Receiving reward’s effect on Ego-depletion

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Travis Johnson and Michael J. Larson, PhD, Psychology and Neuroscience

Introduction

Recent research suggests that willpower–the capacity to exert self-control–is a limited resource that is depleted after exertion (Job, Dweck & Walton, 2010). We aimed to explore the influence of ego-depletion on how we process rewards and punishment. Ego depletion specifically refers to the idea that self-control or willpower draws upon a limited pool of mental resources that can be used up (i.e., depleted) after cognitive exertion. For example, an experiment performed by Kathleen Vohs and Todd Heatherton (2000) demonstrated how ego depletion was relevant in how it affected chronic dieters compared to non-dieters. Their results showed that dieters, in responding to an emotional suppression task, ate more ice cream in a taste test than non-dieters under the same conditions. They concluded that this response was due to the ego-depleting effect of dieting on these individuals. Indeed, research has shown depletion is associated with an uptake of alcohol consumption (Muraven, Collins, & Nienhaus, 2002), overeating (Kahan, Polivy, & Herman, 2003; Vohs & Heatherton, 2000), impulsive spending (Vohs & Faber, 2007), and increased responding to sexual urges (Gailliot & Baumeister, 2007). These studies, among others, show the negative effect that depletion has on daily decisions. Depletion can influence our day-to-day choices and actions in a variety of important ways.

In our study, we explored the link between feelings of depletion and the feedbackrelated negativity (FRN). The FRN is a negative deflection in an electroencephalogram (EEG) that occurs when humans receive positive and negative feedback. The deflection is more negative for negative or unexpected feedback than for positive or rewarding feedback. We tested the following hypothesis in regards to the FRN: Participants in the “depletion” condition will show increased FRN amplitude on reward trials compared to participants in the control condition. Participants in the “depleted” condition will show decreased FRN amplitude on non-reward trials compared to participants in the control condition. Participants in the “depleted” condition will show greater FRN amplitude for food rewards rather than monetary rewards; no differences were expected in the control condition

Methodology

Participants were healthy and free from neurological disease. All participants were recruited through Brigham Young University’s SONA system and flyers throughout the community and BYU faculty. Prior to beginning the depletion task, EEG electrodes were applied. Participants were randomly assigned using a random number generator to either “Depletion”, “Boredom” or a “Control” condition. Approximately one third of the participants were assigned to each condition. After being assigned to a condition, participants were introduced to the first computerized task. In the boredom and depletion conditions, the task involved 4 blocks of 5 minutes during which the participant was presented with strings of 4 digits. In the depletion condition, participants were asked to add 3 to each digit and type their response (Kahneman, 2011); in the boredom condition, they were asked to simply observe the numbers as they come up. In the control conditions participants immediately began the doors task described below.

Participants were then administered the door lottery task. This task consists of 144 trials organized in 4 blocks to provide participants with breaks. Prior to each trial, participants were informed whether on the upcoming trial they would have the chance to earn or lose money, candy (M&Ms), or nothing. They then saw two doors, and were asked to choose one to open. Once the door was opened, they received feedback to indicate whether they won (green arrow pointing up) or lost (red arrow pointing down). There were 48 trials for each type of reward, with positive feedback presented on 50% of the trials. Prior to the task, participants were informed that they would receive 50c for each ‘correct’ money trial and 2 M&M candies for each ‘correct’ candy trial, and lose 25c on each ‘incorrect’ money trial or 1 M&M candy on each ‘incorrect’ candy trial.

Participants wore a 128-electrode sensor EEG net in order to measure the FRN. We measured the FRN deflection in the EEG in response towards stimuli. We proposed that people who were depleted would exhibit greater sensitivity to rewards, as indexed by stronger FRN amplitudes following (reward) feedback. To analyze the data, electroencephalogram data was recorded during the tasks from 128 scalp sites using a geodesic sensor net and Electrical Geodesics, Inc. Electrode placements enabled recording vertical and horizontal eye movements reflecting electrooculographic (EOG) activity. Data from the EEG was referenced to Cz and digitized continuously at 250Hz with a 16-bit analog-to-digital converter.

Results

Participants included 164 total individuals. Of these, there were 58 (30 female) in the depletion group, 50 (27 female) in the boredom group, and 56 (25 female) in the control group. The three groups did not significantly differ male to female ratio, as evidenced by a non-significant chisquare (2) = 1.03, p = 0.60. Groups also did not differ in mean age, F(2,161) = 1.04, p = 0.36. Mean age of the depletion group was 21.0 (+/- 2.0 years), 20.5 (+/- 2.1 years) for the boredom group, and 21.0 (+/- 2.3) for the control group. To analyze the difference between depletion and condition, we conducted a 3-group (depletion, boredom, control) x 3-condition (money, candy, neutral) repeated measures analysis of variance (ANOVA).

Results of the 3×3 ANOVA indicate a significant main effect of condition, F(2,322) = 4.73, p = 0.009, and a significant main effect of group, F(2,161) = 3.69, p = 0.027. The Group by Condition interaction was not significant, F(4,322) = 0.41, p = 0.81. Post-hoc analysis indicated the boredom group showed a significantly more negative FRN than the depletion group (p = 0.03) and trend-level different from the neutral group (p = 0.09). Neutral and depletion were not significantly different (p = 0.89). The money condition was significantly more negative than the neutral condition (p = 0.003), and trended different than the candy condition (p = 0.09). The candy and neutral conditions did not differ (p = 0.19).

Conclusion and Summary

Overall our results showed that there were no interactions between group and condition. There were, however, main effects of group and condition. We hypothesized that the “depletion” condition would show the greatest negative FRN amplitude in response to reward, but discovered that the “boredom” group reacted the strongest to the rewards with the greatest negative feedback. Consistent with predictions, the money task had the largest FRN amplitude. This is possibly due to the need for reward after being in a “depleted” or “ boredom” condition for an extended period of time, with those in the “boredom” condition seemingly being in more of a need for reward. Money may lead to larger FRN amplitude because of the demographics need for money over candy. Future studies should examine the effectiveness of the depletion task and compare with sensitivity to reward.

Scholarly Sources

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