Assessment of Peripheral Visual Acuity in Different Test Conditions

Assessment of peripheral visual acuity (VA) involves the need to split attention between the foveal fixation stimulus and the peripheral test stimulus. One approach to excluding the central fixation task consists of creating an artificial central scotoma using a contact lens with an implanted occluder (4–5 mm in diameter). Using this approach, we measured peripheral VA in three subjects over a range of eccentricities of up to 60° using a specially designed computerized perimetric apparatus using different optotypes – “tumbling E” letters and modified three-bar stimuli. In the conditions of our experiments, occlusion of the fixation stimulus did not itself lead to the expected increase in VA, probably because the effect of the instruction “look directly ahead” was analogous to the effect of the instruction “fix on the visible foveal stimulus.” Furthermore, it can be suggested that the contact lens with occluder had adverse effects because of veiling of the peripheral test stimulus in cases of uncontrollable involuntary decreases in pupil diameter, leading to significant increases in the blind zone. Peripheral VA in all subjects was somewhat higher for modified three-bar optotypes measuring “visual acuity” than for “tumbling E” letters measuring “recognition acuity.” Between-individual variability in the data was greater for smaller eccentricities.