**3. Results**

#### *3.1. Behavioral—Fatigue Symptoms*

The average sleep length during the night preceding the study was 1.7 ± 1.1 h (min = 0 h, max = 3 h) in the fatigued condition, as compared to 6.8 ± 1.1 h (min = 4 h, max = 9 h) in the rested condition. As some of the participants reported napping, we also measured time since last waking up. In the fatigued condition, the time was on average 8.2 ± 6.6 h (min = 2.5 h, max = 26 h), whereas in the rested condition, the times were the following: 2.9 ± 1.0 (min = 2 h, max = 5 h). The drivers had significantly more symptoms of sleepiness and fatigue in the fatigued condition than in the rested one, which confirmed the successful manipulation of their state (see Tables 1 and 2).

As shown in Table 1, the drivers participating in the study were more tired, and consequently reported more severe symptoms of fatigue in the assumed fatigued condition than in the rested one. Significant differences between these two experimental conditions were obtained in three subscales, as well as in the general score in the Japanese Questionnaire.

The results in Table 2 show that the reduced level of self-reported cognitive functioning of the drivers was the most symptomatic (the highest score on the cognitive-attenuation subscale in the fatigued condition compared to other subscales).

**Table 1.** Severity of fatigue symptoms reported in the Japanese Questionnaire (lower score = more severe reported symptoms). The general score was defined as the sum of the results from three subscales. All *p* < 0.001.




#### *3.2. Behavioral—Task Performance*

There were no significant differences in the level of task performance between the fMRI sessions. The percentages of correct answers were 97 ± 3% for the rested condition and 95 ± 5% for the fatigued condition (*p* = 0.06). However, there were noted differences in the response speed. The mean response time was 13% longer for the fatigued condition; particularly for the right hand, the average response time increased from 644 ± 44 ms to 731 ± 130 ms (*p* = 0.01); whereas for the left hand, the mean response time for the rested condition was 657 ± 42 ms, while for the fatigued condition, the mean response time was 743 ± 113 ms (*p* = 0.004).

#### *3.3. Imaging Results (Analysis Level 2)*

#### 3.3.1. Main Effects of the Task in Both Conditions

The results are displayed in Figures 2 and 3, as well as presented in Tables 3 and 4. In both conditions, we observed a mostly overlapping pattern of activations in the lateral frontal, lateral parietal, and medial frontal regions. In both conditions, we also observed overlapping clusters in the left lateral temporo-occipital regions. There were marked differences between both conditions. In the rested condition, the general extent of suprathreshold activation was notably wider and more bilateral than in the fatigued condition. The strongest effects of task in the rested condition were observed in the posterior parietal cortex, as well as in the premotor cortex, extending into the dorsolateral cortex, mainly in the right hemisphere. Bilateral activations were also observed in the anterior insulae and in the medial frontal regions (SMA). In contrast with the fatigued condition, we observed bilateral activations in the polar visual cortex, as well as in the fusiform gyrus.

**Table 3.** Main effects—rested condition. The table lists the approximate anatomical details of the suprathreshold clusters. Z-score represents the maximum Z value within the given cluster. X, Y, and Z values denote MNI coordinates of the maximum Z-value voxel. Cluster volume is given as a voxel count. Anatomical labels were identified using the Harvard-Oxford Cortical and Subcortical Atlases, Juelich Histological Atlas, and Talairach Daemon Labels as provided by FSL software (https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/Atlases (accessed on 20 August 2021)). The characters within parentheses accompanying the anatomical labels indicate the hemisphere—left or right.


**Figure 2.** The main effects of the task in the rested condition. Images were thresholded at Z > 3.1. Note the extensive bilateral activation in the posterior parietal cortices, SMA, premotor cortex, and dorso-lateral prefrontal cortex. Activation in the middle temporal cortex was left-lateralized. The right side of the brain is denoted by letter R.

**Figure 3.** The main effects of the task in the fatigued condition. Images were thresholded at Z > 3.1. Note the more extensive activations in the right hemisphere of the brain (denoted by the letter R) in the posterior parietal cortices, supplementary motor area (SMA), premotor cortex, and dorso-lateral prefrontal cortex in comparison to the rested condition (see Figure 2). Activation in the middle temporal cortex maintained its lateralization in the left hemisphere.

**Table 4.** Main effects—fatigued condition. The table lists the approximate anatomical details of the suprathreshold clusters. Z-score represents the maximum Z value within the given cluster. X, Y, and Z values denote MNI coordinates of the maximum Z-value voxel. Cluster volume is given as a voxel count. Anatomical labels were identified using the Harvard-Oxford Cortical and Subcortical Atlases, Juelich Histological Atlas, and Talairach Daemon Labels as provided by FSL software. The characters within parentheses accompanying the anatomical labels indicate the hemisphere—left or right.


In the fatigued condition, the extent of the suprathreshold clusters was notably smaller. Bilateral activations were seen only in the parietal regions, while in the frontal regions, the activations were mainly observed in the right hemisphere. In neither condition were the activations correlated with the reaction times.
