Abstract
Purpose: :
To measured contrast sensitivity to equiluminant gratings using steady-state visual cortical evoked potential (ssVECP) and psychophysics
Methods: :
Six healthy volunteers were evaluated for the ssVECP and psychophysics (28.3±3.5 years old). The visual stimuli were 6 Hz red-green or blue-yellow horizontal sinusoidal gratings at 8 contrast levels (Visage system, CRS). Chromaticities were at the highest contrast: red, u’=0.288, v’=0.480; green, u’=0.150, v’=0.480; blue= u’=0.219, v’=0.420; yellow= 0.219, v’=0.540. Test stimulus comprised 5o central visual field with 34.3 cd/m2 mean luminance. Stimuli were displayed on a Diamond Pro 2070 (Mitsubishi) monitor. Background area was fixed at the stimulus mean luminance and chromaticity (u’= 0.219, v’ = 0.480). Eight spatial frequencies from 0.2 to 8 cpd were used. Heterochromatic Flicker Photometry (HFP) at 20 Hz was assessed to obtain the equiluminance for each subject in each spatial frequency. To evaluate signal to noise ratio, statistical significance was estimated as in Meigen and Bach (1999), and thresholds were estimated by linear regressions fitted to data points as a function of log contrast. Contrast threshold was obtained by extrapolating second harmonic amplitude values to zero. We estimated the psychophysical contrast threshold using stimuli at 6 Hz and static. Contrast sensitivity was calculated as the inverse function of the pooled cone contrast threshold
Results: :
For red-green measurements ssVEP and both psychophysical CSFs were low-pass functions. The ssVEP CSF was similar to the dynamic psychophysical CSF, while the static CSF was higher from 0.4 up to 6 cpd (ANOVA p<0.05). The blue-yellow CSFs showed no specific tuning shape, however at higher spatial frequencies the ssVEP showed higher contrast sensitivity than the psychophysical CSFs (ANOVA p<0.05)
Conclusions: :
ssVECP can reliably be used to evaluate chromatic CSFs in accordance to psychophysics, mainly if the same temporal properties are applied to the stimulus. Tuning of red-green sensitivity data was similar in comparison with suprathreshold measurements from literature. It could be conjectured that low pass tuning curves observed for red-green stimulus could be related to activity of cortical single opponency receptive fields. Blue-yellow CSFs showed no specific tuning and there was no good correlation between ssVECP and psychophysics
Keywords: color vision • electrophysiology: clinical • visual cortex