## 4 Discussion  
This study first aimed to investigate the feasibility of combining I/E measures to understand if they could provide univocal interpreted pieces of information in an ecological task (i.e., repetitive work) characterized by the manipulation of the imposed mental workload. The second aim was to analyze whether the I/E measures' variations matched the perceptions of users about the workload experienced. Therefore, we intended to verify the participants' awareness regarding the mental demand imposed by the different tasks, because the adaptive changes in the functioning of future symbiotic systems will be based on the modification of I/E measures. Thus, in the case of a match between I/E measures and users' perceptions, the user will comprehend the actions implemented by the system (i.e., transparency). We have defined these two issues in terms of data interpretability and system transparency. We believe that whenever they will be solved, as in a symbiotic system, the HMI collaboration will be certainly improved. Regarding the interpretability, we compared the results concerning the ability of these measures to discriminate between two distinct levels of MWL evoked by two tasks of different levels of difficulty (i.e., single- vs. dual-task). The majority of the considered I/E measures (i.e., psychophysiological, behavioural, performance) were able to distinguish between the difficulty (and thus the MWL) of the two tasks. Although the screwing task does not appear to be extremely difficult (i.e., a limited number of errors in both experimental sessions), when participants had to deal with the interference task (i.e., dual-task condition), they made more errors (i.e., 4.3 vs. 2.7). Furthermore, the outcomes related to this explicit measure reflected one of the considered implicit measures of performance (i.e., time on task: dual-task (DT), 873.36 s; single-task (ST), 692.97 s). Figure 13: Interactions of the regression model: fixation duration \(\times\) task (**a**) and blink frequency \(\times\) task (**b**). In our ecological paradigm, some of the indices concerning ocular behaviour have proven to be equally useful in discriminating the MWL levels, thus demonstrating once again the I/E binomial correspondence. These findings are also in line with the literature, considering laboratory settings, where a shorter duration and a lower number of fixations [21; 23; 26], lower blinks, and a lower saccade rate [22; 26; 27; 28; 39; 40] were shown in dual-task conditions. Therefore, these ocular indices could be used in real-world working tasks with characteristics similar to the present ones. Regarding the psychophysiological measures, the HR has also shown the ability to discriminate between the two levels of task difficulty, proving its reliability to monitor changes in the user's MWL. During the dual-task, the higher level of HR is consistent with previous studies that considered different tasks in terms of imposed mental load [22; 23; 41]. Some of the considered measures show contrasting results with respect to the literature. In the present research, the blink duration showed an increment in the higher mental load condition (i.e., DT), whereas previous studies reported a decrement in such circumstances [22; 24; 42]. Reviewing the DT experimental sessions' video-recordings, several participants were backward counting, keeping their eyes closed for prolonged time intervals. This occurrence affected the average blink duration, as has been noticed in internally focused tasks [43]. Furthermore, the interference task (i.e., backward counting) did not add visual demands on the participants, which have been proven as a factor that shortens blink duration [42; 44; 45]. The expected outcome considering the NNI was a higher dispersion of fixations in the dual-task (i.e., an NNI close to 1). The absence of a difference could be due to the participants' strategy, as observed in the dual-task session's videos.