Ghez Claude, Scheidt Robert, Heijink Hank
Department of Neuroscience, Columbia University Medical Center, New York, NY 10032, USA.
J Neurophysiol. 2007 Dec;98(6):3614-26. doi: 10.1152/jn.00652.2007. Epub 2007 Sep 5.
We previously reported that the kinematics of reaching movements reflect the superimposition of two separate control mechanisms specifying the hand's spatial trajectory and its final equilibrium position. We now asked whether the brain maintains separate representations of the spatial goals for planning hand trajectory and final position. One group of subjects learned a 30 degrees visuomotor rotation about the hand's starting point while performing a movement reversal task ("slicing") in which they reversed direction at one target and terminated movement at another. This task required accuracy in acquiring a target mid-movement. A second group adapted while moving to -- and stabilizing at -- a single target ("reaching"). This task required accuracy in specifying an intended final position. We examined how learning in the two tasks generalized both to movements made from untrained initial positions and to movements directed toward untrained targets. Shifting initial hand position had differential effects on the location of reversals and final positions: Trajectory directions remained unchanged and reversal locations were displaced in slicing whereas final positions of both reaches and slices were relatively unchanged. Generalization across directions in slicing was consistent with a hand-centered representation of desired reversal point as demonstrated previously for this task whereas the distributions of final positions were consistent with an eye-centered representation as found previously in studies of pointing in three-dimensional space. Our findings indicate that the intended trajectory and final position are represented in different coordinate frames, reconciling previous conflicting claims of hand-centered (vectorial) and eye-centered representations in reach planning.
我们之前报道过，伸手动作的运动学反映了两种独立控制机制的叠加，这两种机制分别指定了手部的空间轨迹及其最终平衡位置。我们现在要问，大脑在规划手部轨迹和最终位置时，是否会对手部空间目标保持独立的表征。一组受试者在执行运动反转任务（“切片”）时，学习了围绕手部起始点进行30度的视觉运动旋转，在该任务中，他们在一个目标处反转方向，并在另一个目标处终止运动。这项任务要求在运动过程中准确获取目标。第二组受试者在向单个目标移动并稳定在该目标上（“伸手”）时进行适应训练。这项任务要求准确指定预期的最终位置。我们研究了在这两项任务中的学习如何推广到从未训练过的初始位置做出的动作，以及指向从未训练过的目标的动作。改变初始手部位置对反转位置和最终位置的定位有不同影响：在“切片”任务中，轨迹方向保持不变，反转位置发生位移，而在“伸手”和“切片”任务中，最终位置相对不变。在“切片”任务中跨方向的泛化与期望反转点的以手部为中心的表征一致，正如之前针对此任务所证明的那样，而最终位置的分布与以眼睛为中心的表征一致，这与之前在三维空间指向研究中发现的情况相同。我们的研究结果表明，预期轨迹和最终位置以不同的坐标框架表示，这调和了之前在伸手动作规划中关于以手部为中心（矢量）和以眼睛为中心表征的相互矛盾的说法。
