Detection contours were measured in L and M cone contrast coordinates
for foveal flashes of 200 msec duration and 2.3, 5, 10 and 15 min are
diameter on a bright yellow field. The test dash consisted of simultan
eous incremental and decremental red and green lights in various ampli
tude ratios. At all sizes, the most sensitive detection mechanism was
not a luminance mechanism, but rather a red-green mechanism that respo
nds to the Linear difference of equally, weighted L and M cone contras
ts, and signals red or green sensations at the detection threshold. Bo
th temporal and spatial integration were greater for red-green detecti
on than luminance detection. A coincident, subthreshold, yellow flash
(a luminance pedestal) did not affect the threshold of the red-green m
echanism, Such a pedestal is a sum of equal L and M cone contrast-it r
epresents a vector parallel to the red-green detection contour and thu
s is expected not to stimulate directly the red-green mechanism. When
suprathreshold, the coincident pedestal facilitated chromatic detectio
n by similar to 2x at all tested sizes; intense pedestals did not mask
chromatic detection. This insensitivity to intense luminance pedestal
s further indicates that the red-green mechanism has fixed spectral tu
ning with balanced opponent L and M contrast inputs. This view of fixe
d spectral weights contrasts with the ''variable tuning hypothesis'',
which postulates that the weights change with spatial-temporal variati
ons in the test stimulus. The red-green mechanism, when facilitated by
the suprathreshold luminance pedestal, can account for the color disc
rimination of small, slightly suprathreshold, incremental monochromati
c flashes, so it is not necessary to postulate an array of detectors w
ith variable spectral tuning for small flashes.