授和同事发现猕猴大脑F5区的单个神经元不仅对猴子的抓握做出反应，而且还对人类实验者观察到的相同抓握做出反应。这些神经元被称为“镜像神经元”。我们的研究小组对镜像神经元领域做出了重大贡献，他们发现，在F5区和初级运动皮层中，甚至直接与脊髓运动神经元相连的神经元也可以通过观察动作来调节。运动系统在观察动作的过程中是活跃的，但没有产生任何动作，这一明显的悖论挑战了我们对动作是如何产生的理解，并促使研究确定动作执行与动作观察之间的区别，一个明显的候选者是对身体表面或内部状态变化做出反应的体感系统。体感输入在手静止时的抓握观察和手移动并触摸物体时的抓握执行之间明显非常不同。这种差异可能是一个重要的影响因素，为什么运动观察过程中不发生，尽管镜像神经元的活动存在于运动区。一个戏剧性的例子是“橡胶手”错觉，它说明了体感输入对感知的影响有多么强大：受试者手的轻微划动与人造手的同步划动的观察相结合，迫使受试者相信人造橡胶手是他们自己的，并且他们可以控制它。为了理解在动作观察期间体感输入对运动抑制机制的可能贡献，我们建议研究镜像神经元系统（MNS）的躯体感觉特性。这包括单镜像神经元对轻触、梳头和被动手部运动的反应。F5区的一些神经元（发现镜像神经元的地方）已经被证明对这种刺激有反应，但迄今为止还没有对镜像神经元进行测试。更具体地说，我们将研究我们实验室发现的易化和抑制镜像神经元的体感特性差异。众所周知，在运动过程中，我们对感觉输入（如触摸）不太敏感。这种降低的敏感性表现为与休息时的响应相比，响应于神经的电刺激的神经元信号更小。这种现象被称为感觉衰减，但没有在镜像神经元中进行测试。旨在揭示动作观察期间感觉衰减的非侵入性研究是矛盾的。一些研究报告衰减，而其他增强。为了解决这一争议，关键是要研究局部神经元信号水平上的感觉衰减。在经典的MNS研究中，受试者只是被动地观察演员的动作。在真实的生活中，这往往是一种积极的观察。当我们执行一个动作时，我们同时观察到一个类似的动作，例如在一个共享的运动任务中，比如两个外科医生一起工作。在这里，动作观察发生在躯体感觉输入的存在下，这类似于演员的躯体感觉输入。由动作执行和动作观察同时驱动的MNS会发生什么？为了回答这个问题，我们将结合非人类灵长类动物的联合收割机侵入性神经生理学研究和人类志愿者的非侵入性经颅磁刺激（TMS）研究。

Execution of everyday movements as for example grasping of an object appears and feels effortless but requires a sophisticated mechanical machinery of muscles and joints and a complex control system including spinal cord and brain circuits. There is accumulating evidence that large part of this control system is active during movement observation. More than twenty years ago Prof Rizzolatti and colleagues discovered single neurons in area F5 of macaque monkeys brain responding not only to monkey grasping but also to observation of the same grasp performed by human experimenter. These neurons were called "mirror neurons". Our group made a major contribution to the field of mirror neurons by discovering that in area F5 and in primary motor cortex even neurons directly connected to the spinal motor neurons can be modulated just by observation of an action. This apparent paradox of the motor system being active during action observation without production of any movement challenges our understanding of how movements are generated and prompts research to identify what separate movement execution from action observation.An obvious candidate is somatosensory system that responds to changes to the surface or internal state of the body. Somatosensory input is clearly very different between observation of a grasp when hand is at rest and grasp execution when hand is moving and touching an object. This difference might be an important contributing factor to why movements do not occur during action observation despite mirror neuronal activity present in motor areas. A dramatic examples of how powerful the somatosensory input is for perception is the "rubber hand" illusion: light stroking of subject hand combined with observation of synchronous stroking of an artificial hand forces subjects to believe that an artificial rubber hand is their own and that they can control it. To understand the possible contribution of somatosensory input to the mechanisms of movement suppression during action observation, we suggest to investigate the somatosensory properties of the mirror neuron system (MNS). This includes responses of single mirror neurons to light touch, hair brushing and passive hand movement. Some neurons in area F5, where mirror neurons where discovered, have been shown to respond to such stimulation but it was not so far tested for mirror neurons. And more specifically we will investigate differences in somatosensory properties of facilitation and suppression mirror neurons discovered in our laboratory. It is well established that during movement we are less sensitive to the sensory input, eg touch. This reduced sensitivity manifests itself in a smaller neuronal signal in response to electrical stimulation of the nerve in comparison to the response at rest. This phenomenon is called sensory attenuation but it was not tested for mirror neurons. Non-invasive studies aiming at revealing sensory attenuation during action observation are contradictory. Some studies report attenuation while others enhancement. To resolve this controversy it is critical to investigate sensory attenuation on the level of local neuronal signals.In classical studies of the MNS, subjects just passively observe the actors' movements. In real life, it is quite often an active observation. While we execute an action we simultaneously observe a similar action, e.g. in a shared motor task such as two surgeons working together. Here action observation happens in the presence of somatosensory input, which resembles somatosensory input of an actor. What happens to the MNS simultaneously driven by action execution and action observation? To answer this question we will combine invasive neurophysiological investigation of non-human primates with non-invasive transcranial magnetic stimulation (TMS) studies of human volunteers.