It would be informative to have an electrophysiological method to
study, in an objective way, the effects of mercury exposure and other
neurotoxics on human color vision performance. The purpose of the present
work was to study human color discrimination by measuring chromatic
difference thresholds with visual evoked potential (VEP). Six young normal
trichromats (24 ± 1 years old) and one deutan (26 years old) were
tested. The stimuli consisted of sinusoidal isoluminant chromatic gratings
made from chromaticity pairs located along four different color directions
centered on two reference points. Heterochromatic flicker photometry (HFP)
protocol was used to obtain the isoluminance condition for every subject
and for all chromaticity pairs. Spatial frequency was 2 cycles/deg.
Presentation mode comprised onset (300 ms)/offset (700 ms) periods. As
previously described, we found a negative deflection in the VEP which was
related to the chromatic difference: as chromatic difference increased,
amplitude increased and latency decreased. VEP response amplitude was
plotted against distance in the CIE 1976 color space between the grating
chromaticities and fitted with a regression line. We found color
thresholds by extrapolating the fitting to null amplitude values. The
thresholds were plotted in the CIE 1976 color space as MacAdam ellipses.
In normal trichromats the ellipses had small size, low ellipticity, and
were vertically oriented. In the deutan subject, the ellipses had large
size, high ellipticity, and were oriented towards the deutan copunctal
locus. The VEP thresholds were similar to those obtained using grating
stimuli and psychophysical procedures, however smaller than those obtained
using pseudoisochromatic stimuli (Mollon-Reffin method). We concluded that
transient VEP amplitude as a function of contrast can be reliably used in
objective studies of chromatic discrimination performance in normal and
altered human subjects.
