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A plausible link between the time-on-task effect and the sequential task effect

Mental fatigue can be studied by using either the time-on-task protocol or the sequential task protocol. In the time-on-task protocol, participants perform a long and effortful task and a decrease in performance in this task is generally observed over time. In the sequential task protocol, a first effortful or control task is followed by a second effortful task. The performance in the second task is generally worse after the effortful task than after the control task. The principal aim of the present experiment is to examine the relationship between these two decrements in performance while concomitantly using a sequential task protocol and assessing the performance of the first effortful task as a function of time-on-task. We expect a positive correlation between these two decrements in performance. A total of 83 participants performed a 30-min fatiguing mental task (i.e., a modified Stroop task) or a control task followed by a time-to-exhaustion handgrip task. As expected, this protocol combining the time-on-task and sequential task protocols allowed us to observe (1) a decrease in performance over time during the Stroop task, (2) a worst performance in the handgrip task after the Stroop task by comparison to the control task, (3) a positive correlation between these two effects. The decrease in performance during the Stroop task also correlated with the subjective measures of boredom and fatigue, whereas the detrimental effect observed in the handgrip task did not. Our findings suggest that the two fatigue-related phenomena share a common mechanism but are not completely equivalent.


Results concerning error rates in the Stroop task while differentiating the two types of trial
An ANOVA with Time-on-task (Part 1, Part 2, Part 3, Part 4) and Type of trial (reading the color name, naming the ink color) as within-subjects factors was conducted on the accuracy during the Stroop task. The interaction and the main effect of TOT did not reach significance: F (3, 234) = 1.27, p = .285, ƞ²p = .016, and F (3, 234) = 0.49, p = .689, ƞ²p = .006, respectively. By contrast, the main effect of the Type of trial was significant: F (1, 78) = 68.98, p < .001, ƞ²p = .469. Participants showed a higher accuracy for the 'reading color name' trials (M = 97.5%, SD = 2.8%) than for the 'naming ink color' trials (M = 91.1%, SD = 8.6%).

Correlations between subjective variables and behavioral phenomena of interest
A first series of correlations has been examined between subjective variables (boredom and fatigue) and behavioral effects of interest (time-on task effect, sequential task effect) while mixing the two types of trial for the time-on-task (TOT) effect. In this case the subjective score assessed just after the Stroop task was considered to calculate the correlation (and not the composite score as reported in the article). For these correlations the TOT effect was calculated by subtracting the IES scores of the fourth part from the first part of the Stroop task, and then divided this difference by the IES score of the first part.
TOT effect and boredom score after the Stroop task: r = -.348, p = .002. Sequential task effect and boredom score after the Stroop task: r = -174, p = .126.
TOT effect and fatigue score after the Stroop task: r = -.184, p = .104. Sequential task effect and fatigue score after the Stroop task: r = -.077, p = .503.
A second series of correlations has been examined between subjective variables (boredom and fatigue) and behavioral effects of interest (TOT effect, sequential task effect) while differentiating the two types of trial for the time-on-task effect (see Table A1). For these correlations, the TOT effect was calculated by subtracting the mean reaction time for correct responses when the participants had to name the ink color of the fourth part from the first part of the Stroop task, and then divided this difference by the reaction time of the first part.

Mediation analysis
In order to be sure that boredom mediated the relationship between the time-on-task effect and the sequential task effect, we should first obtain a significant correlation between the three variables. As mentioned in Figure A2, the correlation between the boredom index (i.e., the difference between the boredom score after the Stroop task minus the boredom score after the Video task) and the sequential task effect was marginal, r = -.216, p = .055. By contrast, the correlation between the boredom index and the time-on-task effect was significant: r = -.332, p = .003. Then, a mediation analysis was conducted with bootstrap for 5000 samples. The total effect was significant, β (standardized) = 0.272, p = .013. The indirect effect was not significant: β = 0.047, p = .249. Finally, the direct effect, when controlling for the boredom index, was at the limit of significance: β = 0.25, p = .050. This made the mediation analysis inconclusive, because the direct effect was at the limit of significance. Figure A2: Mediation analysis between the time-on-task effect, the sequential task effect and the boredom index. * = significant correlation between two variables.

A systematic review of studies analyzing the time-on-task effect within the sequential task protocol
This systematic review aimed to examine if studies using the sequential task protocol with a long depleting task (duration ≥ 30 min) analyzed the performance in the depleting task as a function of time-on-task and the correlation between the time-on-task effect and the sequential task effect; two phenomena generally interpreted as manifestations of mental or cognitive fatigue.
To be included in the systematic review, a study should use the sequential task protocol with a depleting task and a control task longer than 20 min. The meta-analysis of Hagger et al. Consequently, the systematic review focused on studies from sport sciences examining the effect of mental fatigue on a subsequent physical performance.
Sources that was used to identify studies for the systematic review included Academic Search Complete, CINAHL with full text, APA PsycINFO, and SPORTDiscuss with full text through the EBSCO platform. The search strategy was the following: AB ((mental N0 fatigue OR cognitive N0 fatigue) AND (control N0 condition OR control N0 treatment OR control N0 trial OR control N0 task)). The flow chart of the systematic review is represented on Figure A3. The selected studies are listed in Table A2. This table clearly showed that out of the 56 listed studies, 40 (71.43%) observed a sequential task effect on the performance of the dependent task, and only 10 (17.86%) reported a time-on-task effect on the performance of the depleting task. By contrast, only 5 studies (8.93%) observed a practice effect on the performance of the depleting task and 10 (17.86%) did not observe a significant effect on this dependent variable. Concerning our main hypothesis, only 8 studies (14.29%) observed both a time-on-task effect and a sequential task effect and were able to calculate a correlation between these two effects, but none of these 8 studies reported this correlation. The small sample size of these 8 studies (10 < N < 25; M = 16.6; SD = 6.2) can explain why they did not report the correlation between the two effects of interest. Finally, among the 56 studies listed in table A2, 32 (57.14%) did not report the performance of the depleting task as a function of time-on-task and consequently were not interested by the effect of mental fatigue that could be observed in the depleting task.