Hazard Detection by Drivers with Paracentral Homonymous Field Loss: A Small Case Series

Abstract

Introduction: Stroke frequently causes homonymous visual field loss. We previously found in a driving simulator that patients with complete homonymous hemianopia had difficulty detecting potential hazards on the side of the field loss. Here we measured the effects of limited paracentral homonymous field loss on detection performance.

Methods: Three patients with paracentral homonymous scotomas, yet meeting vision requirements for driving in the United States, performed a pedestrian detection task while driving in a simulator. Pedestrians appeared in a variety of potentially hazardous situations on both sides of the road. Three age- and gender-matched control participants with normal vision participated for comparison purposes.

Results: Pedestrians appearing in the scotomatous side of the visual field were less likely to be detected, and when they were, reaction times were longer, frequently too late to respond safely.

Conclusions: Although legally permitted to drive in the U.S.A., and possibly in other countries, patients with paracentral homonymous field loss may have impaired hazard detection and may benefit from education about their deficit and a fitness-to-drive evaluation.

Keywords: Low vision; driver licensing; hemianopsia; quadranopia; quadranopsia; quadrantanopia; sectoranopia.

Figure 1

A–C): Binocular central 60° visual field plots for patients with homonymous paracentral scotomas. Shading represents binocular blind areas; rectangular shape indicates approximate area of a typical car windshield (right extent ends near 50°). D): Location and size of pedestrians (−14°, −4°, 4°, & 14°) 3 seconds after appearance on city drives (on highway, pedestrian figures appeared half as large, because they were positioned at twice the distance). Dark polygon shows position of central monitor of driving simulator (irregular bottom from view obstructed by car hood).

Figure 2

Reaction times to pedestrians for patients and three age- and gender-matched control participants. Thick lines show median reaction times, whiskers show inter-quartile range (25% and 75% reaction times).

Figure 3

Percentages of pedestrian appearances for which reactions were timely or untimely, and detection failures.

Figure 4

Case 3’s eye-gaze data for two pedestrians at the large right eccentricity (median filtered with a 5-frame window, 60 Hz). Left: Lateral (top) and vertical (bottom) eye position for a +14° pedestrian. Bottom left graph shows three distinct downward saccades, the last of which caused his upper quadrant to obscure the pedestrian for approximately two seconds (Pedestrian Envelope shows the vertical angle subtended by the pedestrian). Right: Although making four downward saccades, the pedestrian is detected as illustrated by a rightward saccade (~277 seconds after drive start) and the horn-press that quickly followed.