Entropy Compensation in Human Motor Adaptation

Open Access
Hong, Siang Lee
Graduate Program:
Doctor of Philosophy
Document Type:
Date of Defense:
June 14, 2007
Committee Members:
  • Karl Maxim Newell, Committee Chair
  • Dagmar Sternad, Committee Member
  • John Henry Challis, Committee Member
  • Joseph Paul Cusumano, Committee Member
  • entropy
  • motor control
  • dynamic systems
  • isometric
  • perception-action
  • variability
Human movement is inherently variable, due to the number of controllable components that the motor system possesses and the relatively large number of potential movement patterns that can be employed to achieve a given goal. Motor variability has been known to be context-dependent, and thus, the structure of motor variability, as characterized by its probability distributions has been shown to change under different task and environmental constraints. This raises the possibility that the uncertainty or unpredictability contained within motor variability and at the level of the task and environment can be represented as entropies. This dissertation examined human motor adaptation to different task and environmental contexts with a view that this adaptive process can be represented as a process of entropy conservation through compensation. Three experiments were conducted to investigate the hypothesis that human motor adaptation reflects the process of entropy conservation. The first experiment examined the compensatory effects of spatial and temporal properties of visual feedback from the environment on the entropy of isometric force output. In this experiment, the entropy of the fluctuations of an index finger isometric force output under various levels of feedback frequencies (temporal) and visual gain (spatial). Increasing the entropy of the environment through reduced spatial and temporal information resulted in a decrease in the entropy of the force output, as indexed by a decrease Approximate Entropy (ApEn). This finding reflects a compensatory tradeoff between the effects of spatial and temporal entropy of the environment on the force output dynamics. Thus, at a constant level of task constraint, a decrease in entropy in the force output is noted when the entropy of the environment is increased. Experiments 2 and 3 were designed to examine the effects of the task constraint on unimanual and bimanual isometric force production in conjunction with that of the environment. Here, the entropy of the environment was manipulated through feedback frequency, while the task entropy was altered via the ratio of the required force to the error tolerance. In Experiment 2, the entropy of the isometric force signal was measured with ApEn; in Experiment 3 the entropy of the motor output was assessed using the information entropy of the relative phase of the isometric forces generated by the index fingers of the left and right hand. As conservation processes are based on idealized cases, both entropy calculations were made conditional upon the probability of achieving the goal of the action, that is, remaining within the error tolerance bands. In both experiments, nonlinear decreases in the entropy of the force output were observed as the entropies of the task and environment were increased. Compensatory effects of the task and environmental entropies on the entropy of force dynamics were found, and these entropy tradeoffs were represented with a quadratic surface that captured a majority of the variance. These findings show that the context-dependent changes in motor variability in accordance with task and environmental contexts can be characterized through the compensation of entropy across the task, organism, and environment.