## 3 Results
### Multiple Linear Regressions  
A series of multiple linear regressions was carried out to test whether the I/E measures could predict the scores obtained in the scales of the NASA-TLX that showed differences as a function of task difficulty (i.e., mental demand, performance, effort, and frustration). The I/E measures that discriminated the two levels of task difficulty (i.e., task accuracy, time on task, fixation duration, fixation frequency, blink duration, blink frequency, saccade frequency, and heart rate), the task, and all two-way interactions (i.e., I/E measures-task) were inserted as predictors in the regression models. The outcomes of the regression model with the mental demand as the dependent variable indicated that there was a significant collective effect between the considered predictors (F (16, 43) = 4.684, \(p<0.001\), R2 = 0.49). However, the individual predictors and the interactions did not significantly predict the mental demand scores (all \(p_{s}>0.05\)). Similarly, the regression model which considered the frustration scores (F (16, 43) = 2.455, \(p<0.001\), R2 = 0.28) highlighted a collective significant effect but no significant effect of individual predictors or interactions (all \(p_{s}>0.05\)). Considering the other two  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Variables**} & \multicolumn{2}{c}{ST} & \multicolumn{2}{c}{DT} & \multirow{2}{*}{**Tests**} & \multirow{2}{*}{\(p\)} \\  & **M (SD)** & **M (SD)** & & \\ \hline NNI & 0.82 (0.058) & 0.79 (0.063) & \(t\)(29) = \(-\)2.07 & 0.047 \\ Saccade Duration (ms) & 110.92 (18.41) & 111.51 (26.74) & N = 30, W = 220 & 0.81 \\ Pupil Diameter (mm) & 4.26 (0.75) & 4.32 (0.77) & \(t\)(29) = 1.58 & 0.13 \\ EDA (mS) & 3.71 (2.82) & 3.67 (3.06) & N = 30, W = 198 & 0.48 \\ Physical Demand (NASA-TLX) & 48.33 (25.91) & 41.17 (24.62) & N = 20, W = 65 & 0.14 \\ Temporal Demand (NASA-TLX) & 41.33 (21.13) & 49.67 (24.28) & N = 24, W = 213.5 & 0.071 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Means and standard deviations of each variable that did not show a significant difference between the two conditions, the corresponding statistical tests, and \(p\)-values. Note: ST = single-task; DT = dual-task.  
Figure 12: (a) Mean perceived effort and (b) perceived frustration as a function of task difficulty. *** \(p<0.001\).  
regression models, no collective significant effect between the predictors was shown (i.e., performance: F (16, 43) = 1.862, \(p>0.05\), R2 = 0.19); effort: F (16, 43) = 1.859, \(p>0.05\), R2 = 0.19).  
A second multiple linear regression was performed to check if the implicit measures (i.e., time on task, fixation duration, fixation frequency, blink duration, blink frequency, saccade frequency, and heart rate) could predict the explicit one (i.e., task accuracy), also considering the task and all two-way interactions between I/E measures and the task. Results of the regression model indicated that there was a collective significant effect between the considered predictors (F (15, 44) = 4.582, \(p<0.001\), R2 = 0.48). We further examined the individual predictors. The task (B = 26.57, \(t=2.335\), \(p<0.05\)), the heart rate (B = \(-0.052\), \(t=-2.046\), \(p<0.05\)), and the interactions respectively between fixation duration \(\times\) task (B = \(-0.23\), \(t=-3.613\), \(p<0.001\)) and blink Frequency \(\times\) Task (B = \(-0.159\), \(t=-2.241\), \(p<0.05\)) were able to significantly predict the task accuracy. In Figure 13, the significant interactions that emerged in the regression model are depicted.