Predictability in human manipulation of nonlinear dynamic objects

Manipulation of complex objects such as carrying a cup of coffee is ubiquitous in daily activities. To gain insight into the control of such motor skills, the present study examines how humans adapt their amplitude and frequency and force when rhythmically manipulating a nonlinear dynamic object, and what these selections mean in terms of movement strategy. In two experiments, subjects were asked to manipulate an object with an internal degree of freedom - a cup containing a rolling ball, simulating carrying a cup of coffee. The first experiment tested how subjects rhythmically moved the object when paced at its resonant frequency (0.75Hz), leaving amplitude free to choose. The second experiment fixed the oscillation amplitude, but allowed subjects to freely choose the frequency. To evaluate the chosen movements we examined the experimental exerted force and the mutual information between the model-simulated force and kinematics that would emerge if subjects continued their movements with no intermittent correction. The first served to evaluate effort, and the latter as index for predictability of the object dynamics. Importantly, these two criteria were alternatives. Results of experiment 1 showed that amplitude and predictability of object dynamics increased with practice while there was no decrease in effort. In experiment 2 the frequency diverged from resonant frequency and thereby again increased predictability, while keeping effort unchanged. Neither experiment showed evidence for minimum effort as criterion, or a preference for object resonant frequency. Instead, both experiments identified predictability as an important criterion in complex dynamic object manipulation.