Mental Fatigue and Motivation: Effects and mechanisms

Mental fatigue is a condition that occurs after performing a demanding cognitive task for a long time. One of the major theories of mental fatigue, the motivational theory of mental fatigue, suggests that mental fatigue is caused by increased lack of motivation to continue performing an sustained activity.
In this thesis, we investigated the effects of motivation on mental fatigue. In the first experiment, we asked participants to continuously perform a demanding working memory task for 2.5 hours and offered participants monetary rewards for good performance in the experimental condition. In the second experiment, we asked participants who liked playing Sudoku puzzles to play Sudoku puzzles for three hours and alternated two conditions: low-level and high-level motivation. Overall, the results in both experiments suggest that motivation is an essential factor in mental fatigue. Participants exerted more mental effort when motivated to maintain task performance but exerted less mental effort when they became demotivated, causing performance to decrease.
To explain the effects of motivation on mental fatigue, we simulated the results of three different mental fatigue studies and proposed a mechanism named goal competition, which assumes that there is a competition between the main task goal and others: If motivation for the main task decreases, other goals can win the competition, which leads to a drop in motivation and later affects task performance. The models produced the results that we had predicted, supporting the notion of the motivational theory of mental fatigue.