A few studies have assessed eye and head compensatory strategies in patients with glaucoma.19,32 We found that glaucoma patients who were safe to drive showed increased exploration activity in terms of more eccentric head movements, compared with glaucoma subjects who failed the test. Thus, the present simulator study replicated the findings of our recent on-road study and the study of Coeckelbergh et al.32 by means of sophisticated eye and head tracking and suggests that active scanning by means of head movements is an efficient way to compensate for a glaucomatous VFD affecting the binocular visual field.19
These findings are at odds with a recent simulator study, which showed that patients with mild to severe glaucoma displayed similar eye scanning behavior with the control group.35 However, the horizontal field of view in the above study was considerably smaller than that in the present study, only 9 of 23 participants had binocular field loss, the driving session lasted considerably shorter, there was no other traffic apart from static obstacles, and the detection task included targets that are not part of a driving scene, namely, verbalization of letters. Our results regarding longer saccadic amplitudes and more lateral eye position in safe drivers are more consistent with recent studies using video-based hazard perception tasks.18 The authors also reported that patients with binocular glaucomatous VFDs performed more eye movements than control participants.18 An increase in saccade rate in patients with glaucoma was also associated with better performance in a search task when viewing images of everyday scenes.36 A recent study investigating the viewing behavior of patients with binocular glaucomatous VFDs during a supermarket shopping task in a real setting reported that an increase in saccade rates strongly correlated with the ability to quickly find the objects of interest, probably an attempt to compensate for their restricted field of view.37
Additionally, patients who failed the test showed a tendency for longer fixation duration and less fixations per minute compared with patients who passed the test. The smaller number of fixations indicates decreased eye scanning activity, and longer fixation duration appears to be associated with an inability to acquire visual information in a quick and effective manner, as observed in patients who passed the test. Because new information is acquired during fixations, the finding that patients who failed made fewer saccades suggests that they were unable to process as much of the visual scene as patients who passed the test. In a recent study,36 patients with glaucoma had longer average fixation duration compared with control subjects when viewing everyday scenes. However, the latter study did not correlate gaze parameters with performance and the nature of the task was substantially different from the present one, as cognitive demands were minimal in that participants were presented with static, consistent information and were tasked to simply “enjoy” the images.36 Conversely, our results are more consistent within another study regarding eye movement behavior when viewing a dynamic driving scene. In the latter study, glaucoma patients produced more and, thus, shorter fixations than the control subjects when searching for hazards in the Hazard Perception Test.38 Hence, viewing behavior appears to be related not only to compensatory potential but also to the task complexity and quantity of visual information.
Speed control and adaptation to the speed limit is an important skill. Some drivers with glaucoma may try to compensate for their degraded visual ability by reducing their driving speed. We found that safe drivers drove at slower speeds and this strategy possibly provides enough time to scan their visual environment, because eye movements and especially head movements are time-consuming. However, very slow driving can be dangerous, because the vehicle may represent an obstacle for the other drivers.32 Other on-road studies reported adequate speed control in glaucoma subjects, and because of the variable results, we believe that this skill warrants further investigation in larger series of patients.19,33
Despite the total number of 14 participants in this costly study, the number of subjects in each subgroup was relatively small for statistical analysis. Additionally, although we have tried to design and score the driving test as close to real-world conditions as possible, effects arising from the use of a simulator in a virtual environment with a variable degree of fidelity cannot be avoided. We decided to do the experiment in the simulator because of the unique possibility of standardizing the traffic scenario and thereby establishing identical and thus comparable driving conditions for all participants. Finally, the severity of VFDs varied between patients from severe visual field loss to small circumscribed areas. However, a common characteristic was that they fail to meet the European driving regulations.
In conclusion, this study supports the hypothesis that a considerable subgroup of subjects with binocular visual field loss attributed to glaucoma shows a safe driving behavior in a virtual reality environment, because they adapt their viewing behavior by increasing scanning. By means of a driving simulator and sophisticated eye and head tracking, individual performance differences in terms of driving safety were related to visual exploratory behavior. This type of compensation improves traffic safety and may have practical implications in planning individualized driving fitness tests and driver rehabilitation programs.
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