Selective cone suppression by the L-M- and M-L-cone-opponent mechanisms in the luminance pathway

We investigated how transient changes of background color influence the L- and M- (long- and middle-wavelength-sensitive-) cone signals in the luminance pathway. Motion identification thresholds were measured for a drifting sinusoidal grating (1 cycle/deg) modulated along different vector directions in L- and M-cone contrast space. The color of a central 4-deg-diameter region was briefly altered (500 ms) by incrementing or decrementing either L- or M-cone excitation. Incrementing L-cone and decrementing M-cone excitation produced a field that appeared reddish relative to the yellow surround. Likewise, incrementing M-cone and decrementing L-cone produced a field that appeared greenish. Motion identification thresholds were obtained on the yellow field following the brief color transitions. The results show that the threshold for the L-cone direction was selectively elevated by the background substitution of incrementing L-cone and decrementing M-cone excitation (shift toward reddish color). The same substitution, however, did not affect the threshold in the M-cone direction. Similarly, the threshold for the M-cone direction was selectively elevated by the background substitution of incrementing M-cone, decrementing L-cone excitation (shift toward greenish) without affecting the threshold in the L-cone direction. Experiments using the motion quadrature paradigm confirmed that these effects occur within the luminance mechanisms. These results indicate that the activation of L-on plus M-off signals suppresses the L-cone signal and that the activation of L-off plus M-on signals suppresses the M-cone signals in the luminance pathway. We propose a retinal model based on the experimental results.