Motor equivalence in Two-third power law

The observation that different effectors can execute the same movement suggests functional equivalences and abstract representation of action in the CNS. A common motor invariant is the coupling between speed and curvature of limb movements (the 2/3 power law), which is resilient to different sensorimotor contexts. Accordingly, this broad concept proposed a unique and universal internal model which is consistent for all different conditions and it considers same neural representation for all individual behaviors, while based on recent research there are important cognitive, neural, and behavioral variabilities among individuals due to differences in the developmental stage, training or pathologies, etc. Studies showed that studying subjective values is important as they inform us about the decision process in adopting a motor strategy. When subjects face multiple choices, due to the large extrinsic (the lack of visible salient target to reach) and intrinsic (the whole body multi-joint system) redundancy, we can extract different but consistent motor behaviors which define motor style. In order to study these differences, our purpose in this study is to verify the consistency of movement features during a drawing task performed with either hand, by different subjects, and in a pathological population. In the first step, we tested the effect of manual dominance and movement speed on motor performance consisting of an elliptical drawing task. We hypothesize that movement parameters extracted from the task can be categorized either as global and abstract representations within the CNS that are limb-independent or specific limb-dependent representations. Accordingly, the results indicate that the kinematics parameters are differently affected when drawing at different peeds with the non-dominant compared to the dominant hand. Movement duration, velocity-curvature covariation, and maximum velocity were not significantly affected by the hand used, while geometrical features were strongly limb-dependent. However, intra-trial analysis performed over the successive drawing movements reveals a significant effect of the hand side on the variability of the velocity-curvature relationship and maximum velocity. The effects of end-effectors on the movement components suggest differentiated neural strategies, according to a pattern that does not go from the most abstract component to the less abstract, as implied by the idea of hierarchical organization of motor control. In the following analysis, we aim to investigate the existence of an IMS or different motor styles according to the limb-dependency strategies we found in the first step. The results revealed a new aspect of behaviors that would extend our perspective on the previous results. Notably, examining the subject separately some showed a higher difference in the non-dominant relative to the dominant hand for various parameters such as duration which was categorized as an abstract motor component. This finding showed that a drawing task led the subjects to adopt different strategies according to controlling end-effector dependent (spatial) and abstract parameters (temporal) of movement, suggesting the existence of idiosyncratic strategies influencing motor planning due to a clear spatial-temporal trade-off. Finally, we aimed to compare the motor components of drawing movement between dyslexic and healthy children. Dyslexia is distinguished by specific problems dealing with learning to read accurately, which has been generally explained in terms of phonological deficits that are accompanied by sensorimotor deficits. Compared to healthy children, temporal representation and timing organization of children with dyslexia were characterized respectively by increased movement duration and higher asymmetry. Their geometrical features were distinguished by a circular shape, and smaller ellipse size when producing an elliptical movement.