*2.2. fMRI-Visual Stimuli*

The visual stimuli were pairs of filled line drawings of geometric figures (squares, triangles, circles) presented on an LCD screen (see Figure 1). Each figure was filled with one of four possible colors (red, green, yellow, or blue). Stimuli were presented over a grey background (RGB: 64,64,64). A fixation point was presented in the center of the screen (white-filled circle, diameter = 3 pixels). The stimulus consisted of a pair of geometric figures, which were shifted 150 pixels to the left or to the right in regard to the fixation point; this horizontal deflection subtended ±4.5 degrees of visual angle relative to the center of the screen. The LCD screen resolution was 1920 × 1080 pixels (InroomViewingDevice, NordicNeuroLab, Bergen, Norway). The stimuli were back-projected on the screen and viewed through a system of mirrors mounted inside the head coil. Each stimulus pair was presented for 200 ms. Following the presentation of the stimulus pair, the short irregular inter-trial interval of varying duration of 1300 ms to 2300 ms occurred. During this period, only the fixation point was visible.

**Figure 1.** An outline of the cognitive task with two example trials separated by the inter-trial interval. In the actual experiment, 15 trials were presented during each 30 s block. The yellow arrow (not shown during the actual experiment) indicates the target stimulus. The relative size of the stimuli has been enlarged for illustrative purposes. Participants were instructed to respond with the left index finger to each instance of the distracter stimulus, and with the right index finger to each instance of the target stimulus.

#### *2.3. Cognitive Task*

The cognitive task used in the study involved the detection of visual feature conjunctions, namely the shape and the color. The study protocol was based on a blocked design, with each block lasting for 30 s. There was one type of an active block (repeated 6 times during the scanning session), which was interspersed with baseline blocks (7 repetitions). The functional scanning session always began and ended with the baseline block. All participants practiced the task before the scanning session (with three active blocks).

During the active block, 15 pairs of filled geometric figures appeared on the screen, presented on the left or the right side of the fixation point. Each active block was preceded with an instruction screen shown for 2000 ms containing the prompt "Detect the red square" in the center of the screen, displayed in yellow font. The volunteers had to detect the target stimulus in each pair, which was a specific combination of color and shape—a red square (see Figure 1 for the cognitive task outline). The participants had to react to this stimulus with right index finger using an NNL ResponseGrip (NordicNeuroLab, Bergen, Norway). They were instructed to respond with their left index finger to all other combinations of features and shapes, called distracter stimuli; for example, blue squares, red circles etc. Participants were required to respond in each trial.

To decrease participants' expectations about the probability of the target, the proportion of pairs containing the target stimulus varied in each block. In two blocks, the target probability was set to 0.8, in another two, it was decreased to 0.2, and in the remaining two blocks, it was 0.5. The order of the blocks was random. During the so-called baseline blocks, the participants' task was to keep their eyes on the fixation point.

The task difficulty was set to equalize the task performance during both the rested and fatigued conditions. In this way, detected differences between the rested and fatigued conditions could indicate modulation of task-related neural processing evoked by the alteration in tiredness, and not the confounding effects of varying subjective task difficulty.

#### *2.4. Image Acquisition and Processing*

The study was performed with a GE DISCOVERY MR750w with a 3.0 T field. For the presentation of the stimuli, an LCD screen with a horizontal angle of 24◦ was used, which was viewed by the participants through a system of mirrors in the head coil. The initiation of the procedure was synchronized with the first scanner TR by NNL SyncBox (NordicNeuroLab, Bergen, Norway https://nordicneurolab.com/product/responsegrips/ (accessed on 20 August 2021)). The participants responded with NNL grips (NordicNeuro-Lab, Bergen, Norway) by pressing the buttons under the index fingers of the left and right hand.

An 8-channel coil was used in all tests. Structural images were acquired with a 3D Fast Spoiled Gradient Echo (FSPGR BRAVO) sequence (TR = 8.496 ms, TE = 3.26 ms, TI = 450 ms, matrix 256 × 256 × 124, single voxel size: 0.9375 mm × 0.9375 mm × 1.2 mm). Functional images within the fMRI task used the echo planar (EPI) sequence with the following parameters: TR = 2000 ms, TE = 30 ms, 64 × 64 × 35 matrix, single voxel size: 3.125 × 3.125 × 3.5 mm, 196 repetitions, 4 dummy scans at the beginning of each scanning session.
