Experimental paradigm
cognitive fatigue
The experiment consisted of a modified fMRI version of a feedback-based, paired-associate word learning task (Paired-word association task procedure. Participants first learned associations between arbitrary target words and response options by memorizing the highlighted correct response (Phase 1). They then repeated the same trials in the scanner (Phase 2), but were asked to indicate the correct response (which was no longer highlighted) for each target word. After making their response, they were presented with a feedback stimulus that signaled response accuracy and varied by feedback condition (monetary feedback, non-monetary feedback, or no feedback). During monetary feedback trials, “+$1.00” written in green font was presented after correct responses, and “−$0.50” written in red font was presented after incorrect responses. During non-monetary feedback trials, a green checkmark and red “X” symbol were presented after correct and incorrect responses, respectively. During no feedback trials, a black “#” symbol was presented after every response, regardless of accuracy, thus providing no information about performance. Participants also rated their current cognitive fatigue levels every ten trials using a visual analog scale anchored between 0 (not at all mentally fatigued) and 10 (extremely mentally fatigued). The final phase (Phase 3) served as a test of associative memory, during which feedback was no longer provided. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)Phase 1 (study phase) was completed on a computer outside the scanner and consisted of 150 trials. During each trial, participants viewed a target word and two response options below it. All words were matched for word length, which ranged between four and eight letters, and were also semantically dissimilar from each other. One of the response options was surrounded by a green box, which indicated the correct response for the target word. Participants were instructed to remember the correct word pair. Each trial during Phase 1 lasted four seconds.fMRI data were acquired during Phase 2 of the task. 150 trials from the study phase were presented again in a randomized order (four seconds per trial) – this time, without a box appearing around the correct response option. Participants were instructed to select the correct paired associate that they learned from the previous phase using one of two buttons. Participants were presented with a feedback stimulus for one second (The final phase (test phase) tested associative memory, and was completed outside the scanner. Participants viewed the same 150 trials in a randomized order and made their selections via keypress. No feedback appeared after their responses. None of the participants verbally reported difficulty performing the task, nor did they display any overt behavioral tendencies indicating a lack of understanding during the task.