The participants were split into two groups, to perform a hard and easy version of the task, with the level of difficulty depending on the time between the letters and the number of changes in the sequence. All of them performed the same task repeatedly over a period of more than six hours. The hard version required retaining larger amounts of information in working memory, so those performing it experienced more cognitive fatigue.

Between trials, the researchers measured cognitive fatigue by asking the participants to make simple decisions, such as whether they would like to receive a small amount of money immediately or a larger one later, the assumption being that cognitive fatigue will reduce their self-control so that they are more impulsive.

They found that those who performed the hard version of the task were indeed slightly more impulsive. The scans revealed that this was also associated with an 8% increase in glutamate levels in the lateral prefrontal cortex, which is well-known to play an important role in reward and decision-making. This increase was not seen in participants who performed the easy version of the task.