Abstract
In a conventional view of motor control, the human brain might employ an optimization principle that leads a stereotypical motor behavior which we observe as an averaged behavioral data over subjects. In this scenario, the inter-individual motor variability is considered as an observation noise. Here, we challenged this view. We considered a motor control task where the human participants manipulated arm force by coordinating shoulder and elbow torques and investigated the muscle-tuning function that represents how the brain distributed the ideal joint torques to multiple muscles. In the experimental data, we observed large inter-individual variability in the profile of a muscle-tuning function. This contradicts with a well-established optimization theory that is based on minimization of muscle energy consumption and minimization of motor variability. We then hypothesized the inter-subject differences in the structure of the motor cortical areas might be the source of the across-subjects variability of the motor behavior. This was supported by a voxel-based morphometry analysis of magnetic resonance imaging; The inter-individual variability of the muscle tuning profile was correlated with that of the gray matter volume in the premotor cortex which is ipsilateral to the used arm (i.e., right hemisphere for the right arm). This study suggests that motor individuality may originate from inter-individual variation in the cortical structure.
Highlights
A theory of motor control postulates that, in all individuals, the brain employs a certain principle that produces stereotypical motor behavior to achieve a given task
The optimization principle predicts a stereotypical muscle tuning function no matter which optimization principle adopted by the model
Studies of motor control have strongly relied on the hypothesis that the brain might employ a common cost function across all human participants to make a motor plan for a certain task
Summary
A theory of motor control postulates that, in all individuals, the brain employs a certain principle that produces stereotypical motor behavior to achieve a given task. The data from individuals often deviate slightly from the ideal motor trajectory predicted by a theoretical model, even though the averaged trajectory fits very well with the model prediction. This inter-individual variability in motor behaviors contradicts the theory; the cause of this diversity in movement trajectory has not yet been examined in depth. The contemporary consensus in the motor control community regarding the mechanisms of how the brain solves this redundancy problem is that the brain may employ an optimization principle that minimizes the cost and maximizes the task performance (Todorov and Jordan, 2002; Izawa and Shadmehr, 2008, 2011) to keep consistence of intra-individual motor behavior. Ample evidence has suggested that both the motor costs that penalize energy consumptions (Emken et al, 2007; Izawa and Shadmehr, 2008, 2011) and the accuracy costs that penalize the influence of motor noise (Harris and Wolpert, 1998) on the precision of motor movement are essential components of the optimization principle the human brain employs (O’Sullivan et al, 2009)
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