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M. Laron, H. Cheng, B. Zhang, L. J. Frishman; The Effect of Contrast on the Multifocal Visual Evoked Potential (mfVEP). Invest. Ophthalmol. Vis. Sci. 2008;49(13):3311. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To study effects of contrast and eccentricity on the mfVEP.
MfVEPs were recorded from one eye of 7 healthy subjects (age 28 ± 6, VA 20/25 or better) using a 60 sector cortically scaled pattern reversal dartboard stimulus (VERIS). Mean luminance was fixed at 78 cd/m2, and the Michelson contrast of the checks in the dartboard was varied over 6 steps (10, 25, 35, 50, 75, and 95%). Two 7 min runs were averaged for each contrast, and analyzed with customized software.1 For each subject, mean signal-noise-ratio (SNR)2 of the 60 sectors was calculated for each contrast. Average SNR was calculated for 6 concentric rings of the dartboard of increasing eccentricity (1 to 23 deg radius). Latencies3 were compared across contrast levels and eccentricities.
The mean SNR of the subjects for 95% contrast ranged from 3.6 to 7.0. The effect of contrast on SNR, for the range of contrasts used, was adequately described by a function relating SNR and log contrast (i.e. an exponential saturation of the response). For all individuals, the overall mean SNR increased linearly with logarithmic increase in contrast (R2: 0.82 -0.98). At each eccentricity, the average SNR for all subjects increased linearly with log contrast as well. The slope of the SNR vs. log contrast function decreased linearly with increasing eccentricity (R2 of 0.97; p = 0.0003). For example, average SNR in the central (foveal) ring increased from 1.5 at 10% contrast to 6.0 at 95% (slope of 4.6 for SNR vs. log contrast), whereas average SNR in the most peripheral ring increased from 1.4 at 10% contrast to 3.2 at 95% (slope = 1.7). The contrast required to achieve 63% (1-e-1) of the maximum, asymptotic, SNR was 44% in the central ring, and ranged from 24% to 18% in more peripheral rings. Mean latency for all subjects decreased 6 (SE ± 1.6) ms from the lowest to the highest contrast tested. No significant change in latency was observed with eccentricity.
For the mfVEP, the increase in SNR with rising contrast is largest in the fovea and declines with eccentricity. Latency is relatively immune to changes in contrast and eccentricity.1. Hood DC and Greenstein VC, 2003. 2. Fortune B et al., 2004. 3. Hood DC et al., 2004.
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