Reschechtko Sasha, Zatsiorsky Vladimir M, Latash Mark L
Department of Kinesiology, Rec.Hall-267, The Pennsylvania State University, University Park, PA, 16802, USA.
Exp Brain Res. 2017 Jan;235(1):1-14. doi: 10.1007/s00221-016-4768-4. Epub 2016 Sep 6.
Manipulating objects with the hands requires the accurate production of resultant forces including shear forces; effective control of these shear forces also requires the production of internal forces normal to the surface of the object(s) being manipulated. In the present study, we investigated multi-finger synergies stabilizing shear and normal components of force, as well as drifts in both components of force, during isometric pressing tasks requiring a specific magnitude of shear force production. We hypothesized that shear and normal forces would evolve similarly in time and also show similar stability properties as assessed by the decomposition of inter-trial variance within the uncontrolled manifold hypothesis. Healthy subjects were required to accurately produce total shear and total normal forces with four fingers of the hand during a steady-state force task (with and without visual feedback) and a self-paced force pulse task. The two force components showed similar time profiles during both shear force pulse production and unintentional drift induced by turning the visual feedback off. Only the explicitly instructed components of force, however, were stabilized with multi-finger synergies. No force-stabilizing synergies and no anticipatory synergy adjustments were seen for the normal force in shear force production trials. These unexpected qualitative differences in the control of the two force components-which are produced by some of the same muscles and show high degree of temporal coupling-are interpreted within the theory of control with referent coordinates for salient variables. These observations suggest the existence of two classes of neural variables: one that translates into shifts of referent coordinates and defines changes in magnitude of salient variables, and the other controlling gains in back-coupling loops that define stability of the salient variables. Only the former are shared between the explicit and implicit task components.
用手操作物体需要精确产生包括剪切力在内的合力；有效控制这些剪切力还需要产生垂直于被操作物体表面的内力。在本研究中，我们调查了在需要产生特定大小剪切力的等长按压任务中，稳定力的剪切分量和法向分量的多手指协同作用，以及这两个力分量的漂移情况。我们假设，根据非控制流形假设中试验间方差的分解评估，剪切力和法向力在时间上会有相似的演变，并且也会表现出相似的稳定性。在稳态力任务（有和没有视觉反馈）和自定节奏的力脉冲任务中，要求健康受试者用手的四根手指精确产生总剪切力和总法向力。在剪切力脉冲产生和关闭视觉反馈引起的无意漂移过程中，这两个力分量都显示出相似的时间分布。然而，只有明确指示的力分量通过多手指协同作用得以稳定。在剪切力产生试验中，未观察到法向力的力稳定协同作用和预期协同作用调整。这两个力分量由一些相同的肌肉产生且表现出高度的时间耦合，对它们控制上这些意外的定性差异，在基于显著变量参考坐标的控制理论中进行了解释。这些观察结果表明存在两类神经变量：一类转化为参考坐标的变化并定义显著变量大小的变化，另一类控制反向耦合回路中的增益，这些增益定义了显著变量的稳定性。只有前者在明确任务和隐式任务组件之间共享。
