Contrast-response functions of the multifocal steady-state VEP (MSV)

Objectives: To measure contrast-response functions (CRFs) for 9 visual field (VF) regions and nonlinear interactions between regions using a multifocal steady-state VEP (MSV). Methods: Ten normal adults were tested (51.7 ± 16.9. yr, 5 females). Stimuli resembling those of the Frequency Doubling Technology (FDT) perimeter were presented in 9 VF regions simultaneously, which were modulated at incommensurate temporal frequencies (mean 19.7. Hz). Responses were recorded to 11 contrasts from 3% to 89%, using 8 scalp electrodes. Two repeats of a 20. s duration stimulus were averaged for each contrast. Results: The CRFs were log-linear except for a depression near 7% contrast (p= 0.0008), which was prominent in the central VF. The effects of VF region, stimulus frequency and recording electrode were significant (all p< 0.016). Significant responses at frequencies corresponding to interactions between VF regions also appeared. Electrodes that were best for the interactions and second harmonic responses differed, suggesting different cortical sources. Conclusions: Despite short recording durations the saturating CRFs meant that significant responses could be measured to low contrasts, and be distinguished from nonlinear interactions. Significance: Recording MSVs to low contrast FDT-like stimuli might be useful for quantifying damage by glaucoma and other visual disorders.