GRASP STABILITY DURING MANIPULATIVE ACTIONS

The control of adequate contact forces between the skin and an object (grasp stability) is examined for two classes of prehensile actions th at employ a precision grip: lifting objects that are ''passive'' (subj ect only to inertial forces and gravity) and preventing ''active'' obj ects from moving. For manipulating either passive or active objects th e relevant fingertip forces are determined by at least two control pro cesses. ''Anticipatory parameter control'' is a feedforward controller that specifies the values for motor command parameters on the basis o f predictions of critical characteristics, such as abject weight and s kin-object friction, and initial condition information. Through vision , for instance, common objects can be identified in terms of the finge rtip forces necessary for a successful lift according to previous expe riences. After contact with the object, sensory information representi ng discrete mechanical events at the fingertips can (i) automatically modify the motor commands, (ii) update sensorimotor memories supportin g 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 ce ntral nervous system monitors specific, more or less expected peripher al sensory events to produce control signals that are appropriate for the task at its current phase. The control is based on neural modellin g of the entire dynamics of the control process that predicts the appr opriate output for several steps ahead. This ''discrete-event, sensor- driven control'' is distinguished from feedback or other continuous re gulation. Using these two control processes, slips are avoided at each digit by independent control mechanisms that specify commands and pro cess sensory information on a local, digit-specific basis. This scheme obviates explicit coordination of the digits and is employed when ind ependent nervous systems lift objects. The force coordination across d igits is an emergent property of the local control mechanisms operatin g over the same time span.