Neurological ballistic movements: Sampled data or intermittent open-loop control

A neural model for generating and learning a rapid movement sequence

In this article, a neural model for generating and learning a rapid ballistic movement sequence in two-dimensional (2D) space is presented and evaluated in the light of some considerations about handwriting generation. The model is based on a central nucleus (called a planning space) consisting of a fully connected grid of leaky integrators simulating neurons, and reading an input vector [symbol: see text] (t) which represents the external movement of the end effector. The movement sequencing results in a succession of motor strokes whose instantiation is controlled by the global activation of the planning space as defined by a competitive interaction between the neurons of the grid. Constraints such as spatial accuracy and movement time are exploited for the correct synchronization of the impulse commands. These commands are then fed into a neuromuscular synergy whose output is governed by a delta lognormal equation. Each movement sequence is memorized originally as a symbolic engram representing the sequence of the principal reference points of the 2D movement. These points, called virtual targets, correspond to the targets of each single rapid motor stroke composing the movement sequence. The task during the learning phase is to detect the engram corresponding to a new observed movement; the process is controlled by the dynamics of the neural grid.

A kinematic theory of rapid human movements. Part I. Movement representation and generation

This paper proposes a kinematic theory that can be used to study and analyze rapid human movements. It describes a synergy in terms of the agonist and antagonist neuromuscular systems involved in the production of these movements. It is shown that these systems have a log-normal impulse response that results from the limiting behavior of a large number of interdependent neuromuscular networks, as predicted by the central limit theorem. The delta log-normal law that follows from this model is very general and can reproduce almost perfectly the complete velocity patterns of an end-effector. The theory accounts for the invariance and rescalability of these patterns, as well as for the various observations that have been reported concerning the change in maximum and mean velocities, time to maximum velocity, etc., under different experimental conditions. Movement time, load effects, and control strategies are discussed in a companion paper.

Peripheral regulation of stiffness during arm movements by coactivation of the antagonist muscles

Two experiments investigated whether unexpected and differential loading of a rapid, unsighted arm movement resulted in the central nervous system (CNS) regulating limb stiffness by modifying the associated neuromuscular activity. In Experiment 1, subjects completed multiple, spring-loaded training trials until a prespecified criterion of learning was attained. On selected trials, the spring load was unexpectedly replaced by an inertial load. Results indicated that to maintain positional accuracy during this inertial load trial, limb stiffness was increased by coactivating the antagonist muscles, i.e. by changing the associated neuromuscular activity from a predominantly triphasic pattern to one of coactivation. In Experiment 2, the sequence of loading was reversed producing a change in the required limb stiffness from a relatively high to low level. This change was observed as a pattern of coactivation being replaced by a triphasic activity pattern. These results support the notion that limb stiffness is regulated primarily through modification of the neuromuscular activity pattern prior to movement termination. It was also demonstrated that the size of the unexpected load did not affect the basic activation pattern selected by the CNS. It is proposed that the signal which triggers the CNS to regulate limb stiffness is based on peripheral information generated as a result of agonist activity occurring during the first part of the movement.