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

Quanz et al. (Sci Rep 14:16797, 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}.

Twelve participants with nystagmus were included in this analysis. VA_VEP was determined for pattern-pulse steady-state VEP stimulation (Quanz et al. in Sci Rep 14:16797, 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 (ΔVA_Psych = VA_{Psych_Pulsed} - VA_{Psych_Stat}) was tested.

VA_{Psych_Stat} (0.43 ± 0.06 logMAR) and VA_{Psych_Pulsed} (0.45 ± 0.06 logMAR, P = 0.15) did not differ from each other, but from VA_VEP (0.26 ± 0.08 logMAR, P = 0.02 and P = 0.01, respectively). There was no correlation of ΔVA_Psych with fixation instability (r² = 0.002, P = 0.89).

Pulsed stimulation appears not to be the reason for the VA_VEP overestimation in nystagmus. 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.