Driving with central field loss I: effect of central scotomas on responses to hazards

Abstract

Objectives: To determine how central field loss (CFL) affects reaction time to pedestrians and to test the hypothesis that scotomas lateral to the preferred retinal locus will delay detection of hazards approaching from that side.

Methods: Participants with binocular CFL (scotoma diameter, 7°-25°; visual acuity, 0.3-1.0 logMAR) using lateral preferred retinal fixation loci and matched controls with normal vision drove in a simulator for approximately 1½ hours per session for 2 sessions a week apart. Participants responded to frequent virtual pedestrians who appeared on either the left or right sides and approached the participant's lane on a collision trajectory that, therefore, caused them to remain in approximately the same area of the visual field.

Results: The study included 11 individuals with CFL and 11 controls with normal vision. The CFL participants had more detection failures for pedestrians who appeared in areas of visual field loss than did controls in corresponding areas (6.4% vs 0.2%). Furthermore, the CFL participants reacted more slowly to pedestrians in blind than nonscotomatous areas (4.28 vs 2.43 seconds, P < .001) and overall had more late and missed responses than controls (29% vs 3%, P < .001). Scotoma size and contrast sensitivity predicted outcomes in blind and seeing areas, respectively. Visual acuity was not correlated with response measures.

Conclusions: In addition to causing visual acuity and contrast sensitivity loss, the central scotoma per se delayed hazard detection even though small eye movements could potentially compensate for the loss. Responses in nonscotomatous areas were also delayed, although to a lesser extent, possibly because of the eccentricity of fixation. Our findings will help practitioners advise patients with CFL about specific difficulties they may face when driving.

Figure 1

Binocular visual field plots for each subject and their individual reaction times for the 4 pedestrian eccentricities ( to appearances at each eccentricity, median 22). Reaction times for the group of normally-sighted control participants are shown at bottom right. CFL subjects S1& S2 have scotomas to the left of their PRL in visual field space and were predicted to have longer reaction times to the-4° pedestrians than to pedestrians at the other 3 eccentricities; predictions for each participant are shown with gray highlight over the relevant eccentricities. Box lengths show the 25% to 75% extent and whiskers show the maximum extent of cases that are not outliers. Percentages under each plot show detection rates.

Figure 2

Median reaction times for seeing and blind areas. Data for each participant on city and highway drives are connected by straight lines. CFLs had reaction times longer than controls, and longer to pedestrians in their blind than seeing areas (above diagonal). As expected, NVs had similar reaction times in “blind” and seeing areas. CFL medians were longer on rural highway drives (filled circles shifted up and right).

Figure 3

Proportion of untimely reactions for seeing vs. blind areas. Data for each participant are connected by straight lines. Participants with CFL had much higher untimely reaction rates than controls, particularly in their blind areas and on rural highways. Controls also had more untimely reactions in rural highway than in city drives.