A survey of 1022 Chinese college students revealed that using a computer for more than 3 hours per day was one of the risk factors for asthenopia. Another study from the United States reported similar results, arguing that asthenopia occurred in 90% of users exposed to computers for more than three hours a day. In the present study, all participants used computers for at least 4 hours per day, and 81.7% had asthenopia. Dry eye was the most common symptom (90.3%), followed by decreased vision (75.3%), itchy eyes (73.1%), and blurred vision (61.3%). In 2021, Zayed et al used the same questionnaire (CVS-Q) to investigate 108 IT practitioners and found an asthenopia rate of 82.41%. In addition, Touma et al found that 67.8% of 457 American college students reported at least one symptom of asthenopia, among which blurred vision was the most common, accounting for 27.0%.
Compared with asthenopia, which occurred in 81.7% of cases, only 4 (4.3%) participants had accommodation dysfunction in our study. It seemed that accommodation dysfunction could not reflect the status of asthenopia sensitively. In the present study, the CVS-Q total score was only correlated with positive relative accommodation (PRA). Positive relative accommodation represents the ability of the accommodative system to see near target under maximum strain, as measured by the addition of negative lenses in front of both eyes on a far correction basis. The affected positive relative regulation indicated that the ability to regulate tension worsened. Wang et al used a different questionnaire; nevertheless, they obtained the result consistent with ours, suggesting that asthenopia was significantly positively correlated with positive relative accommodation. They also found that asthenopia was significantly negatively correlated with negative relative accommodation, while there was no significant correlation with accommodation sensitivity.
The association of the visual disturbance and accommodation was tested. Previous studies suggested that asthenopia was largely due to uncorrected or inadequate accommodation of ametropia  and that college students with at least one ocular or visual complaint had significantly less accommodative amplitude and were near the point of convergence than those who without fatigue. Still, our results revealed no significant correlation between accommodation function and the scores of visual symptoms, such as difficulty in seeing things at close range. It seems that in healthy young people, the index of accommodative function cannot always sensitively reflect the symptoms of visual impairment. Accommodation was closely related to symptoms such as near vision difficulties only in the population meeting the diagnostic criteria of accommodative dysfunction. Also, the accuracy of optometry or accommodation examination is 0.25D. Compared with the measurement accuracy of 0.4 ° of the eye tracker and the sampling frequency of 120 Hz, the accuracy of the results of accommodation examination may be limited.
Moreover, we found significant positive correlations between the symptoms of difficulty in seeing near things and the total duration of fixation, total interview time, and reading speed, though these correlations were not very strong. These results suggest that the more severe the symptoms of difficulty in seeing things at a close distance, the longer it takes to locate the target location and complete reading. We found a positive correlation between perceived decreased visual acuity and the number of regressive saccades, defined as the number of saccades in each line of text that were read in the opposite direction (from right to left). A regressive saccade is a process of returning to the previous text to retrieve information, which breaks the original semantics and syntax. In reading scenes, about 10–25% of eye movements belong to regressive saccade. Regressive saccades are very important for reading and for the process of retrieving information. The positive correlation between the visual disturbance score and the regressive saccade indicated that the increase in regressive saccade would compensate for the missing text information during reading when the visual signal is insufficient. There was a weak positive correlation between the double vision and the number of unknown saccades, which referred to the number of saccades unrelated to the acquisition of text information in each line of text. Previous studies reported double vision to be associated with eye diseases such as strabismus and glaucoma[27, 28]. In this study, the positive correlation between unknown saccades and double vision implied the probability of gaze position deviation caused by the increased object image ghosting.
Secondly, the association between ocular disturbance and accommodation was analyzed, revealing no significant correlation. The eye discomfort in asthenopia is mainly caused by dry eye syndrome[18, 29], which is characterized by tear film instability, tear hyperosmolality, ocular surface inflammation, and corneal nerve abnormalities. In addition, uneven tear film distribution may also lead to visual fluctuation or blurring, which affects performance in visual tasks. Van and colleagues found that among older adults, those with dry eye were more likely to experience reading difficulties; however, there was no significant decrease in reading speed compared with normal controls. Nonetheless, real-time tear film cannot be detected during reading. In the present study, burning, itching, tearing, and red-eye symptoms were found to be significantly correlated with the number of fixation points and the number of regressive and unknown saccades.
During the reading process that lasted some 20 minutes, the eye movement parameters in the first reading material and the last reading material were compared, revealing that a number of eye movement parameters significantly decreased, including the average duration of fixation, the total duration of fixation, the number of fixation counts, the total visit time, the number of visits and the reading speed. Also, a number of eye movement parameters significantly increased, including average visit time and the number of unknown saccades.
“Visit” includes the process from the first fixation appearing in the region of interest to the first fixation moving out of the region of interest, i.e., the sum of all fixations and saccades in this range. Katz et al. found that an increase in the number and duration of visits to specific regions of interest was associated with increased visual attention. In the current study, both the number of visits and the total duration of visits decreased, but the average duration of visits increased, which may be related to the increase in attention concentration.
As mentioned above, fixation duration and frequency are important predictors of reading fluency, and prolonged fixation duration indicates a decline in text readability. Our results revealed that as the reading progressed, the sum and mean fixation duration decreased, and the number of fixation count in the area of interest also decreased, implying lower and shorter fixation. Similarly, in a non-reading scenario, Wang et al used an eye tracker to record 25 subjects watching videos on a computer and found decreased eye movements (fixation number and fixation duration during the first hour) and the increased unknown saccades . Nevertheless, in another study, the eye movements of 38 subjects during a 40-minute simulated computer task were observed, and the changes in mean fixation time were not significant, while the number of unknown saccades increased. Smith et al observed eye movements in 14 glaucoma patients during reading in 2014 and found more meaningless saccades in poor vision eyes than in good vision eyes. Also, the increase in meaningless saccades slowed down the reading speed .
To sum up, in a short reading time of about 20 minutes, the visit time and frequency changed, the number and time of fixation decreased, the unknown saccade increased, and the reading speed slowed down. Compared with studies on longer visual tasks reported over recent years [12, 15],this study further found that the eye movement parameters significantly changed over a short period. As in such a relatively short period of time, asthenopia may not rise so significantly to be detected by the subject; this proves the potential of eye-tracking technology for sensitive assessment of asthenopia. Among the above eye movement parameters, the decrease in fixation duration and counts may be one of the potential indicators related to asthenopia.
This study has a few limitations. First, we only included young people. In further research, different age groups should be recruited, as well as incipient presbyopia population or people from different educational backgrounds or pseudophakic eyes. Also, the sample size should be expanded. Second, a dry eye should be examined for participants, as ocular surface signs could provide more information.