SEPARABLE RED-GREEN AND LUMINANCE DETECTORS FOR SMALL FLASHES

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.