Department of Human Movement Sciences, Vrije Universiteit Amsterdam, Amsterdam, The Netherlands.
J Neurophysiol. 2019 Oct 1;122(4):1578-1597. doi: 10.1152/jn.00123.2019. Epub 2019 Jul 24.
It is tempting to describe human reach-to-grasp movements in terms of two, more or less independent visuomotor channels, one relating hand transport to the object's location and the other relating grip aperture to the object's size. Our review of experimental work questions this framework for reasons that go beyond noting the dependence between the two channels. Both the lack of effect of size illusions on grip aperture and the finding that the variability in grip aperture does not depend on the object's size indicate that size information is not used to control grip aperture. An alternative is to describe grip formation as emerging from controlling the movements of the digits in space. Each digit's trajectory when grasping an object is remarkably similar to its trajectory when moving to tap the same position on its own. The similarity is also evident in the fast responses when the object is displaced. This review develops a new description of the speed-accuracy trade-off for multiple effectors that is applied to grasping. The most direct support for the digit-in-space framework is that prism-induced adaptation of each digit's tapping movements transfers to that digit's movements when grasping, leading to changes in grip aperture for adaptation in opposite directions for the two digits. We conclude that although grip aperture and hand transport are convenient variables to describe grasping, treating grasping as movements of the digits in space is a more suitable basis for understanding the neural control of grasping.
将人类伸手抓握的动作描述为两个相对独立的视动通道，一个将手的运动与物体的位置联系起来，另一个将握力开口与物体的大小联系起来，这种描述很诱人。然而，我们对实验工作的回顾表明，这种框架存在问题，原因不仅仅在于指出了两个通道之间的依赖关系。大小错觉对握力开口没有影响，而且握力开口的可变性并不取决于物体的大小，这表明大小信息不用于控制握力开口。另一种选择是将握力形成描述为控制手指在空间中的运动而产生的。当用手抓握物体时，每个手指的运动轨迹与它移动到自身相同位置轻敲时的轨迹非常相似。当物体发生位移时，这种相似性在快速响应中也很明显。这篇综述为多效应器的速度-准确性权衡开发了一种新的描述，并将其应用于抓握。对每个手指轻敲运动进行棱镜诱导适应，会转移到该手指抓握时的运动中，这为两个手指的适应方向提供了相反的变化，为手指在空间中的运动框架提供了最直接的支持，导致握力开口的变化。我们得出结论，尽管握力开口和手的运动是描述抓握的方便变量，但将抓握视为手指在空间中的运动是理解抓握神经控制的更合适基础。
