Hemianopic and Quadrantanopic Field Loss, Eye and Head Movements, and Driving

Abstract

There has been considerable debate in the literature regarding the driving safety of individuals with homonymous hemianopic and quadrantanopic field defects.1–5 An important consideration for understanding their driving performance is the extent to which individuals with these field defects might adopt patterns of eye and head movements that assist them to compensate for their field loss. If this were the case, it would provide justification for exploring the potential for predicting whether an individual with these field defects might have the potential for safe driving and for training scanning behaviors as a means of improving driver safety in these individuals.6–10 Numerous studies have explored the eye and head movements and scanning behavior of persons with homonymous hemianopia in well-controlled laboratory-based settings; however, none have been conducted under real-world driving conditions. These laboratory-based studies have shown that persons with hemianopic field defects mainly look toward their blind hemifield when undertaking a range of tasks, including counting dots,11–13 viewing natural and degraded images,14 viewing randomly presented13 and moving targets15 within a virtual reality environment, but not when assembling wooden models in a static environment.11 Martin et al.11 explain their findings by suggesting that these compensatory strategies of biasing gaze in the direction of the blind hemifield are most evident in dynamic and unpredictable environments, where subjects cannot rely on their spatial memory to locate salient objects. This hypothesis was recently supported by Hardiess et al.,13 who found that the differences in gaze patterns between hemianopes who performed visual search tasks at “adequate” or “inadequate” levels were dependent on the level of complexity of the search task. They suggested that the poorer performance of the inadequate performers on the more complex task was due to reduced working memory. Given that driving is a complex and dynamic task, where drivers cannot rely on their spatial memory to locate salient objects, we hypothesize that individuals with hemianopic defects might similarly adopt head and eye movements that bias fixation toward the blind field while driving, and that those who adopt these strategies will be able to successfully compensate for their field defects and exhibit safer driving performance. In addition, few studies have assessed the on-road driving characteristics of hemianopic and quadrantanopic drivers, including speed, braking, acceleration, cornering, and lane keeping, which might also differentiate between safe and unsafe drivers. Szlyk et al.1 in an interactive driving simulator study reported higher numbers of lane boundary crossings for a small sample of persons with hemianopia compared with controls, while on-road studies have also reported problems with steering stability and lane keeping.2,5 Bowers et al.16 in a driving simulator study also showed that hemianopic persons adopted a lane position toward their seeing field, therefore providing a safety margin on their blind side. However, this finding has not been verified for actual on-road driving performance, an issue that is addressed in this study. The aim of the present study was thus to compare the patterns of eye and head movements, lane keeping, and vehicle control of drivers with homonymous hemianopia and quadrantanopia to that of age-matched drivers with normal visual fields while driving under real-world conditions. We also compared the eye and head movements of those hemianopic and quadrantanic drivers rated as safe to drive with those rated as unsafe. We hypothesized, based on the evidence of previous studies, that persons with hemianopia would make more head movements into their blind field as a means of compensating for their field defects and that this would be more evident in those rated as safe to drive. We also hypothesized that those rated as unsafe to drive would adopt a lane position in the direction of their seeing field to avoid their blind side, while those rated as safe to drive would maintain a relatively central lane position.