Date of Award
Doctor of Philosophy (PhD)
Alaa A. Ahmed
The goal of our actions should be to maximize reward while minimizing effort and time. An individual’s movements in response to changing costs and rewards can reveal how they value these contributing variables.
The motivation of this dissertation is to better understand how reward, effort, and time interact to determine an individual’s movement preferences. The first study used an objective measure of effort, metabolic cost, to quantify subjective effort valuation in healthy young adults in a low-effort reaching task. Participants were idiosyncratic in how they valued effort with the group as a whole representing effort objectively. The second study investigated the costs of effort and time in gait choice by probing preferences to walk or run across changing relative distances. I found that effort alone could not explain choices. Participants also considered time, with their tendency to discount time predicting running velocity. These first two studies demonstrate that effort is essential in explaining movement behaviors with its exact contribution varying across individuals.
In the third and fourth studies I investigated the role of reward in discounting effort costs in movements within individuals. The third study consisted of young adults reaching towards alternating quadrants where some quadrants were paired with reward. Expectation of reward led participants to react earlier and move faster, discounting the cost of effort to arrive at the reward sooner. Movements toward reward were also less variable, violating the traditional speed-accuracy trade-off. For the final study, I probed whether aging would influence an individual’s willingness to alter reaching behavior in response to reward. Older adults exhibited a reduced response to reward. While reward led to faster reaction times, I did not observe an increase in movement speed. I found that metabolic cost of reaching was not elevated in older adults compared to young, suggesting that the reduced response in older adults could not be explained by an increased effort cost, but rather a reduced sensitivity to reward.
Collectively, these studies advance our understanding of how reward, time, and effort interact in explaining movement preferences in changing cost and reward landscapes both within and across human populations.
Summerside, Erik Mikael, "On the Interactions of Reward, Time, and Effort in Human Movement" (2018). Integrative Physiology Graduate Theses & Dissertations. 68.