Schepens Eye Research Institute scientists have found that--when tested in a driving simulator--patients with hemianopia (blindness in one half of the visual field in both eyes) have significantly more difficulty detecting pedestrians (on their blind side) than normally sighted people. These results, published in the November 2009 issue of Investigative Ophthalmology and Visual Science, fly in the face of some recent on-road studies that have found most people with hemianopia safe to drive.
"The results are important because they mean we need to continue to look carefully at people with this condition and evaluate them individually to determine their fitness to drive," says Dr. Eli Peli, principal investigator of the study and senior scientist at Schepens Eye Research Institute.
In at least 22 states and many other countries, people with hemianopia are prohibited from driving because they do not meet the visual field requirements for licensure.
"Our study urges caution in opening the door for people with hemianopia to start or continue driving again," says first author, Dr. Alex Bowers, who is an Assistant Scientist at Schepens. She and Peli believe the Schepens study, because it was conducted in the safety of a driving simulator over a longer period of time and "miles" than a typical "on-road" study, provides new insights into detection failures of drivers with hemianopia. These detection failures might not be revealed in a short on-road test in which there is little control over the appearance of potential "blind-side" hazards.
More than a million people suffer from hemianopia, a condition in which one half of the visual field in both eyes is blinded, usually the result of a stroke or head injury. People with hemianopia often do not know what they can't see and frequently bump into and trip over objects while walking. Driving is a whole other challenge.
Peli and his research team compared 12 subjects with hemianopia to 12 visually normal people, matched by age, sex, and years of driving experience. All subjects drove for about 120 minutes along city roads and rural highways. During the journey pedestrians appeared at random intervals along the roadway and at intersections.
Subjects pressed the horn button every time they saw a pedestrian. The team then measured detection rates and response times based on these horn-press responses.