## 1 Introduction  
Motivation is the primary driver of goal-directed behavior, and highly motivated individuals can make continuous efforts to achieve their final goals. In the psychological literature, motivation has been grouped into two classes depending on the type of motivator [1]. One can be motivated to take certain actions guided by external, often tangible, reinforcers, such as money and food. Motivation induced by reward is termed \"external motivation\" [1]. By contrast, humans and other species can be committed to certain activities owing to the enjoyment and interest in the activity itself, regardless of whether the activity leads to the attainment of an external reward. This type of motivation is termed \"intrinsic motivation\" [1]. Since a pioneering study on intrinsic motivation in rhesus monkeys [2], several studies have been conducted on human participants. These studies have consistently linked high levels of intrinsic motivation to performance improvement and expertise in many domains of mental and physical activity [2, 3]. An overarching goal of educational engineering is to develop learning-assistive technology to help people learn novel materials efficiently. Considering the close link between students' intrinsic motivation and academic achievement [4, 5], it is indispensable to develop an educational system that sustains students' intrinsic motivation for learning. Importantly, there are great individual differences in learning styles and the mastery of academic subjects. Thus, educational materials should be personalized for each learner to enhance their intrinsic motivation to learn. One promising solution to achieve this is real-time monitoring of intrinsic motivation. By objectively evaluating temporal fluctuations in intrinsic motivation, the presentation schedule and type of educational materials can be adaptively determined to sustain an appropriate level of motivation in learners, thereby increasing learning efficiency and productivity. There are several established ways to ascertain whether a person is engaged in an ongoing activity out of intrinsic motivation or not. Activities out of intrinsic motivation are often accompanied by positive feelings of curiosity, enjoyment and interest and are more likely to be deemed as \"fun\" rather than \"work\" [3]. Based on these observations, researchers have often utilized self-reports of subjective feeling during an ongoing activity or participant categorization of the activity as \"fun\" or \"work\" as a surrogate measure of intrinsic motivation [6, 7]. As a more objective measure of intrinsic motivation, some researchers relied on the time one spends on the activity of interest without any tangible reward [8]. In addition to the measurement of intrinsic motivation, experimental methodology to manipulate the state of intrinsic motivation is valuable in investigations into the mechanisms of intrinsic motivation. However, many previous studies on intrinsic motivation relied on surveys or field experiments [3]. Thus, there are few alternatives to experimentally induce states with high and low intrinsic motivation. Today, one of the most widely accepted methods to experimentally manipulate the state of intrinsic motivation is the stopwatch (SW)/watch-stop (WS) paradigm [9] first introduced in a seminal functional magnetic resonance imaging (fMRI) study investigating the neural mechanisms of the undermining effect [10], i.e., the detrimental effect of external rewards on intrinsic motivation [8]. In the stopwatch (SW) task in this paradigm, participants were instructed to stop a stopwatch at a predefined time by mentally tracking the time elapsed. This game-like task was enjoyable and induced a high level of intrinsic motivation. In the watch-stop (WS) task, the stopwatch stopped automatically, and the participants were instructed to respond by pressing a button as soon as the stopwatch stopped counting. Compared with the SW task, the WS task was less engaging and even boring, and it induced low levels of intrinsic motivation.