Investigation of motor adaptation to movement versus object parameters

In this study healthy human subjects (n=10) manually controlled a rotary handle to track a sinusoidally moving target as displayed on a computer screen. During movement, either the apparent handle inertia or tracking frequency changed to a higher or lower value. We analyzed the initial performance recovery following task perturbation using a linear fit of the velocity tracking error trends. For both types of task perturbations, we found significant increases in the intercept of the line fit (paired t-tests, two-tailed: p<.05) compared to trials with no change. We also found that adaptation rates indicated by the slope of the line fit of the tracking velocity error were larger for frequency changes than for apparent changes of the inertia for parameter increases (p=0.029, paired t-tests, one-tailed) and parameter decreases (p=0.055, paired tests, one-tailed). Our results provide evidence that humans use low impedance control that is task-specific to object parameters such as inertia. In addition, the results provide evidence that the adaptation to motion parameter changes and object parameter changes are different control processes.