Feedforward Control Processes

Almost everyone will recall having fallen victim to an older sibling, cousin, or friend who passed us an empty box while pretending it was very heavy. When we took the box, our arms flailed upwards. This trick demonstrates that when we interact with objects, we anticipate the forces required to complete the task. Although it may occasionally result in large movement errors, anticipatory or feedforward control is essential for skilled object manipulation. Feedback control is important when our predictions are erroneous or, as in reactive tasks, when predictions are unavailable. However, because of the long time delays, feedback control cannot support the swift and skilled coordination of fingertip forces observed in most manipulation tasks that involve ordinary "passive" objects. Instead, the brain relies on feedforward control mechanisms that take advantage of the stable and predictable physical properties of these objects. These mechanisms parametrically adapt force motor commands to the relevant physical properties of the target object.

Figure 3 illustrates parametric anticipatory adjustments of motor output to object weight, friction between the object and skin, and shape of the contact surface. The task is to lift a test object from a support surface, hold it in air for a couple of seconds, and then replace it. To accomplish this task, the vertical load force increases until liftoff occurs, stays constant during the hold phase, and then starts to decrease when the object contacts the support surface during replacement. When lifting objects of different weight (Fig. 3A), people scale the rate of increase of both grip force and load force to object weight such that lighter and heavier objects tend to be lifted in about the same amount of time. The scaling occurs prior to liftoff— before sensory information about object weight becomes available-and is therefore predictive. To deal with changes in friction, the motor system adjusts the balance between grip force and load force. As shown in Fig. 3B, when lifting equally weighted objects of varying slipperiness, people scale the rate of increase of grip force while keeping the rate ofchange ofload force constant. Thus, the ratio of these force rates is a controlled parameter that is set to the current frictional conditions. A similar scaling of the grip-to-load force ratio is observed when object shape is varied. A larger

Figure 3 Feedforward adjustments of motor output to object weight (A), frictional conditions (B), and object shape (C) in a task in which a test object is lifted with a precision grip, held in air, and then replaced. The top graphs show horizontal grip force, vertical load force, and the vertical position of the object as a function of time for two superimposed trials. The bottom graphs show the relation between load force and grip force for the same trials. The dashed line indicates the minimum grip-to-load force ratio required to prevent slip. The gray area represents the safety margin against slip. After contact with the object (left most vertical line, top), grip force increases by a short period while the grip is established. A command is then released for simultaneous increases in grip and load force (second vertical line). This increase continues until the load force overcomes the force of gravity and the object lifts off (third vertical line). After replacement of the object and table contact occurs (fourth line), there is a short delay before the two forces decline in parallel (fifth line) until the object is released (sixth line) (adapted with permission from Johansson, R. S., andWestling, G., Exp. Brain Res. 56,550-564,1984 by Springer-Verlag; Johansson, R. S., and Westling, G., Exp. Brain Res. 71, 59-71, 1988. Copyright © 1988 by Springer-Verlag; and Jenmalm, P., and Johansson, R. S., J. Neurosci. 17, 4486-4499, 1997 Copyright © 1997 by the Society for Neuroscience).

Figure 3 Feedforward adjustments of motor output to object weight (A), frictional conditions (B), and object shape (C) in a task in which a test object is lifted with a precision grip, held in air, and then replaced. The top graphs show horizontal grip force, vertical load force, and the vertical position of the object as a function of time for two superimposed trials. The bottom graphs show the relation between load force and grip force for the same trials. The dashed line indicates the minimum grip-to-load force ratio required to prevent slip. The gray area represents the safety margin against slip. After contact with the object (left most vertical line, top), grip force increases by a short period while the grip is established. A command is then released for simultaneous increases in grip and load force (second vertical line). This increase continues until the load force overcomes the force of gravity and the object lifts off (third vertical line). After replacement of the object and table contact occurs (fourth line), there is a short delay before the two forces decline in parallel (fifth line) until the object is released (sixth line) (adapted with permission from Johansson, R. S., andWestling, G., Exp. Brain Res. 56,550-564,1984 by Springer-Verlag; Johansson, R. S., and Westling, G., Exp. Brain Res. 71, 59-71, 1988. Copyright © 1988 by Springer-Verlag; and Jenmalm, P., and Johansson, R. S., J. Neurosci. 17, 4486-4499, 1997 Copyright © 1997 by the Society for Neuroscience).

ratio is used when the grip surfaces are tapered upward compared to downward (Fig. 3C).

In each example shown in Fig. 3, grip force increases and decreases in phase with (and thus predicts) changes in vertical load force. This parallel coordination of grip force and load force ensures grasp stability. The grip force at any given load force is controlled such that it exceeds the corresponding minimum grip force, required to prevent slip, by a small safety margin (gray areas in the bottom of Fig. 3). This minimum grip force depends on the weight of the object, the friction between the object and skin, and the shape (e.g., angle) of the contact surfaces.

This parallel coordination of grip force and load force is a general feedforward control strategy and is not specific to any particular task or grip configuration. Parallel force coordination is observed when grasping with two or more digits of the same hand or both hands, when grasping with the palms of both hands, and even when gripping objects with the teeth. Moreover, it does not matter whether the object is moved by the arm or, for example, by the legs as when jumping with the object in hand. Importantly, the parallel coordination of grip and anticipatory load force is not restricted to common inertial loads. People also adjust grip force in parallel with load force when pushing or pulling against immovable objects and when moving objects subjected to elastic and viscous loads. Figure 4 illustrates parallel coordination of grip and load forces under varying load conditions. People alternately pushed and pulled an object instrumented for force sensors and attached to a simple robot that could simulate various types of opposing loads acting tangential to the grasp surfaces (Fig. 4A). Figures 4B and 4C show kinematic and force records obtained under three different load conditions: an acceleration-dependent inertial load, a velocity-dependent viscous load, and an elastic load that largely depended on position but also contained viscous and inertial components. In all three cases, the grip force normal to the grasp surfaces changes in parallel with the magnitude of the load force tangential to the grasp surface. Importantly, the relationship between arm movement motor commands and the load experienced at the fingertips depends on the type of load being moved. Thus, to adjust grip force in parallel with load

Figure 4 Kinematic and force records from one subject under the three load conditions. Shaded regions indicate the primary kinematic variable on which load depended. Under all three load conditions, grip force (GF) is adjusted in parallel with fluctuations in load force (LF), with the resultant load tangential to the grasp surface. The dashed vertical lines indicate movement onset (modified with permission from Flanagan, J. R., and Wing, A. M., J. Neurosci. 17, 1519-1528, 1997. Copyright © 1997 by the Society for Neuroscience).

Figure 4 Kinematic and force records from one subject under the three load conditions. Shaded regions indicate the primary kinematic variable on which load depended. Under all three load conditions, grip force (GF) is adjusted in parallel with fluctuations in load force (LF), with the resultant load tangential to the grasp surface. The dashed vertical lines indicate movement onset (modified with permission from Flanagan, J. R., and Wing, A. M., J. Neurosci. 17, 1519-1528, 1997. Copyright © 1997 by the Society for Neuroscience).

force under the different load conditions, people had to alter the mapping between the motor command driving arm movement and that driving the grip force.

In most everyday tasks, destabilizing loads acting on the grasp include not only linear load forces but also torques tangential to the grasped surfaces. Such torsional loads occur whenever we tilt an object around a grip axis that does not intersect the vertical line through the object's center of mass. In addition, torque loads arise in many natural manipulatory tasks due to changes in the orientation of the grip axis with respect to gravity. For example, this occurs when we hold a book flat by gripping it between the fingers beneath and the thumb above (vertical grip axis) and then rotate it by a pronation movement to put it in a bookshelf (horizontal grip axis). Because we rarely take a book such that the grip axis passes through its center of mass, a torque will develop in relation to the grasp. Importantly, the sensorimotor programs for object manipulation account for torsional loads by predicting the consequences of object rotation both when we rotate objects around the grip axis and when we rotate the grip axis in the field of gravity. Rotational slips are prevented by automatic increases in grip force that parallel increases in tangential torque. The sensorimotor programs thus model the effect of the total load in terms of linear forces, tangential torques, and their combination.

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