Vigor of reaching movements: Reward discounts the cost of effort

Erik M. Summerside, Reza Shadmehr, Alaa A. Ahmed

Research output: Contribution to journalArticle

Abstract

Making a movement may be thought of as an economic decision in which one spends effort to acquire reward. Time discounts reward, which predicts that the magnitude of reward should affect movement vigor: we should move faster, spending greater effort, when there is greater reward at stake. Indeed, saccade peak velocities are greater and reaction-times shorter when a target is paired with reward. In this study, we focused on human reaching and asked whether movement kinematics were affected by expectation of reward. Participants made out-and-back reaching movements to one of four quadrants of a 14-cm circle. During various periods of the experiment only one of the four quadrants was paired with reward, and the transition from reward to nonreward status of a quadrant occurred randomly. Our experiment design minimized dependence of reward on accuracy, granting the subjects wide latitude in self-selecting their movement speed, amplitude, and variability. When a quadrant was paired with reward, reaching movements had a shorter reaction time, higher peak velocity, and greater amplitude. Despite this greater vigor, movements toward the rewarded quadrant suffered from less variability: both reaction times and reach kinematics were less variable when there was expectation of reward. Importantly, the effect of reward on vigor was specific to the movement component that preceded the time of reward (outward reach), not the movement component that followed it (return reach). Our results suggest that expectation of reward not only increases vigor of human reaching but also decreases its variability. NEW & NOTEWORTHY Movements may be thought of as an economic transaction where the vigor of the movement represents the effort that the brain is willing to expend to acquire a rewarding state. We show that in reaching, reward discounts the cost of effort, producing movements with shorter reaction time, higher velocity, greater amplitude, and reduced reaction-time variability. These results complement earlier observations in saccades, suggesting a common principle of economics across modalities of motor control.

Original languageEnglish (US)
Pages (from-to)2347-2357
Number of pages11
JournalJournal of Neurophysiology
Volume119
Issue number6
DOIs
StatePublished - Jun 1 2018

Fingerprint

Reward
Costs and Cost Analysis
Reaction Time
Saccades
Economics
Biomechanical Phenomena

Keywords

  • Effort
  • Reaching movements
  • Reward
  • Variability
  • Vigor

ASJC Scopus subject areas

  • Neuroscience(all)
  • Physiology

Cite this

Vigor of reaching movements : Reward discounts the cost of effort. / Summerside, Erik M.; Shadmehr, Reza; Ahmed, Alaa A.

In: Journal of Neurophysiology, Vol. 119, No. 6, 01.06.2018, p. 2347-2357.

Research output: Contribution to journalArticle

Summerside, Erik M. ; Shadmehr, Reza ; Ahmed, Alaa A. / Vigor of reaching movements : Reward discounts the cost of effort. In: Journal of Neurophysiology. 2018 ; Vol. 119, No. 6. pp. 2347-2357.
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