Kavli Affiliate: Reza Shadmehr
| Authors: Paul Hage, In Kyu Jang, Vivian Looi, Mohammad Amin Fakharian, Simon P Orozco, Ehsan Sedaghat-Nejad, Jay Pi and Reza Shadmehr
| Summary:
We prefer to decline effortful rewards, but if the circumstances require it, we will move only slowly to harvest them. Why should economic variables such as reward and effort affect movement vigor? In theory, our decisions and movements both contribute to a measure of fitness in which the objective is to maximize rewards minus efforts, divided by time. To test this idea, we engaged marmosets in a foraging task in which on each trial they decided whether to work by making saccades to visual targets, thus accumulating food, or to harvest by licking. We varied the effort cost of harvest by moving the food tube with respect to the mouth. Theory predicted that the subjects would respond to the increased effort costs by working longer, stockpiling food before commencing harvest, but reduce their movement vigor to conserve energy. Indeed, in response to the increased effort costs of harvest, marmosets increased their work duration but reduced their movement vigor. These changes in decisions and movements coincided with changes in pupil size. As the effort cost of harvest declined, work duration decreased, the pupils dilated, and lick and tongue vigor increased. Thus, when acquisition of reward became effortful, there was a global change in the state of the brain: the pupils constricted, the decisions exhibited delayed gratification, and the movements displayed reduced vigor.