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

Abstract

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.

A cross-species neural integration of gravity for motor optimisation

“…The metabolic rate was shown to influence body size, resource use, rate of senescence and survival probability (Berghänel et al, 2015; Brown et al, 2004; DeLong et al, 2010; Munch and Salinas, 2009; Strotz et al, 2018; Voorhies and Ward, 1999). Preserving muscle effort may thus represent an essential pursuit for the brain (Baraduc et al, 2013; Bramble and Lieberman, 2004; Carrier et al, 2011; Cheval et al, 2018a, 2018b; Farshchiansadegh et al, 2016; Huang et al, 2012; Inzlicht et al, 2018; Kurzban et al, 2013; Lee et al, 2016; Lieberman, 2015; Mazzoni et al, 2007; Morel et al, 2017; Pageaux, 2016; Pageaux and Gaveau, 2016; Selinger et al, 2015; Shadmehr et al, 2016; Walton et al, 2006; Wang and Dounskaia, 2012).…”

Humans sacrifice decision-making for action execution when a demanding control of movement is required

“…Cos and colleagues showed 547 that when humans make rapid choices between reaching actions, they tend to choose the one that 548 carries the lowest biomechanical cost (Cos et al, 2011. Morel and colleagues found that 549 biomechanics affects action selection too, but among duration, amplitude, direction and force, they 550 observed that movement duration is perceived as the greatest cost by subjects (Morel et al, 2017). 551…”

Models, movements, and minds: bridging the gap between decision making and action

“…99 Other recent work further shows that the costs associated with motor control bias decision making between actions. [100][101][102] While the impact of motor costs on decision making is not limited to action-based models-for instance, good-based models can account for motor costs through learning or association-the representation of options in a sensorimotor space provides a convincing and direct way for motor information to influence value.…”