Imitating the effect of amblyopia on VEP-based acuity estimates.

PURPOSE: Acuity testing based on visual evoked potentials (VEPs) overestimates acuity in patients with amblyopia. We hypothesized that distortion and fragmentation of the stimulus in amblyopia impede recognition of optotypes, while it leaves the pattern onset response in the VEP mostly unaffected, resulting in overestimation of acuity.

METHODS: Acuity VEPs were recorded in visually normal participants with the stimulus degraded by patterned polymethyl methacrylate panes, which induce distortion and fragmentation. For comparison, frosted panes were used to induce blur through wide-angle scattering. Standard psychophysical optotype acuity was recorded under the same conditions.

RESULTS: With the distorted and fragmented stimuli, the VEP consistently overestimated acuity relative to psychophysical optotype acuity. With blurred stimuli, both measures were in good agreement.

CONCLUSIONS: The data support the assumption that stimulus distortion and fragmentation leave VEP-based measures of acuity relatively unaffected, resulting in a discrepancy between measures of acuity that are based on checkerboard VEPs on one hand and psychophysical optotype recognition on the other hand. The technique of stimulus degradation described here provides a simple and efficient way of imitating effects that are known from amblyopia and may thus serve as a tool in the evaluation of vision tests.