Abstract
The brain has evolved an internal model of gravity to cope with life in the Earth's gravitational environment. How this internal model benefits the implementation of skilled movement has remained unsolved. One prevailing theory has assumed that this internal model is used to compensate for gravity's mechanical effects on the body, such as to maintain invariant motor trajectories. Alternatively, gravity force could be used purposely and efficiently for the planning and execution of voluntary movements, thereby resulting in direction-depending kinematics. Here we experimentally interrogate these two hypotheses by measuring arm kinematics while varying movement direction in normal and zero-G gravity conditions. By comparing experimental results with model predictions, we show that the brain uses the internal model to implement control policies that take advantage of gravity to minimize movement effort.
Highlights
It is always fascinating to witness the ability of acrobats and dancers to accomplish complex and elegant movements, graciously interacting with gravito-inertial forces
symmetry ratio (SR) corresponds to the relative timing of peak velocity and allows quantifying whether arm kinematics remains invariant or changes with movement direction
It is broadly believed that the brain develops and uses internal models of the sensor and effector dynamics, as well as physical laws of motion, to optimally interact with the external environment (Shadmehr and Mussa-Ivaldi, 1994; Conditt et al, 1997; Gribble and Ostry, 1999; Wolpert and Ghahramani, 2000; Pigeon et al, 2003; Todorov, 2004; Ahmed et al, 2008; Scott, 2012)
Summary
It is always fascinating to witness the ability of acrobats and dancers to accomplish complex and elegant movements, graciously interacting with gravito-inertial forces. A neural representation of gravity is created and stored through an internal model (Papaxanthis et al, 1998a; Angelaki et al, 1999; Merfeld et al, 1999; McIntyre et al, 2001; Angelaki et al, 2004; Indovina et al, 2005; Miller et al, 2008; Crevecoeur et al, 2009; Gaveau and Papaxanthis, 2011; Laurens et al, 2013a, 2013b). The neural representation of gravity is thought to solve this ambiguity by multisensory statistical inference (Angelaki et al, 1999; Merfeld et al, 1999; Angelaki et al, 2004; Laurens et al, 2013b). Whether and how an internal model of gravity benefits the planning and execution of skilled movement remains unknown
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