Facilitation between the luminance and red–green detection mechanisms: enhancing contrast differences across edges

Previous studies have shown that detection of a red–green test pattern, such as a spot or grating, may be facilitated two to three times by a suprathreshold luminance pedestal of the same shape. We measured facilitation between the red–green (RG) and luminance (LUM) detection mechanisms using sine and square-wave gratings. Facilitation of RG by luminance pedestals was 3-fold for in phase sine-wave gratings of 0.8 cpd and a remarkable 7-fold for square-wave gratings. The latter facilitation was greatly reduced at intermediate relative phases and was generally reduced at higher spatial frequencies. We show that on a uniform field, the red or green regions of low spatial frequency test patterns are detected approximately independently, but in the presence of the LUM pedestal RG becomes sensitive to the red–green difference across the luminance edges. Under optimal conditions (with the low-frequency, square-wave luminance pedestal) this increased red–green sensitivity corresponds to a wavelength discrimination threshold as small as ∼0.04 nm. This conversion of RG into an ‘edge detector’ may explain why facilitation is twice as large for square-wave gratings (bipolar patterns) than spots (unipolar patterns). The reverse facilitation, that of LUM by the red–green pedestal, is weaker and the results suggest that this is because LUM is initially sensitive to the light–dark difference across luminance edges even in the absence of the red–green pedestal.