Admiraal M A, Medendorp W P, Gielen C C A M
Department Medical Physics and Biophysics, University of Nijmegen, Geert Grooteplein 21, 6525 EZ Nijmegen, the Netherlands.
Exp Brain Res. 2002 Jan;142(2):181-92. doi: 10.1007/s00221-001-0897-4. Epub 2001 Nov 30.
The three rotational degrees of freedom of the head and the upper arm exceed the number needed in a two-dimensional (2-D) facing or pointing task, respectively. Previous studies reported a reduction of the number of degrees of freedom from three to two, with one degree of freedom being a unique function of the other two (Donders' law). This study investigated whether three-dimensional (3-D) orientations of the head and arm are the same at rest and during movement for corresponding pointing or facing directions. Two separate experiments were performed: one focused on head orientations, the other focused on upper arm orientations. We instructed subjects to direct the nose or to point the extended arm in the direction of targets, which appeared in a quasi-random order at 2-s intervals. The head and upper arm orientations at rest were described by a 2-D surface with a scatter less than 3 or 4 degrees, respectively. Both for the arm and the head, orientations started and ended near the 2-D surface, but for a number of the target pairs, the orientations deviated from those predicted by the 2-D surface during movement in a way that was consistent and reproducible for movements between each target pair. For upper arm movements, we often found that deviations of arm orientations from the 2-D surface increased with increasing movement velocity. Such a positive correlation between deviation and movement velocity was not found for head movements. These results clearly indicate violations of Donders' law during movement and argue against several models for movement control found in the literature.
头部和上臂的三个旋转自由度分别超过了二维（2-D）面向或指向任务所需的数量。先前的研究报告称，自由度数量从三个减少到两个，其中一个自由度是另外两个自由度的独特函数（东德斯定律）。本研究调查了头部和手臂在静止和运动时对应指向或面向方向的三维（3-D）取向是否相同。进行了两个独立的实验：一个聚焦于头部取向，另一个聚焦于上臂取向。我们指示受试者将鼻子对准或伸直手臂指向目标方向，目标以准随机顺序每隔2秒出现一次。静止时头部和上臂的取向分别由散度小于3度或4度的二维表面描述。对于手臂和头部，取向在运动开始和结束时都靠近二维表面，但对于一些目标对，在运动过程中，取向偏离了二维表面预测的方向，且这种偏离对于每个目标对之间的运动是一致且可重复的。对于上臂运动，我们经常发现手臂取向与二维表面的偏差随着运动速度的增加而增大。而头部运动未发现偏差与运动速度之间存在这种正相关关系。这些结果清楚地表明运动过程中违反了东德斯定律，并对文献中发现的几种运动控制模型提出了质疑。
