Chromatic spatial contrast sensitivity estimated by visual evoked cortical potential and psychophysics

Abstract

The purpose of the present study was to measure contrast sensitivity to equiluminant gratings using steady-state visual evoked cortical potential (ssVECP) and psychophysics. Six healthy volunteers were evaluated with ssVECPs and psychophysics. The visual stimuli were red-green or blue-yellow horizontal sinusoidal gratings, 5° × 5°, 34.3 cd/m2 mean luminance, presented at 6 Hz. Eight spatial frequencies from 0.2 to 8 cpd were used, each presented at 8 contrast levels. Contrast threshold was obtained by extrapolating second harmonic amplitude values to zero. Psychophysical contrast thresholds were measured using stimuli at 6 Hz and static presentation. Contrast sensitivity was calculated as the inverse function of the pooled cone contrast threshold. ssVECP and both psychophysical contrast sensitivity functions (CSFs) were low-pass functions for red-green gratings. For electrophysiology, the highest contrast sensitivity values were found at 0.4 cpd (1.95 ± 0.15). ssVECP CSF was similar to dynamic psychophysical CSF, while static CSF had higher values ranging from 0.4 to 6 cpd (P < 0.05, ANOVA). Blue-yellow chromatic functions showed no specific tuning shape; however, at high spatial frequencies the evoked potentials showed higher contrast sensitivity than the psychophysical methods (P < 0.05, ANOVA). Evoked potentials can be used reliably to evaluate chromatic red-green CSFs in agreement with psychophysical thresholds, mainly if the same temporal properties are applied to the stimulus. For blue-yellow CSF, correlation between electrophysiology and psychophysics was poor at high spatial frequency, possibly due to a greater effect of chromatic aberration on this kind of stimulus.

Influence of Stimulus Size on Simultaneous Chromatic Induction

“…Reflecting this early visual processing, the cells in the cortex respond to higher spatial frequencies of the L/M-modulating stimuli than those in the S-cone ( Wandell and Silverstein, 2003 ; Solomon et al, 2004 ; Johnson et al, 2010 ). Psychophysical experiments revealed that observers can detect higher spatial stimuli that modulate the L/M-cone chromaticity than those that modulate the S-cone chromaticity ( Mullen, 1985 ; Webster et al, 1990 ; McKeefry et al, 2001 ; Mullen and Kingdom, 2002 ; Barboni et al, 2013 ). This suggests that the spatial resolution of color vision is higher for the L/M-cone chromaticities than for the S-cone chromaticities.…”

Visual evoked potentials to change in coloration of a moving bar

“…Several studies have demonstrated that the color aberration and isoluminance value related to retinal eccentricity vary depending on the target extent and spatial frequency ( Bilodeau and Faubert, 1997 ; Barboni et al, 2013 ). However, Bilodeau and Faubert (1997) have shown that while they manipulated with spatial frequency and size of the target, the isoluminance values within central 20 degrees did not change.…”