Three-rods test as drivers' license vision test from the viewpoint of reproducibility, eye deviation, and functional visual acuity

BACKGROUND: Three-rods test is required as depth perception vision test to obtain motor vehicle license to drive taxies, buses, and trucks in Japan. Functional visual acuity is measured automatically by successive visual target presentation in a fixed period of time. This study examined three-rods test from the viewpoint of reproducibility, eye deviation, and functional visual acuity to assess the feasibility for drivers' license vision test. METHODS: At three-rods test, a central rod was moved at the speed of 50 mm/sec forward and backward automatically against two fixed rods on both sides inside an illuminated box. An examinee at the distance of 2.5 m observed the rods inside the box from a small window and pushed a button to stop the central rod in alignment with the fixed rods. Erred distance of the central rod from the fixed rods as a mean of 4 measurements was used. At functional visual acuity test, an examinee moved a joystick to the same direction as Landolt-C opening as a visual target which was sequentially presented every 2 seconds for 30 times in 1 minute. RESULTS: The mean erred distance of three-rods test was reproducible between two tests done on separate occasions (n = 44, ρ = 0.679, P < 0.0001, Spearman rank correlation). Exophoria induced by wearing 4-prism-diopter base-out prism did not significantly influence the mean erred distance while vertical diplopia induced by wearing 4-prism-diopter base-up prism disrupted the measurement (n = 9). The mean erred distance of three-rods test was better correlated with functional visual acuity tested with both eyes open than with conventional visual acuity with both eyes open (n = 17, ρ = 0.2 versus ρ = 0.179). CONCLUSION: In the context of the small sample size in the present study, the three-rods test was reproducible, and testable in the presence of phoria, but not testable in diplopia, indicating the feasibility as a depth perception vision test.