Task constraints act at the level of synergies and at the level of end-effector kinematics in manual reaching and manual lateral interception.

Abstract

To coordinate the redundant degrees of freedom (DOF) in the action system, synergies are often proposed. Synergies organi e DOF in temporary task-specific units emerging from interactions amongst task, organism, and environmental constraints. We examined whether task constraints affect synergies, end-effector kinematics, or both. To this end, we compared synergies and end-effector kinematics when participants (N = 15) performed discrete movements of identical amplitude in manual reaching (stationary targets) and manual lateral interception (moving targets, with different angles of approach). We found that time-velocity profiles were roughly symmetric in reaching, whereas they had a longer decelerative tail and showed an angle-ofapproach effect in interception. Uncontrolled Manifold analyses showed that in all conditions joint angle variability was primarily co-variation, indicating a synergistic organi ation. The analysis on the clusters of joint angle configurations demonstrated differences between reaching and interception synergies, whereas more similar synergies were used within interception conditions. This implies that some task constraints operate at the level of synergies while other task constraints only affect end-effector kinematics. The results support a two-step process in the organi ation of DOF, consisting of synergy formation and further constraining of synergies to produce the actual movement, as proposed by Kay (1988).

On the psychological origins of tool use

“…First, our earlier studies regarding synergies in rod reaching (Valk et al, 2016;van der Steen and Bongers, 2011) can be taken one step further. A defining characteristic of a synergy is not just co-variation of joint angles, but also the actual location in joint space where the synergy emerges (Tuitert et al, 2020;Wissing et al, 2020). Studying whether, and if so how, the location in joint space where the synergy emerges depends on the tool's properties could further our understanding of the roles of tool constraints in the emergence of synergies.…”

On the psychological origins of tool use

“…First, our earlier studies regarding synergies in rod reaching (Valk et al, 2016;van der Steen and Bongers, 2011) can be taken one step further. A defining characteristic of a synergy is not just co-variation of joint angles, but also the actual location in joint space where the synergy emerges (Tuitert et al, 2020;Wissing et al, 2020). Studying whether, and if so how, the location in joint space where the synergy emerges depends on the tool's properties could further our understanding of the roles of tool constraints in the emergence of synergies.…”

Action and perception manifolds have gradients that may play a role in learning

“…Note that task constraints affect the shape of these energy gradients, and thus the location on the UCM at which the task is performed. For instance, reaching to a target or intercepting an object at that same target location implies different task constraints and Tuitert et al [2] showed that the joint angle configuration in the arm at the target was at a different location on the UCM for the two tasks. This indicates that different constraints affect the RCs at different levels and create energy gradients in spaces of elemental variables.…”