Medendorp W P, Crawford J D, Henriques D Y, Van Gisbergen J A, Gielen C C
Department of Medical Physics and Biophysics, University of Nijmegen, NL 6525 EZ Nijmegen, The Netherlands.
J Neurophysiol. 2000 Nov;84(5):2302-16. doi: 10.1152/jn.2000.84.5.2302.
This study addressed the question of how the three-dimensional (3-D) control strategy for the upper arm depends on what the forearm is doing. Subjects were instructed to point a laser-attached in line with the upper arm-toward various visual targets, such that two-dimensional (2-D) pointing directions of the upper arm were held constant across different tasks. For each such task, subjects maintained one of several static upper arm-forearm configurations, i. e., each with a set elbow angle and forearm orientation. Upper arm, forearm, and eye orientations were measured with the use of 3-D search coils. The results confirmed that Donders' law (a behavioral restriction of 3-D orientation vectors to a 2-D "surface") does not hold across all pointing tasks, i.e., for a given pointing target, upper arm torsion varied widely. However, for any one static elbow configuration, torsional variance was considerably reduced and was independent of previous arm position, resulting in a thin, Donders-like surface of orientation vectors. More importantly, the shape of this surface (which describes upper arm torsion as a function of its 2-D pointing direction) depended on both elbow angle and forearm orientation. For pointing with the arm fully extended or with the elbow flexed in the horizontal plane, a Listing's-law-like strategy was observed, minimizing shoulder rotations to and from center at the cost of position-dependent tilts in the forearm. In contrast, when the arm was bent in the vertical plane, the surface of best fit showed a Fick-like twist that increased continuously as a function of static elbow flexion, thereby reducing position-dependent tilts of the forearm with respect to gravity. In each case, the torsional variance from these surfaces remained constant, suggesting that Donders' law was obeyed equally well for each task condition. Further experiments established that these kinematic rules were independent of gaze direction and eye orientation, suggesting that Donders' law of the arm does not coordinate with Listing's law for the eye. These results revive the idea that Donders' law is an important governing principle for the control of arm movements but also suggest that its various forms may only be limited manifestations of a more general set of context-dependent kinematic rules. We propose that these rules are implemented by neural velocity commands arising as a function of initial arm orientation and desired pointing direction, calculated such that the torsional orientation of the upper arm is implicitly coordinated with desired forearm posture.
本研究探讨了上臂的三维（3-D）控制策略如何依赖于前臂的动作。受试者被指示将与上臂成一直线连接的激光指向各种视觉目标，使得上臂的二维（2-D）指向方向在不同任务中保持恒定。对于每个这样的任务，受试者保持几种静态上臂-前臂配置中的一种，即每种配置都有一组特定的肘部角度和前臂方向。使用三维搜索线圈测量上臂、前臂和眼睛的方向。结果证实，东德斯定律（三维方向向量在二维“表面”上的行为限制）并不适用于所有指向任务，即对于给定的指向目标，上臂扭转变化很大。然而，对于任何一种静态肘部配置，扭转变化都显著减小且与先前的手臂位置无关，从而形成一个类似东德斯定律的、较窄的方向向量表面。更重要的是，这个表面的形状（它将上臂扭转描述为其二维指向方向的函数）取决于肘部角度和前臂方向。对于手臂完全伸展或肘部在水平面内弯曲的指向，观察到一种类似利斯廷定律的策略，即以牺牲前臂与位置相关的倾斜为代价，尽量减少肩部相对于中心的旋转。相反，当手臂在垂直平面内弯曲时，最佳拟合表面显示出一种类似菲克扭转的情况，随着静态肘部弯曲程度的增加而持续增加，从而减少前臂相对于重力的与位置相关的倾斜。在每种情况下，这些表面的扭转变化保持恒定，这表明在每个任务条件下，东德斯定律都同样适用。进一步的实验表明，这些运动学规则与注视方向和眼睛方向无关，这表明手臂的东德斯定律与眼睛的利斯廷定律并不协调。这些结果再次表明，东德斯定律是控制手臂运动的一个重要主导原则，但也表明其各种形式可能只是一组更普遍的、依赖于上下文的运动学规则的有限表现。我们提出这些规则是由神经速度命令实现的，这些命令根据初始手臂方向和期望的指向方向产生，其计算方式使得上臂的扭转方向与期望的前臂姿势隐含地协调。
