The mfERG response topography with scaled stimuli: effect of the stretch factor.

In multifocal electroretinogram recordings, the stimulus array is usually scaled with eccentricity to compensate for cone density changes. The strength of this scaling is parameterized by the so-called "stretch factor" (SF). In this study, we determined the quantitative influence of the SF striving for equal response densities over the entire stimulus area. VERIS 4.8 software was used to record multifocal ERGs from 11 normals with 61 hexagons and a 60 degrees stimulus diameter. Six recordings were obtained per subject with SFs 0, 12, 23, 25, 27, and 29. For analysis, we calculated the mean hexagon area per eccentricity and the average amplitude for elements of equal eccentricity (rings 1-5 = R1-R5), normalized to R2. As expected, recordings with SF = 0 showed the steepest amplitude drop-off with eccentricity (relative amplitudes of 1.90 (R1/R2), 1.0 (R2/R2), 0.67 (R3/R2), 0.51 (R4/R2), 0.38 (R5/R2). Using the preset SF = 12 in our set-up resulted in R1-amplitudes 1.7x larger than the peripheral amplitudes (relative amplitudes 1.39, 1.0, 0.88, 0.87, 0.82). SF = 23 compensated best for eccentricity, yielding relative amplitudes of 1.2, 1.0, 1.02, 1.08, and 1.11. Deviations increased again with higher SFs. To minimize amplitude variability over the stimulated retinal area, the optimal SF needs to be adjusted to the individual recording set-up.