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B. D. Gomes, S. L. Johnson, G. S. Souza, M. E. C. Fitzgerald, L. L. Silveira; Chromatic Discrimination Measured With mfVEPs: Luminance Contrast Influence Evaluated With a Signal to Noise Protocol. Invest. Ophthalmol. Vis. Sci. 2009;50(13):5878.
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© ARVO (1962-2015); The Authors (2016-present)
Asymmetries with respect to sensitivity to chromatic stimulation have been described with pattern visual evoked potential. The aim of the present study was to investigate these phenomena using multifocal visual evoked potential (mfVEP).
Four subjects (26 years-old ± 3) with normal or corrected to normal visual acuity were monocularly tested. Stimulus was a standard dartboard pattern array covering 40° of visual angle, and consisting of 120 sectors. The checks were scaled across eccentricity to compensate for the cortical magnification factor variation. Stimuli were presented in a FMSII microdisplay, 75 Hz frame-rate (2 frames / step) (EDI, San Mateo, CA). Checks reversal was driven by a binary m-sequence with an exponent of 15. Chromaticities were modulated to red (CIE 1931: x = 0.5, y = 0.352) and green (CIE 1931: x = 0.21, y= 0.508). Luminance increments were applied to obtain 100, 50, 25, 12.5, and 0% Michelson contrasts. Luminance increments started either from the red (LR-LG/LR+LG) or green phosphor (LG-LR/LG+LR). For each luminance condition first-order (1k) kernel and first slice of the second-order kernel (2.1k) were studied. 1k and 2.1k waveforms from the three central rings comprising 8, 16, and 24 checks, respectively, were analyzed by a signal-to-noise ratio (SNR) protocol using the rooting mean squares of the signal and noise periods, 45-195 and 325-475 ms, respectively. The SNR values for each sector were compared with a threshold SNR value estimated for each luminance contrast condition in order to accept them as a valid response.
As expected for pattern reversal stimuli, responses energy was concentrated in the second-order kernel. Only the first and second central rings evoked responses consistently above threshold SNR values. SNR responses increased from the isoluminance condition to reach a peak at 50% contrast in the green side and 12.5% contrast in the red side of the tested range of luminance contrasts. At 100% contrast, red-black darts evoked larger responses than green-black darts (one way ANOVA (F = 4.6): 10.8).
mfVEP responses exhibited an asymmetric behavior when red-green dart stimulus moved from the isoluminance condition towards a pure red or green luminance contrast.
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