Impact of pulsed stimulation on objective and subjective visual acuity measurements in nystagmus

Purpose Quanz et al. (2024) reported that participants with nystagmus had higher objective visual evoked potential visual acuity estimates (VA _VEP ) by 0.12 logMAR relative compared to standard psychophysical VA (VA _{Psych_Stat} ). The cause of this modest, but significant VA _VEP overestimation remains unclear. Here we investigated its association with the pattern-pulse stimulation mode applied for steady state VEP recording for VA _VEP estimation. Specifically, we tested whether psychophysical visual acuity to pulsed optotypes (VA _{Psych_Pulsed} ) also exceeds standard optotype VA _{Psych_Stat} . Methods Twelve participants with nystagmus were included in this analysis. VA _VEP was determined for pattern-pulse steady-state VEP stimulation (Quanz et al., 2024) using EP2000, psychophysical VA was determined to stationary (VA _{Psych_Stat} ) and to pulsed (VA _{Psych_Pulsed} ) Landolt-C optotypes employing a modified version of the Freiburg vision Test (FrACT). Pulsed stimulus timing was identical for VEP and VA (40 ms on and 93 ms off, i.e at 7.5 Hz). In a separate measurement, fixation stability within the central 4° was determined using microperimetry (Nidek MP-1), and the eye with the stronger fixation instability was selected for the analysis (12 eyes). LogMAR differences were assessed with a paired t-test and the correlation of fixation stability and VA differences (DVA _Psych = VA _{Psych_Pulsed} – VA _{Psych_Stat} ) was tested. Results VA _{Psych_Stat} (0.43 ± 0.06 logMAR) and VA _{Psych_Pulsed} did not differ from each other (0.45 ± 0.06 logMAR, P = 0.15), but from VA _VEP (0.26 ± 0.08 logMAR, P = 0.02 and P = 0.01, respectively). There was no correlation of DVA _Psych with fixation instability (r ²  = 0.002, P = 0.89). Conclusion Pulsed stimulation appears not to be the reason for the VA _VEP overestimation. Further research should address whether differences in the spatial stimulus properties might be of relevance, as VA _Psych is tested with optotypes, VA _VEP with extended patterns.