Purpose: The event-related potential P300 has been proposed for objective acuity estimation. In contrast to the acuity VEP, which relies on grating or checkerboard stimuli, the P300 can be recorded to small stimulus differences, making it suitable to use optotype stimuli. This may result in a better agreement with subjective measures of acuity than that found with VEP-based estimates. We tested the feasibility of using Landolt C optotypes with a P300 acuity paradigm and assessed the relationship between subjective and objective acuity estimates for both optotype and grating stimuli.
Methods: Dioptric blur (+1, +2, and +4 diopters) was used to degrade vision. Separate oddball sequences with Landolt Cs and with gratings, respectively, were used to record the P300. Thresholds were estimated by fitting a sigmoid curve to P300 amplitudes as a function of Landolt C size or grating coarseness, respectively. Psychophysical thresholds were also obtained for both stimulus types.
Results: There was a sizable discrepancy in psychophysical acuity estimates between Landolt C and grating stimuli, probably due to spurious resolution with gratings. With both stimulus types, reliable P300 acuity estimates were obtained. These reflected the psychophysical discrepancy. Conversion factors (Landolt C, 2.3; grating, 1.5) allowed for translating P300-based acuity to psychophysical acuity.
Conclusions: We demonstrate, to our knowledge for the first time, that objective acuity estimation with Landolt C optotypes is feasible. Both psychophysical and P300-based estimates share the same stimulus-type dependence. P300-based acuity estimation with optotypes may thus be particularly useful with visual impairments that differentially affect grating and optotype acuity.