Burstedt M K, Edin B B, Johansson R S
Department of Physiology, University of Umeå, Sweden.
Exp Brain Res. 1997 Oct;117(1):67-79. doi: 10.1007/s002210050200.
We investigated the coordination of fingertip forces in subjects who lifted an object (i) using the index finger and thumb of their right hand, (ii) using their left and right index fingers, and (iii) cooperatively with another subject using the right index finger. The forces applied normal and tangential to the two parallel grip surfaces of the test object and the vertical movement of the object were recorded. The friction between the object and the digits was varied independently at each surface between blocks of trials by changing the materials covering the grip surfaces. The object's weight and surface materials were held constant across consecutive trials. The performance was remarkably similar whether the task was shared by two subjects or carried out unimanually or bimanually by a single subject. The local friction was the main factor determining the normal:tangential force ratio employed at each digit-object interface. Irrespective of grasp configuration, the subjects adapted the force ratios to the local frictional conditions such that they maintained adequate safety margins against slips at each of the engaged digits during the various phases of the lifting task. Importantly, the observed force adjustments were not obligatory mechanical consequences of the task. In all three grasp configurations an incidental slip at one of the digits elicited a normal force increase at both engaged digits such that the normal:tangential force ratio was restored at the non-slipping digit and increased at the slipping digit. The initial development of the fingertip forces prior to object lift-off revealed that the subjects employed digit-specific anticipatory mechanisms using weight and frictional experiences in the previous trial. Because grasp stability was accomplished in a similar manner whether the task was carried out by one subject or cooperatively by two subjects, it was concluded that anticipatory adjustments of the fingertip forces can emerge from the action of anatomically independent neural networks controlling each engaged digit. In contrast, important aspects of the temporal coordination of the digits was organized by a "higher level" sensory-based control that influenced both digits. In lifts by single subjects this control was mast probably based on tactile and visual input and on communication between neural control mechanisms associated with each digit. In the two-subject grasp configuration this synchronization information was based on auditory and visual cues.
(i) 使用右手食指和拇指；(ii) 使用左右食指；(iii) 与另一名受试者合作，使用右手食指。记录了施加在测试物体两个平行握持表面的法向力和切向力以及物体的垂直运动。在各试验组之间，通过更换覆盖握持表面的材料，独立改变物体与手指间每个表面的摩擦力。在连续试验中，物体的重量和表面材料保持不变。无论任务是由两名受试者共同完成，还是由一名受试者单手或双手完成，表现都非常相似。局部摩擦力是决定每个手指与物体界面处法向力与切向力比值的主要因素。无论抓握方式如何，受试者都会根据局部摩擦条件调整力的比值，以便在提起任务的各个阶段，在每个参与的手指上都保持足够的防滑安全裕度。重要的是，观察到的力的调整并非任务必然的机械结果。在所有三种抓握方式中，一个手指的偶然滑动会导致两个参与手指的法向力增加，从而使不滑动手指处的法向力与切向力比值恢复，而滑动手指处的该比值增加。物体提起前指尖力的初始发展表明，受试者利用前一次试验中的重量和摩擦经验，采用了特定手指的预期机制。由于无论任务是由一名受试者完成还是由两名受试者合作完成，抓握稳定性的实现方式相似，因此得出结论，指尖力的预期调整可能源于控制每个参与手指的解剖学上独立的神经网络的作用。相比之下，手指时间协调的重要方面是由影响两个手指的 “更高层次” 的基于感觉的控制来组织的。在单名受试者提起物体的情况下，这种控制很可能基于触觉和视觉输入以及与每个手指相关的神经控制机制之间的通信。在两名受试者的抓握方式中，这种同步信息基于听觉和视觉线索。
