Does the cost function of human motor control depend on the internal metabolic state?

Abstract

Optimal feedback control theory (OFCT) has been very successful in explaining human motor coordination in a principled manner [1,2]. OFCT derives motor control policies from the minimization of a cost function, which predicts a large variety of movement data [3]. However, very little is known about the nature of this cost func-tion. From the many proposed costs (which take the same mathematical form) two stand out as being moti-vated in a principled manner: noise [4,5] and " effort " (incl. muscle fatigue and metabolic demand [6]). These two cost are evolutionarily sensible as they maximize an organism's fitness: increasing task-relevant precision and energy efficiency. It was recently suggested [6] that noise and effort are weighed against each other when determining motor coordination. However, the neuronal implementation or representation of such costs in the brain remains unclear [7]. We test the hypothesis that this trade-off may be directly affected by our internal metabolic state (e.g. blood glucose level). We test the hypothesis if a subject's internal metabolic state has an impact on the strategy for motor control. Specifically, low metabolic levels could bias motor coordination across redundant muscle groups from larger, metaboli-cally more costly muscles towards smaller muscles to increase efficiency. We performed preliminary experiments by conducting reaching experiments under a dietary regime. 5 right-handed participants 21-27 years old performed reaching movements in a virtual reality arm movement-tracking rig (visual stimuli were projected via a mirror system onto the plane of hand movement). Air sleds on a surrounding table supported the participant's arm to allow frictionless movement. The task began when participants moved their hand to a visual workspace centre; then a 1.5cm radius target sphere appeared 15cm away in one of 8 directions (total of 800 trials, randomly ordered directions). Subjects chose when to start reaching towards the target, having to come to a stop within the target sphere within movement dura-tions of 75 to 125ms. Feedback was given in the form of a score that increased for successful trials and decreased conversely. Each experiment involved two morning ses-sions on separate days. 3 subjects followed their normal eating/drinking routine for the first session then fasted from 8.00pm the evening before the second session and vice versa for the 2 other subjects. Blood glucose mea-surements were taken before and after each session using a personal blood glucose monitoring system. Our preliminary data suggests a systematic shift in the coordination of muscle groups acting on each joint based on available energy levels. We found statistically significant changes in shoulder and elbow joint utilisa-tion (integrated absolute change in joint angle) in all 5 subjects and in up to 5 out of 8 target directions depending on the metabolic state. Moreover, we can model the subject's reaching trajectories under both metabolic conditions using OFCT by a change in the weight of the metabolic cost term as a function of inter-nal metabolic state. Our preliminary findings are impor-tant as we link for the first time metabolic state to neuronal computations underlying motor control.