Birznieks I, Burstedt M K, Edin B B, Johansson R S
Department of Physiology, Umeâ University, SE-901 87 Umeâ, Sweden.
J Neurophysiol. 1998 Oct;80(4):1989-2002. doi: 10.1152/jn.1998.80.4.1989.
Previous studies on adaptation of fingertip forces to local friction at individual digit-object interfaces largely focused on static phases of manipulative tasks in which humans could rely on anticipatory control based on the friction in previous trials. Here we instead analyze mechanisms underlying this adaptation after unpredictable changes in local friction between consecutive trials. With the tips of the right index and middle fingers or the right and left index fingers, subjects restrained a manipulandum whose horizontal contact surfaces were located side by side. At unpredictable moments a tangential force was applied to the contact surfaces in the distal direction at 16 N/s to a plateau at 4 N. The subjects were free to use any combination of normal and tangential forces at the two fingers, but the sum of the tangential forces had to counterbalance the imposed load. The contact surface of the right index finger was fine-grained sandpaper, whereas that of the cooperating finger was changed between sandpaper and the more slippery rayon. The load increase automatically triggered normal force responses at both fingers. When a finger contacted rayon, subjects allowed slips to occur at this finger during the load force increase instead of elevating the normal force. These slips accounted for a partitioning of the load force between the digits that resulted in an adequate adjustment of the normal:tangential force ratios to the local friction at each digit. This mechanism required a fine control of the normal forces. Although the normal force at the more slippery surface had to be comparatively low to allow slippage, the normal forces applied by the nonslipping digit at the same time had to be high enough to prevent loss of the manipulandum. The frictional changes influenced the normal forces applied before the load ramp as well as the size of the triggered normal force responses similarly at both fingers, that is, with rayon at one contact surface the normal forces increased at both fingers. Thus to independently adapt fingertip forces to the local friction the normal forces were controlled at an interdigital level by using sensory information from both engaged digits. Furthermore, subjects used both short- and long-term anticipatory mechanisms in a manner consistent with the notion that the central nervous system (CNS) entertains internal models of relevant object and task properties during manipulation.
先前关于在单个手指与物体界面处指尖力对局部摩擦力的适应研究，主要集中在操作任务的静态阶段，在此阶段人类可以依靠基于先前试验中摩擦力的预期控制。在此，我们转而分析连续试验之间局部摩擦力发生不可预测变化后这种适应背后的机制。受试者用右手食指和中指指尖或左右食指指尖，控制一个水平接触面并排设置的操作手柄。在不可预测的时刻，以16N/s的速度向接触面施加沿远侧方向的切向力，直至达到4N的平稳值。受试者可以自由使用两指上法向力和切向力的任何组合，但切向力的总和必须抵消施加的负载。右手食指的接触面是细粒度砂纸，而协同手指的接触面在砂纸和更光滑的人造丝之间变化。负载增加会自动触发两指的法向力响应。当手指接触人造丝时，受试者在负载力增加期间允许该手指发生滑动，而不是提高法向力。这些滑动导致负载力在手指间分配，从而使法向力与切向力的比值能够根据每个手指处的局部摩擦力进行适当调整。这种机制需要对法向力进行精细控制。尽管在更光滑表面上的法向力必须相对较低以允许滑动，但同时不滑动手指施加的法向力必须足够高以防止操作手柄失控。摩擦力的变化对负载斜坡之前施加的法向力以及触发的法向力响应大小的影响在两指上类似，即当一个接触面为人造丝时，两指的法向力都会增加。因此，为了使指尖力独立适应局部摩擦力，通过使用来自两个参与手指的感觉信息，在指间水平上控制法向力。此外，受试者使用了短期和长期的预期机制，其方式与中枢神经系统（CNS）在操作过程中拥有相关物体和任务属性的内部模型这一概念相一致。
