## 4 Discussion

Table 4: Detailed statistics for Figure 5. Three-way repeated measures ANOVA test with pos-hoc comparisons using Tukey HSD method. * Shows significance at the 0.05 level. when participants used PPLs compared to single-vision lenses. On the other hand, the study from [PERSON] et al. [28] recorded the eye movements of 38 presbyopes using the Tobii X3-120 eye tracking system while participants were using two different PPL designs. Participants were asked to read aloud a text displayed on a monitor screen at centered and off-axis gaze directions located at 0.67 m. The results showed greater fixation time and the number of fixations in off-axis gaze directions in comparison with the central position. Finally, the study from [PERSON] et al. [30] recorded eye movements using a Tobii Pro Glasses 3 device on 28 participants using different PPLs. Participants were asked to read the text at far and near distances. The results showed that fixation time and the number of fixations were affected by the PPL design. All of these studies showed statistically significant differences in eye movements associated with the unwanted refractive errors present in the lateral areas of the PPLs. However, all these methods are based on reading tasks whose difficulty could vary from one experiment to another. To eliminate this uncertainty, we have used, as a reading test, the standard eye charts that are used to evaluate visual acuity under the same standardized conditions in which VA is clinically measured. So, in this paper, we propose a simple way to enhance the gold standard evaluation of VA by incorporating new metrics based on the characteristics of eye movements. To our knowledge, this is the first time an eye-tracking system is used while measuring VA and while using different PPL designs, and this method has proven to be sensitive enough to identify differences between designs and gaze direction. In this study, we also incorporated the analysis of two well-known factors that affect visual performance. Firstly, it is obvious that recognition difficulty depends on the eye chart size. In this sense, when the letter became smaller, the task difficulty increased. As expected, results confirmed that with a smaller optotype size, there was a statistically significant longer test duration, longer fixation time, and higher fixation count than eye charts with a larger optotype size. Secondly, it is well known that unwanted refractive results showed statistically significant longer test duration, longer complete fixation time, and a greater number of fixations for off-axis gaze directions in comparison with the central gaze direction. Future studies could improve the experimental setup by incorporating changes that enable the evaluation of eye movements in a more natural setting. Currently, the assessment of far-distance VA involves using a 21.5-inch screen positioned 5.25 m away from the subject's eyes, resulting in a narrow horizontal field of view of 4.2\({}^{\circ}\). To assess a wider field of view, the subject must be rotated in three different gaze directions while using a chin rest to prevent head motion, which adds complexity to the experiment. Instead, a larger screen with a head tracking system would be a better alternative to the current rotation platform with a chin rest, as it would provide a wider field of view and eliminate the need for rotation. As explained in the flowchart for participant enrollment, 30% of participants were discarded because their recordings did not meet the quality criteria. It would be interesting to study the reasons for the data loss and account for them in future work and also to redefine a quality criterion that could be implemented in the optical practice without compromising the results.