Rhythmic Manipulation of Objects with Complex Dynamics: Predictability over Chaos

The study of object manipulation has been largely confined to discrete tasks, where accuracy, mechanical effort, or smoothness were examined to explain subjects' preferred movements. This study investigated a rhythmic manipulation task, which involved continuous interaction with a nonlinear object that led to unpredictable object behavior. Using a simplified virtual version of the task of carrying a cup of coffee, we studied how this unpredictable object behavior affected the selected strategies. The experiment was conducted in a virtual set-up, where subjects moved a cup with a ball inside, modeled by cart-and-pendulum dynamics. Inverse dynamics calculations of the system showed that performing the task with different amplitudes and relative phases required different force profiles and rendered the object's dynamics with different degrees of predictability (quantified by Mutual Information between the applied force and the cup kinematics and its sensitivity). Subjects (n = 8) oscillated the virtual cup between two targets via a robotic manipulandum, paced by a metronome at 1 Hz for 50 trials, each lasting 45 s. They were free to choose their movement amplitude and relative phase between the ball and cup. Experimental results showed that subjects increased their movement amplitudes, which rendered the interactions with the object more predictable and with lower sensitivity to the execution variables. These solutions were associated with higher average exerted force and lower object smoothness, contradicting common expectations from studies on discrete object manipulation and unrestrained movements. Instead, the findings showed that humans selected strategies with higher predictability of interaction dynamics. This finding expressed that humans seek movement strategies where force and kinematics synchronize to repeatable patterns that may require less sensorimotor information processing.Author Summary: Daily actions frequently involve manipulation of tools and objects, such as stirring soup with a spoon or carrying a cup of coffee. Carrying the cup of coffee without spilling can be challenging as the coffee can only be indirectly controlled via moving the cup. Interaction forces between the cup and the coffee can be complex and, due to the nonlinear dynamics, lead to chaotic behavior. If unpredictable, an individual has to continuously react and compensate for the unexpected behavior. We hypothesized that when interacting continuously with such a complex object, humans seek a strategy that keeps the object's behavior predictable to avoid excessive sensorimotor processing. Using a simplified model of the cup of coffee, we analyzed forces and their relation to hand movements for rhythmic manipulation. Subjects practiced the modeled task in a virtual set-up for 50 trials. Experimental results show that subjects establish repeatable and predictable patterns. Importantly, the experiment was set up such that this increase in predictability was paralleled by an increase in exerted forces and a decrease in smoothness. Hence, this result showed that in complex object manipulation humans did not prioritize mechanical efficiency, but sought predictable interactions.