What makes a reach movement effortful? Physical effort discounting supports common minimization principles in decision making and motor control

• Published in: PLoS biology Vol. 15; no. 6; p. e2001323
• Main Authors: Morel, Pierre, Ulbrich, Philipp, Gail, Alexander
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 06.06.2017; Public Library of Science (PLoS)
• Subjects: Accident Prevention; Adult; Algorithms; Amplitudes; Arm; Bayes Theorem; Biology and Life Sciences; Biomechanics; Choice Behavior; Cognitive science; Cost function; Costs; Data collection; Decision Making; Decision theory; Delay Discounting; Discrimination (Psychology); Expected utility; Expected values; Germany; Humans; Markov Chains; Medicine and Health Sciences; Metabolism; Models, Biological; Models, Psychological; Monte Carlo Method; Motor ability; Motor Activity; Motor Skills; Motor task performance; Muscle Contraction; Neuroscience; Neurosciences; Optimization; Physical Exertion; Physiological aspects; Psychological aspects; Research and Analysis Methods; Roles; Social Sciences; Young Adult; Germany; Musculoskeletal mechanics Decision making Biomechanics Curve fitting Experimental design Friction Shoulders Vision
• ISSN: 1545-7885; 1544-9173; 1545-7885
• DOI: 10.1371/journal.pbio.2001323

When deciding between alternative options, a rational agent chooses on the basis of the desirability of each outcome, including associated costs. As different options typically result in different actions, the effort associated with each action is an essential cost parameter. How do humans discount physical effort when deciding between movements? We used an action-selection task to characterize how subjective effort depends on the parameters of arm transport movements and controlled for potential confounding factors such as delay discounting and performance. First, by repeatedly asking subjects to choose between 2 arm movements of different amplitudes or durations, performed against different levels of force, we identified parameter combinations that subjects experienced as identical in effort (isoeffort curves). Movements with a long duration were judged more effortful than short-duration movements against the same force, while movement amplitudes did not influence effort. Biomechanics of the movements also affected effort, as movements towards the body midline were preferred to movements away from it. Second, by introducing movement repetitions, we further determined that the cost function for choosing between effortful movements had a quadratic relationship with force, while choices were made on the basis of the logarithm of these costs. Our results show that effort-based action selection during reaching cannot easily be explained by metabolic costs. Instead, force-loaded reaches, a widely occurring natural behavior, imposed an effort cost for decision making similar to cost functions in motor control. Our results thereby support the idea that motor control and economic choice are governed by partly overlapping optimization principles.