Preparing to move: Setting initial conditions to simplify interactions with complex objects

Abstract

Humans dexterously interact with a variety of objects, including those with complex internal dynamics. Even in the simple action of carrying a cup of coffee, the hand not only applies a force to the cup, but also indirectly to the liquid, which elicits complex reaction forces back on the hand. Due to underactuation and nonlinearity, the object’s dynamic response to an action sensitively depends on its initial state and can display unpredictable, even chaotic behavior. With the overarching hypothesis that subjects strive for predictable object-hand interactions, this study examined how subjects explored and prepared the dynamics of an object for subsequent execution of the target task. We specifically hypothesized that subjects find initial conditions that shorten the transients prior to reaching a stable and predictable steady state. Reaching a predictable steady state is desirable as it may reduce the need for online error corrections and facilitate feed forward control. Alternative hypotheses were that subjects seek to reduce effort, increase smoothness, and reduce risk of failure. Motivated by the task of ‘carrying a cup of coffee’, a simplified cup-and-ball model was implemented in a virtual environment. Human subjects interacted with this virtual object via a robotic manipulandum that provided force feedback. Subjects were encouraged to first explore and prepare the cup-and-ball before initiating a rhythmic movement at a specified frequency between two targets without losing the ball. Consistent with the hypotheses, subjects increased the predictability of interaction forces between hand and object and converged to a set of initial conditions followed by significantly decreased transients. The three alternative hypotheses were not supported. Surprisingly, the subjects’ strategy was more effortful and less smooth, unlike the observed behavior in simple reaching movements. Inverse dynamics of the cup-and-ball system and forward simulations with an impedance controller successfully described subjects’ behavior. The initial conditions chosen by the subjects in the experiment matched those that produced the most predictable interactions in simulation. These results present first support for the hypothesis that humans prepare the object to minimize transients and increase stability and, overall, the predictability of hand-object interactions.

Skilled motor control of an inverted pendulum implies low entropy of states but high entropy of actions

“…However, it remains to be understood to what extent this result extends to other kinds of skilled behavior that may have different characteristics and whether the prevalence of low entropy of states or of actions is task-dependent. A large body of research has shown that when learning and performing skills that require the manipulation of objects, people use sophisticated strategies; for example, they select appropriate initial conditions for the control task that simplify subsequent interactions [53] and make the object dynamics predictable [54]. Moreover, in keeping with classical work of Bernstein [55], recent studies found that people and animals modulate various aspects of the variability of their actions and sensations during learning in adaptive ways; for example, they counteract the maladaptive effects of noise by channeling it into task-irrelevant dimensions and increase their movement variability when their performance is poor [50,[56][57][58].…”

Novel Platform for Quantitative Assessment of Functional Object Interactions After Stroke

“…With the development of novel metrics, previous work found that neurotypical individuals aimed to make interactions with the cup-and-ball system more predictable, quantified by mutual information. Participants increased the mutual information about the object dynamics by exploiting resonance frequencies [15], [17], converging to the optimal phase of the system [9], stabilizing internal dynamics [11], [12], and appropriately initializing the object [10], [18]. Counter to common expectations, subjects did not minimize expended effort, nor increase the smoothness of the object kinematics [9], [10], [15].…”

“…Many objects that individuals interact with in daily life have internal dynamics that must be managed for successful use [9], [10], [11]. Such interactions pose challenges to coordination that are absent in unconstrained movements which have been predominant foci of motor neuroscience research [12].…”

“…1C, D). The cup-and-ball system can display highly varying and potentially chaotic dynamics, resulting in complex interaction forces [9], [10]. Importantly, these nonlinear forces evolve rapidly such that corrections based on perceived errors are largely insufficient due to latencies in the sensorimotor loop [16].…”

Escape of two-DOF dynamical system from the potential well

“…Evidence of the fractal behavior has also been found in the escapes governed by different regions of initial conditions (IC) and system control parameters [20]. Clearly, the prediction of the dynamical behavior for any region in the IC space is of crucial importance for all the above applications, as well as for design and even optimization purposes [21]. An extended numerical investigation was performed to map the escape region for 3DOF models.…”