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C. M. Poloschek, A. Fuchs, M. Bach; The mfERG Response Topography With Scaled Stimuli - Effect of the Stretch Factor. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5992.
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© ARVO (1962-2015); The Authors (2016-present)
In multifocal electroretinogram recordings the stimulus array is typically scaled with eccentricity to account for the varying cone density over the stimulus field. The factor by which the size of every hexagon is changed is commonly called "stretch factor". In this study we determined the quantitative influence of the stretch factor (SF), striving for equal response densities over the entire stimulus area.
VERIS 4.8 software was used to record multifocal ERGs from 11 normal subjects (mean age 32 ± 13 years) with a resolution of 61 hexagons and a stimulus diameter of 30 degrees of visual angle. 6 recordings were obtained per subject with SFs 0, 12, 23, 25, 27 and 29. For analysis, the average amplitude for elements of equal eccentricity (rings 1-5 = R1-R5) was calculated and the average ring values were normalized to R2.
Recordings without a stretch factor had relative amplitudes of 1.88R1/R2, 1.0R2/R2, 0.66R3/R2, 0.51R4/R2, 0.38R5/R2. Relative amplitudes using SF 12 were 1.39, 1.0, 0.88, 0.87, and 0.82 respectively. SF 23 had the least deviation from the relative amplitude of 1.0 with ratios of 1.2, 1.0, 1.06, 1.08, and 1.11. Deviations increased again with higher SFs (see figure).
As expected, the unscaled stimulus array showed the steepest amplitude drop off with eccentricity. SF 23 came closest to an equal response amplitude over the stimulus field. There was no stretch factor that provided equal amplitudes at all eccentricities of the stimulus: small stretch factors will result in a marked amplitude drop off; as the stretch factor increases, this drop off inverts to an amplitude rise. Deviations from the ideal stretch factor can be compensated for by a correction factor calculated as the absolute difference to the relative amplitude 1.0. This can be of special value to assess clinical recordings in which amplitude reduction caused by localized pathology needs to be identified correctly.
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