Simulating Partial Conduction Delays in VEPs Via Asynchronous and Contrast-Modulated Stimulation

To investigate the origin of P100 double peaks occasionally observed in patients with optic neuritis by experimentally inducing timing asynchronies between visual field locations and modulating stimulus strength via contrast reduction in transient and steady-state VEPs (trVEP and ssVEP).

The trVEPs and ssVEPs were recorded using a checkerboard stimulus vertically split into nasal and temporal half-fields. Timing asynchrony was induced by delaying the temporal half-field by 24, 35, or 47 ms; stimulus strength was modulated by reducing the temporal half-field contrast to 20% or 8%. Full-field and half-field responses were quantitatively analyzed and qualitatively compared to characterize the superposition effects induced by these stimulus changes.

In trVEPs, contrast reduction produced smaller, broader P100 responses with modest peak-time delays and only sporadic double-peak formation. Conversely, induced temporal half-field delays robustly generated P100 double peaks (incidence: 50% at 24 ms, 85% at 35 ms, and 100% at 47 ms). Quantitative and qualitative analyses demonstrated that the P100 peaks of the half-field responses aligned temporally with the two peaks of the full-field double peak. In ssVEPs, increasing delays reduced the absolute harmonic magnitude and altered the second-to-first harmonic ratio (decreased at 24 and 35 ms; increased at 47 ms).

Spatially localized timing asynchronies reliably reproduce double-peaked P100 morphologies, supporting differential conduction speeds within the optic nerve as a plausible mechanism in patients with optic neuritis.

Linking experimentally induced conduction-delay effects to VEP morphology enhances the clinical interpretation of atypical VEP waveforms in suspected optic neuritis.