Visual evoked potentials measure luminance contrast masked by color noise.

Abstract

Purpose : Luminance contrast is influenced by color (chromatic) noise. We investigated the sensitivity of the visual evoked cortical potential (VEP) to chromatic and spatial noise to determine its utility as a measure of these influences. Essentially, do changes in VEP amplitude evidence that our visual system detects changes in luminance without chromatic cues?

Methods : Five adults (four males, 20-30yrs) underwent a visual acuity test, Ishihara color test, and basic disease evaluation by indirect ophthalmoscopy to ensure normative vision. The stimulus controlled for mean luminance (10cd/m²⁾ and composed of chromatically and spatially varied mosaic circles on a mosaic field gave the perception of various spatial gratings. Four spatial frequency gratings alternating at 60% contrast were temporally reversed at 0.5Hz. The target covered 6° of the visual field. O1 and O2 surface electrodes were used to record VEPs from each participant at each stimulus permutation. VEP amplitudes from each participant were averaged for each stimulus type and data from trichromatic participants were analyzed using repeated-measures ANOVA for linear trends.

Results : One participant was missing 2x2-grating data at the O1 recording site and was excluded from these analyses (O1 N=4, O2 N=5). Typical waveform features (P1, N1, P2) were consistent across participants. At O1, a quadratic analysis produced large effect sizes, which we interpreted given our sample size limitations: VEP amplitudes decreased with lower spatial frequencies, then increased with higher spatial frequencies for P1 and N1 values (p=.252, η²=0.40 for P1; p=.267, η²=0.38 for N1;). The opposite was true for P2 values (p=.255, η²=0.40 for P2). At O2, large effect sizes were present for a linear trend (also non-significant): VEP at P1 and P2 increased in amplitude with increasing spatial frequency (p=.610, η²=0.63 for P1; p=.235, η²=0.33 for P2); no trend was evident at N1. No pairwise amplitude comparisons were statistically significant, although large O1 N1, O2 P1, and O2 N1 η² values implied differences with a larger N.

Conclusions : The cortical responses to luminance contrast masked by chromatic noise were reliable among the subjects and this pilot study suggests VEP sensitivity to the combined impact of luminance and color on the visual system. Future work should expand on statistical trends observed here to confirm reliability of the VEP’s sensitivity to these measures.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.