The peripheral visual field is marked by a deterioration in color
sensitivity, sometimes attributed to the random wiring of midget
bipolar cells to cone photoreceptors in the peripheral retina (Mullen, 1991; Mullen &
Kingdom, 1996). Using psychophysical methods, we explored
differences in the sensitivity of peripheral color mechanisms with
detection and discrimination of 2-deg spots at 18-deg eccentricity, and
find evidence for a postreceptoral locus for the observed loss in
sensitivity. As shown before, observers' sensitivity to green was
lower than to red in the periphery, although the magnitude of this
effect differed across observers. These results suggest that the
asymmetry in peripheral sensitivity occurs at a postreceptoral site,
possibly a cortical one. In addition, noise masking was used to
determine the cone inputs to the peripheral color mechanisms. The
masked detection contours indicate that the red and green mechanisms in
the periphery respond to the linear difference of approximately equally
weighted L- and M-cone contrasts, just as they do in the fovea. Thus,
if the midget retinal ganglion system is responsible for red/green
color perception in the fovea, it is likely to be responsible at 18-deg
eccentricity as well.