Fitts' Law in early postural adjustments.

We tested a hypothesis that the classical relation between movement time and index of difficulty (ID) in quick pointing action (Fitts\u2019 Law) reflects processes at the level of motor planning. Healthy subjects stood on a force platform and performed quick and accurate hand movements into targets of different size located at two distances. The movements were associated with early postural adjustments that are assumed to reflect motor planning processes. The short distance did not require trunk rotation, while the long distance did. As a result, movements over the long distance were associated with substantial Coriolis forces. Movement kinematics and contact forces and moments recorded by the platform were studied. Movement time scaled with ID for both movements. However, the data could not be fitted with a single regression: Movements over the long distance had a larger intercept corresponding to movement times about 140 ms longer than movements over the shorter distance. The magnitude of postural adjustments prior to movement initiation scaled with ID for both short and long distances. Our results provide strong support for the hypothesis that Fitts\u2019 Law emerges at the level of motor planning, not at the level of corrections of ongoing movements. They show that, during natural movements, changes in movement distance may lead to changes in the relation between movement time and ID, for example when the contribution of different body segments to the movement varies and when the action of Coriolis force may require an additional correction of the movement trajectory

A central back-coupling hypothesis on the organization of motor synergies: a physical metaphor and a neural model

We offer a hypothesis on the organization of multi-effector motor synergies and illustrate it with the task of force production with a set of fingers. A physical metaphor, a leaking bucket, is analyzed to demonstrate that an inanimate structure can show apparent error compensation among its elements. A neural model is developed using tunable back-coupling loops as means of assuring error compensation in a task-specific way. The model demonstrates non-trivial features of multi-finger interaction such as delayed emergence of force stabilizing synergies and simultaneous stabilization of the total force and total moment produced by the fingers. The hypothesis suggests that neurophysiological structures involving short-latency feedback may play a central role in the formation of motor synergies

Signs of long-term adaptation to permanent brain damage as revealed by prehension studies of children with spastic hemiparesis

This chapter focusses on signs of long-term adaptation to permanent brain damage in children with spastic hemiparesis. First, we recognize that adaptation processes may occur at various time scales. Then, we formulate a tentative strategy to infer signs of adaptation from behavioral data. Subsequently, a series of studies on grasping movements in children with hemiparetic cerebral palsy is reviewed. The series initially addressed a macroscopic aspect of human prehension, i.e. the planning of a suitable grip type, but then gradually focussed on more detailed aspects of the control of grasping movements, viz. the kinematics of the transport and manipulation components of grasps, interjoint coordination, and, finally, force control. We conclude that the deviations in the motor behaviour of children with spastic hemiparesis reflect many signs of adaptation since the deviations are often based on efficiency principles that demonstrate a core feature of adaptation, viz., the exploitation of redundancy