Johansson R S, Cole K J
Department of Physiology, University of Umeå, Sweden.
Can J Physiol Pharmacol. 1994 May;72(5):511-24. doi: 10.1139/y94-075.
The control of adequate contact forces between the skin and an object (grasp stability) is examined for two classes of prehensile actions that employ a precision grip: lifting objects that are "passive" (subject only to inertial forces and gravity) and preventing "active" objects from moving. For manipulating either passive or active objects the relevant fingertip forces are determined by at least two control processes. "Anticipatory parameter control" is a feedforward controller that specifies the values for motor command parameters on the basis of predictions of critical characteristics, such as object weight and skin-object friction, and initial condition information. Through vision, for instance, common objects can be identified in terms of the fingertip forces necessary for a successful lift according to previous experiences. After contact with the object, sensory information representing discrete mechanical events at the fingertips can (i) automatically modify the motor commands, (ii) update sensorimotor memories supporting the anticipatory parameter control policy, (iii) inform the central nervous system about completion of the goal for each action phase, and (iv) trigger commands for the task's sequential phases. Hence, the central nervous system monitors specific, more or less expected peripheral sensory events to produce control signals that are appropriate for the task at its current phase. The control is based on neural modelling of the entire dynamics of the control process that predicts the appropriate output for several steps ahead. This "discrete-event, sensor-driven control" is distinguished from feedback or other continuous regulation. Using these two control processes, slips are avoided at each digit by independent control mechanisms that specify commands and process sensory information on a local, digit-specific basis. This scheme obviates explicit coordination of the digits and is employed when independent nervous systems lift objects. The force coordination across digits is an emergent property of the local control mechanisms operating over the same time span.
针对两类采用精确抓握的预抓握动作，研究了皮肤与物体之间足够接触力的控制（抓握稳定性）：提起“被动”物体（仅受惯性力和重力作用）以及阻止“主动”物体移动。对于操纵被动或主动物体，相关的指尖力至少由两个控制过程决定。“预期参数控制”是一种前馈控制器，它根据对关键特征（如物体重量和皮肤与物体间的摩擦力）的预测以及初始条件信息来指定运动命令参数的值。例如，通过视觉，可以根据以往经验，依据成功提起物体所需的指尖力来识别常见物体。与物体接触后，代表指尖离散机械事件的感官信息能够（i）自动修改运动命令，（ii）更新支持预期参数控制策略的感觉运动记忆，（iii）向中枢神经系统告知每个动作阶段目标的完成情况，以及（iv）触发任务后续阶段的命令。因此，中枢神经系统监测特定的、或多或少预期的外周感觉事件，以产生适合当前任务阶段的控制信号。这种控制基于对控制过程整体动态的神经建模，该模型能提前几步预测合适的输出。这种“离散事件、传感器驱动控制”有别于反馈或其他连续调节。通过独立的控制机制，在每个手指上避免打滑，这些机制在局部、特定手指的基础上指定命令并处理感官信息。该方案避免了手指间的明确协调，并且在独立神经系统提起物体时采用。手指间的力协调是在同一时间跨度内运行的局部控制机制的一种涌现特性。
