Katherine M. Deutsch

Task goals and change in dynamical degrees of freedom with motor learning.

In this article, the authors examined the hypothesis that the direction of the change (increase or decrease) in the dynamical degrees of freedom (dimension) regulated as a function of motor learning is task-dependent. Adult participants learned 1 of 2 isometric force-production tasks (Experiment 1: constant force output; Experiment 2: sinusoidal force output) over 5 days of practice and a 6th day with augmented information withdrawal. The results showed that over practice, the task goal induced either an increase (Experiment 1) or a decrease (Experiment 2) in the dimension of force output as performance error was reduced. These findings support the proposition that the observed increase or decrease in dimension with learning is dependent on both the intrinsic dynamics of the system and the short-term change required to realize the task goal.

On Learning to Move Randomly

Abstract In 2 experiments, the authors examined whether and to what degree young adults can learn to produce random planar motion of the index finger or fingers. Three different types of information feedback were provided to the participants (N = 8 in each experiment) over up to 5 days of practice across the 2 experiments. The results from both experiments revealed that the participants produced a relatively low level of movement randomness in finger motion and that they did not learn through practice to enhance the stochastic properties of their movement under any feedback conditions. The findings provide further evidence that there are relatively tight constraints on the number of dimensions that are regulating single-limb planar motion and that those constraints are not susceptible to change through typical learning protocols.

Dynamics of self-injurious behaviors

Self-injurious behavior was examined in a case study of head-banging by an 8-year-old girl with profound mental retardation and an autistic disorder. Trajectories of the arm movements and impact forces of the head blows were determined from a dynamic analysis of videotapes. Results revealed a high degree of cycle-to-cycle consistency in the qualitative dynamics of the limb motions, with one hand motions being faster than those with two hands (inphase and antiphase) and the motions with the helmet about 25% faster than those without the helmet. The impact force of SIBs as a percentage of body weights are near the low end of forces generated in boxing blows and karate hits.

Tremor Frequency Profile as a Function of Level of Mental Retardation

The characteristic slowness of movement initiation and execution in adult individuals with mental retardation may be driven by the slower frequency profile of the dynamics of the system. To investigate this hypothesis, we examined the resting and postural finger tremor frequency profile (single and dual limb) of adults as a function of level of mental retardation (moderate, severe, profound). There was a progressive increase in the contribution of slow frequency components to the enhanced amplitude of tremor as a function of mental retardation, particularly in the group with profound mental retardation. Findings support the hypothesis of mental retardation inducing a slower frequency to the system dynamics that may fundamentally drive the characteristic slowness of movement behavior.

Adaptive force control in grasping as a function of level of developmental disability.

BACKGROUND The adaptation to the task demands of grasping (grip mode and object mass) was investigated as a function of level of developmental disability. METHODS Subjects grasped objects of different grip widths and masses that were instrumented to record grip forces. RESULTS Proportionally, fewer participants from the profound compared with moderate and severe disability groups were able to complete the prehensile tasks. Nevertheless, all participants who completed the task showed adaptive grasping behaviour in terms of level and variability of force produced. There was higher absolute and relative force variability in low mass tasks that was enhanced with greater level of developmental disability. CONCLUSIONS The findings show task relevant adaptive grasping control with inhibition of force output at very-low-force conditions being the primary performance deficit of the profound disability group as a function of level of developmental disability.